Negative CO_2 emissions for long-term mitigation of extremes in land hydrological cycle

Domain: biology
Published: 2026-03-23
Editorial outcome: major_revision

Abstract
Global warming has profound effects on the terrestrial hydrological cycle, leading to alterations in regional extreme weather patterns. While terrestrial precipitation responses under continued greenhouse gas emissions are well established, the responses of terrestrial precipitation and vegetation feedbacks under climate mitigation scenarios remain uncertain. Here, we investigate terrestrial precipitation changes under idealized negative and zero CO emissions scenarios using the Community Earth System Model 2 version 2 (CESM2). Terrestrial precipitation increases by approximately 1.1% at the peak CO concentration (~725 ppm), but shows even greater increases of about 1.9% and 2.5% under substantially lower atmospheric CO concentrations following zero (~600 ppm) and negative (~430 ppm) CO emissions, respectively. Our results suggest that enhanced transpiration from terrestrial vegetation largely contributes to this increase under the negative emissions scenario. Furthermore, despite a substantial increase in terrestrial precipitation, extreme precipitation events and droughts become less severe globally under the negative emissions scenario, even compared to the zero emissions scenario. While near-term mitigation is essential to curb immediate warming, these findings suggest that sustained negative emissions could be effective for achieving long-term reductions in hydrological extremes and enhancing terrestrial water availability.

Introduction
Anthropogenic activities have led to a notable increase in carbon dioxide (CO ) emissions1. The Paris Agreement aims to mitigate global warming by reducing emissions to achieve net-zero; however, increased temperatures are still anticipated2,3. Furthermore, to minimize the damage from future climate changes, it is necessary to reduce CO concentrations through negative carbon emissions4, along with considering large-scale climate intervention methods, such as climate engineering or geoengineering5. Despite these efforts, the climate system may not be able to return to its current state even if CO levels are restored to current levels, meaning that the climate system is potentially irreversible6-10.

So far, many studies have investigated the reversibility of the climate system, identified hysteresis behavior, and explored associated dynamical processes. These include the Atlantic Meridional Overturning Circulation (AMOC)11-13, Intertropical Convergence Zone14, monsoon15,16, carbon uptake17,18, vegetation19, storm tracks20, Hadley cell21, drought area18, extreme precipitation22,23, global heat transport24, global mean surface temperature25-27 and regional precipitaion25,27-29. Among these climate systems, understanding the irreversible and hysteresis behavior of the hydrological cycle is particularly crucial for climate adaptation and mitigation policies, as its changes can have significant impacts on water availability, agriculture, and ecosystems. However, current knowledge of the hydrological cycles and associated extreme precipitation events that respond to increasing and decreasing CO concentrations is still immature, so further research is needed to better quantify the impacts of climate change on the terrestrial hydrological cycle, water availability, and their extremes to inform effective policy decisions.

The plant physiological effect has climate feedback by altering the temperature and precipitation: For instance, the greening of the Earth due to CO fertilization and global warming has caused the evaporative cooling effect due to increased leaf area30. However, under high CO concentrations, the partial closure of plant stomata reduces plant transpiration, offsetting the fertilization effect in vegetation-rich areas31-34. This plant stomatal response causes surface warming34-37 and significantly impacts the precipitation patterns37,38 by changing the surface energy budget39. While this physiological effect can contribute to reduced precipitation in some regions, the rise in CO levels increases near-surface temperature, thereby enhancing atmospheric moisture content by increasing the saturation water vapor pressure of air40,41. The combination of these feedbacks from the increased CO emissions results in an overall increase in global precipitation. However, how plant physiological feedback plays a role in our climate system for the zero and negative CO emissions remains uncertain. This study will show that the plant physiological effect enhances a future hysteresis behavior of the global land precipitation after the net-negative emissions.

To examine climate reversibility and hysteresis, we conducted two idealized CO emission-driven experiments under the positive, negative, and zero emission scenarios with nine ensemble members (Methods and Supplementary Fig. 1). The first scenario, referred to as Z (zero emissions scenario), achieves zero emissions by the year 2125 by first increasing emissions linearly from the year 2001 to 2050 and subsequently decreasing them linearly to reach zero in 2124 (Supplementary Fig. 1a). The rate of increase until 2050 closely follows that of the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) scenario. In this scenario, atmospheric CO concentrations peak at 728 ppm in 2124, approximately 1.9 times the initial level of 383 ppm in 2000. The second scenario, referred to as N (negative emissions scenario), follows the same emission trajectory as Z until 2124 but then continues with negative emissions, reducing atmospheric CO concentrations return to the present-day level of 383 ppm by 2197 (Supplementary Fig. 1b). Additionally, we performed a long-term integration experiment under zero CO emissions until 2400 for both scenarios, extending for over 200 years, to determine whether climate system can revert to its current state, exploring climate reversibility. These experiments allow us to investigate the response of the global hydrological cycle under zero and negative emissions scenarios, as well as to evaluate the potential for reversibility of these changes.

Results
Land precipitation overshoot in the zero and negative emissions scenarios

Figure 1 illustrates the evolution of atmospheric CO concentrations, mean surface air temperature (SAT), and precipitation changes in response to the CO emissions scenarios, compared to the reference period (2001–2030). SAT increases over both ocean and land areas (excluding Antarctica) and then decreases in response to changes in CO concentrations (Fig. 1a). SAT peaks at approximately 2.6 K over land and 2.0 K over ocean at the highest CO concentration (728 ppm). After starting zero emissions in 2125 (Supplementary Fig. 1a), as CO concentration decreases slowly in Z (gray line in Fig. 1a), influenced by the carbon cycles among the atmosphere, ocean, and land42.

The land SAT decreases slightly until 2250, while the ocean SAT remains stable during the same period (orange and sky-blue lines in Fig. 1a). Subsequently, SAT continuously increases until the end of the experiment, even as CO concentrations decrease from roughly 728 to 600 ppm. In contrast, in N , SAT decreases as CO concentrations reduce through the negative emissions until 2197 (red and blue lines in Fig. 1a). After CO concentration returns to its initial level (383 ppm) (dark gray line in Fig. 1a), SAT increases until 2300, accompanied by 47 ppm increase in CO concentration (i.e., 430 ppm) during this period. The rebounded warming observed in Z and N can be attributed to the AMOC change11,13,43, which is weakened by increased CO concentrations but begins to gradually return to its initial state as the CO levels decrease (Supplementary Fig. 2)11,44. Another contributing factor to the warming is the release of heat stored in the ocean, which effectively raises global temperatures, particularly in high-latitude regions45.

Over the ocean, precipitation aligns with ocean SAT in both Z and N (sky-blue and blue lines in Fig. 1a, b), which relates to the warming-induced increase in water vapor supplied from the ocean. Similarly, land precipitation follows the trends of SAT but with differing amplitudes, particularly noticeable in N (red and orange lines in Fig. 1b). It is noted that the land precipitation in N (~2.6%) overshoots around 2300 compared to Z (~1.9%) and the peak CO concentrations period (MAX, 2091–2120; ~1.1%), increasing by more than twofold. This land precipitation overshoot is particularly striking because the land and ocean SAT at that time is lower than those of Z and MAX (Fig. 1a). This indicates that while the increased land precipitation is tied to the delayed climate responses of the ocean14,45—where sustained warming continues to drive atmospheric moisture changes—the magnitude of the overshoot cannot be solely attributed to an increase in water vapor content from air temperature increases. The increased land precipitation in N remains considerably high until the end of the experiment, resulting in similar amounts of land precipitation as in Z from 2300 to 2399 despite the considerable SAT difference (red and orange lines in Fig. 1b). These results suggest that not only the warming-induced water vapor from the ocean but also land-atmosphere coupled processes play significant roles in influencing land precipitation following the negative emissions.

Impact of physiological feedback on hydrological responses

To understand the land precipitation overshoot, we analyzed the land moisture budget, which includes precipitation, evapotranspiration, moisture flux convergence, and change in precipitable water (Methods). In N , transported moisture contributes little to the increase in precipitation during RS (purple line in Fig. 2a), while evapotranspiration rises considerably, suggesting a link to the increase in land precipitation during RS (orange and green lines in Fig. 2a). Evapotranspiration fluctuates in response to changes in CO concentrations due to its dependence on temperature affecting saturated humidity.

Evapotranspiration shows symmetrical responses to increases and decreases in CO concentrations during the positive and negative emission periods (green line in Fig. 2a). However, from 2200 to 2399, the evapotranspiration in N displays a dramatic increase, accounting for approximately 75% of the changes in land precipitation during RS (orange and green lines in Fig. 2a). In Z , moisture flux convergence mostly shows negative values during the restoring period (RS, 2300–2399), indicating weakening of moisture transport from the adjacent oceans except towards the late 2300s (purple line in Supplementary Fig. 3a), contributing to a decrease in precipitation. Concurrently, evapotranspiration continuously rises when land precipitation rises (green line in Supplementary Fig. 3a).

Evapotranspiration comprises water vapor fluxes from the ground (soil evaporation), wetted leaf and stem areas (evaporation of water intercepted by the canopy, also known as vegetation evaporation), and dry leaf surfaces via stomata (transpiration)46. We have analyzed these three components of evapotranspiration and found that their relative contributions vary significantly with CO emissions (Fig. 2b and Supplementary Fig. 3b). Soil evaporation changes similarly to land precipitation but with weak variations (beige line in Fig. 2b). Vegetation evaporation exhibits patterns similar to land precipitation but with greater amplitude than soil evaporation (blue in Fig. 2b), associated with increased vegetation due to the CO fertilization effect and global warming, known as the Greening Earth30 (Supplementary Fig. 4). The change in transpiration, however, is the most dominant and closely follows the changes in CO concentrations rather than land precipitation (Figs. 1b and 2b). In N , transpiration continuously increases after 2220 despite stabilized CO concentration, contributing to increased land precipitation (red line in Fig. 2b). This indicates that the rise in evapotranspiration is largely driven by changes in vegetation, which combine vegetation evaporation and transpiration (Fig. 2b). Notably, the vegetation evaporation during RS in N is lower than during MAX, suggesting that the overshoot in land precipitation is closely associated with the transpiration. As the CO concentrations rise, transpiration decreases due to stomatal closure (Supplementary Fig. 5). While the change in transpiration appears symmetrical to the CO forcing, in N , it becomes the primary contributor to the increase in evapotranspiration (Fig. 2b). This underscores the critical role of transpiration in amplifying the overshoot of land precipitation.

Transpiration changes in N show an overall positive difference worldwide (Fig. 2c), particularly noticeable between RS and MAX (blue and red shadings in Fig. 2b). These changes are predominantly observed in densely vegetated areas, defined as regions where the leaf area index (LAI) exceeds 50% during the reference period (black dots in Fig. 2c). In the vegetation-rich regions such as South-Central America, South America, Central Africa, and Southeast Asia, transpiration significantly increases in RS compared to MAX. In both scenarios, the differences in transpiration during RS are substantial (red and orange lines in Fig. 2c). As CO levels return to the present-day values, the associated transpiration changes contribute to increased precipitation, in contrast to MAX, when transpiration changes tended to offset precipitation increases (green shading in Fig. 2c). Furthermore, the pronounced increases in transpiration coupled with substantial rises in land precipitation are of sufficient magnitude to account for precipitation changes in certain regions (Fig. 2c and Supplementary Fig. 6a). This indicates the importance of land-atmosphere coupled processes in shaping precipitation patterns and highlights their role in the climatic response to changing CO concentrations.

The increased transpiration in N is related to the physiological feedback of the terrestrial vegetation to CO concentration changes. Transpiration is influenced by several factors, but total leaf area and stomatal closure are most important for response to changes in CO concentration. Figure 3 shows responses of the land mean LAI, transpiration, and precipitation corresponding to the SAT and CO concentration, which can help separate global warming and vegetation physiological effects. Under the positive emissions, as the CO concentration and temperature rise, LAI also increases, possibly due to the combined impact of the fertilization and climate effects (Fig. 3a). Conversely, LAI decreases as the CO concentration and temperature decline through the negative emissions. Interestingly, LAI in N continues to rise with warming despite a relatively constant CO concentration (around 430 ppm), mainly observed in high latitude regions due to the increased temperature and precipitation during this period (Supplementary Fig. 7). In contrast, the change in LAI in Z is minimal due to the compensating effects between reduced CO concentrations and increased temperatures (Supplementary Fig. 4). It is noted that LAI in Z still remains higher than in N , possibly due to higher CO concentrations and warm conditions.

While LAI is positively correlated to the CO concentration, transpiration decreases with increasing CO concentration (Fig. 3b). This reduction is primarily due to the stomatal closure47,48 (Supplementary Fig. 5). As atmospheric CO levels rise, plants partially close their stomata to minimize water loss, thereby reducing transpiration31,32,49,50–an effect that often outweighs the CO fertilization effect (i.e. increases in LAI) at higher CO concentrations33,34,37,51,52. After achieving zero emission, transpiration in Z gradually recovers as CO levels decrease, while transpiration in N becomes much stronger than that at its initial state due to higher temperature.

A concurrent rise in both land precipitation and transpiration is projected in N , marking a distinct departure from the positive emission phase, where transpiration decreased despite enhanced precipitation (Fig. 3b,c). This increase in transpiration further amplifies the precipitation response because transpiration is an important source for the precipitable water, accounting for approximately 53 to 60% of evapotranspiration in both observations and models46. In the meantime, more precipitation can lead to more transpiration through increased soil moisture, suggesting a two-way feedback. Additionally, the increase in LAI in N likely enhances transpiration, leading to more precipitation. With stable CO concentrations, this additional precipitation provides the water necessary for vegetation growth, promoting more vegetation. In turn, this enhanced vegetation further increases water vapor supply through transpiration, creating a positive feedback loop that results in increased precipitation, even under low CO concentrations and cooler ocean and land temperatures.

To further address the critical role of the stomata feedback in the land precipitation overshoot, an additional experiment (N ) was conducted. In this experiment, the atmospheric CO concentration was fixed at 430 ppm, corresponding to RS, while the CO concentration influencing the vegetation module was set to 725 ppm, consistent with MAX (Methods). Therefore, only the vegetation experiences higher CO concentration, while the climate is influenced by the lower CO concentration. When higher CO concentrations are prescribed to the vegetation, land precipitation decreased, which is closer to the precipitation in MAX (Fig.3c and Supplementary Table 1), representing only 35% of the increase seen in RS. Notably, transpiration decreases to levels similar to those during MAX (Fig.3b). Although the increase in vegetation evaporation, associated with abundant vegetation, partially offsets the reduction in transpiration, total evapotranspiration declines compared to RS, contributing to the reduction in land precipitation (Supplementary Table 1). This experiment demonstrates that the reduction in transpiration due to the high CO environment considerably contributes to the decrease in land precipitation, suggesting the critical role of the physiological feedback in the land precipitation overshoot.

Changes in extremes of land hydrological cycle

Under global warming simulation, increased precipitation often tends to be directly linked to an increase in extreme precipitation events53. Concerns might arise that the augmented precipitation from the negative emissions could also exacerbate extreme precipitation events. To address this, we analyzed regional changes in the heavy precipitation/pluvial index, defined as the 1-day maximum precipitation (Methods), by comparing RS and MAX. In Z , regions including Eastern North America, South America, Europe, and Australia exhibit an increase in heavy precipitation alongside overall precipitation rises during RS compared to MAX (Fig. 4a and Supplementary Fig. 6a). In contrast, under N , most areas show a significant reduction in heavy precipitation (Fig. 4b). Despite the increase in precipitation compared to MAX, extreme events decrease in North South America, South American Monsoon, Western and Central Europe, and Southern East Asia regions (Supplementary Fig. 6a). Globally, the fraction of area, where heavy precipitation in the reference period is larger than 30 mm/day, affected by heavy precipitation increases by about 23.4% in MAX relative to the reference period, and this increase reaches 38.1% under Z (Fig. 4c). However, under N , it remains at 14.5% despite higher overall land precipitation levels. In other words, under the negative emissions, the overall increase in precipitation is not directly proportional to increase in heavy precipitation; indeed, despite higher overall land precipitation increases, heavy precipitation events are relatively less severe.

Moreover, the negative emissions tend to alleviate the risk of near-surface soil moisture deficits (droughts). In Z , despite an overall increase in total land precipitation, intensified droughts remain prevalent in several regions (Supplementary Fig. 8a). Regions with reduced drought severity are those that have experienced increased precipitation during RS compared to MAX, consistently accompanied by a rise in heavy precipitation events (Fig. 4a and Supplementary Fig. 8a). In contrast, in N , although regions with increased precipitation also show a reduction in droughts (Supplementary Figs. 6a and 8b), these increases are not universally associated with elevated heavy precipitation (Fig. 4b). This indicates that the negative emissions lead to the alleviation of either heavy precipitation or drought in most regions, except for the Sahara and Northern Europe.

The changes in heavy precipitation and drought following the negative emissions are closely linked to precipitation changes driven by physiological effects, such as increased leaf area and enhanced evapotranspiration. The expansion of leaf area improves water capture and storage, thereby increasing water availability (i.e. increase in water storage) (Supplementary Table 1). Simultaneously, increased evapotranspiration continuously supplies water vapor into the atmosphere, leading to more frequent precipitation events, evidenced by the increase in the number and area of wet days (Fig. 4d and Supplementary Fig. 9) and the total amount of precipitation (Fig. 1b). This increase in precipitation frequency potentially contributes to a reduction in extreme precipitation and droughts occurrences by distributing rainfall more evenly, thereby enhancing the efficiency of water resources. This underscores the importance of negative emissions strategies in reducing potential future climate impacts and highlights the necessity of incorporating these strategies into climate projections and developing effective mitigation and adaptation strategies.

Methods
Model configuration
The Community Earth System Model 2 (CESM2) was utilized to conduct an idealized CO emission-driven simulation. This model is composed of physical components, including atmosphere, ocean, land, and cryosphere, which are fully integrated with the land and ocean carbon cycles60. The atmospheric model is the Community Atmosphere Model version 6 (CAM6), featuring a horizontal resolution of ~1° with 32 vertical levels. The ocean and sea ice models are the Parallel Ocean Program version 2 (POP2)61 and the Community Ice CodE version 5 (CICE5)62, respectively. The ocean components used a nominal ~1° horizontal resolution (meridional resolution ~0.27° near the equator) with 60 vertical levels. The Community Land Model Version 5 (CLM5)46 has the same horizontal resolution as the atmosphere model. It characterizes the carbon and hydrological cycles of the land in CESM2, which has experienced significant advancements through the incorporation of new and updated processes and parameterizations. Notable enhancements include an accurate representation of historical cumulative CO uptake and the seasonal cycle of net ecosystem production46,63,64. The ocean carbon cycles in CESM2 are depicted by the Marine Biogeochemistry Library (MARBL)65.

CLM5 has been evaluated as part of an ensemble of simulations—including prescribed and prognostic vegetation states, multiple forcing datasets, and previous model versions (CLM4 and CLM4.5)—using the International Land Model Benchmarking (ILAMBv2) package46. It simulates LAI and evapotranspiration at a level comparable to dataset and shows improved performance in capturing observed relationships between precipitation, LAI, and evapotranspiration. The CO fertilization effect on LAI has been demonstrated in CLM5 through perturbed physics ensemble simulations66 and historical single-forcing experiments67. In addition, under the SSP5-8.5 high-emission scenario, CLM5 simulates decreased stomatal conductance and evapotranspiration, along with increased LAI, consistent with expectations under elevated CO concentrations54. These studies indicate that the model is capable of reasonably representing between vegetation and climate feedbacks.

Experimental design
To explore the impacts of different CO emission trajectories, we designed idealized simulations based on both zero and negative emissions scenarios (Supplementary Fig. 1). In our model, anthropogenic CO emissions increase linearly from 2000 to 2050, following the SSP5-8.5 scenario, with an annual increase of 1.09 GtCO2. After peaking in 2050, emissions decrease at the same rate until they reach zero by the year 2124. In the zero emissions scenario (Z ), emissions remain at zero until the end of the simulation in 2400 (Supplementary Fig. 1a). In the negative emissions scenario (N ), emissions continue to decrease at the same rate after reaching zero, until the global mean surface atmospheric CO concentration returns to its initial value of approximately 383 ppm by the year 2197 (Supplementary Fig. 1b). Following this, CO emissions are maintained at zero from 2197 until the end of simulation in 2400.

This simulation is driven by CO emissions, where atmospheric CO concentrations are determined not only by prescribed anthropogenic CO emissions but also by CO2 fluxes from land and ocean, capturing changes in CO concentration through carbon cycle-climate feedback. All non-CO factors, such as land use and non-CO greenhouse gas forcings, are held constant at the present-day (2000) levels. A total of nine ensemble members were conducted for each scenario, with slightly different initial conditions, initialized monthly from January to October (excluding September) in the year 2000, using the CESM2 esm-hist experiment.

To analyze the role of transpiration in land precipitation, an additional idealized experiment, referred to as the N scenario, was conducted. The experiment utilized the same components as in the N , with initial conditions set to the year 2300, based on the first ensemble member. The atmospheric CO concentration was fixed at 430 ppm—representative of conditions from 2300 to 2399—while the CO concentration received by the land was fixed at 725 ppm, corresponding to the period of maximum CO concentration (e.g., 2091 to 2120). This setup enables the quantification of CO -physiological forcing on land hydrology due to elevated CO levels at the surface under similar climate conditions and allows for a comparison of the resulting changes in precipitation with those observed under a negative emission scenario. The experiment was carried out over 200 years, with the first 100 years used as a spin-up period and the subsequent 100 years analyzed.

Definition of indices
Atmospheric moisture budget
The atmospheric moisture budget can be expressed in the following form:
\frac{\partial w_{sfc}}{\partial t} = E - P - \int_{top}^{sfc} \nabla_p (\mathbf{u}q) \frac{dp}{g} (1)
Where g is the gravitational acceleration, q is specific humidity, p is atmospheric pressure (sfc and top indicate the earth’s surface and top of atmosphere, respectively), \nabla_p () is the horizontal divergence in pressure coordinates, \mathbf{u} is the horizontal wind vector, P is the precipitation rate, E is the evaporation rate, and \frac{\partial w}{\partial t} is the tendency of total precipitable water. The last term of the right side is the total moisture flux convergence. A positive value is observed when atmospheric moisture is exported from the region. The change in total precipitable water is calculated as the difference between E minus P and moisture flux convergence.

Terrestrial water balance
The terrestrial water balance can be expressed as68:
\frac{dS_L}{dt} = P_L - E_L - R_L (2)
Where \frac{dS_L}{dt} represents the change in soil water storage, P_L is the land precipitation, E_L is the evapotranspiration, and R_L is the runoff.

Heavy precipitation index
The Expert Team on Climate Change Detection and Indices (ETCCDI) has well-defined a series of extreme precipitation indices. Generally, these indices can be classified into three main types: intensity, frequency, and duration. In this study, we selected the widely accepted and commonly used heavy precipitation index of maximum 1-day precipitation (Rx1day). The total fraction of areas with heavy precipitation is calculated as the percentage difference between the specific period and the reference period, divided by the reference period. This calculation is performed only for regions where the heavy precipitation index during the reference period exceeds 30 mm per day. Other indices, such as the number of days with precipitation greater than 30 mLm (R30mm) and the maximum consecutive 5-day precipitation (Rx5day), are analyzed but are not presented as they show similar patterns to Rx1day. Eight ensemble members are used for each scenario to analyze the heavy precipitation index.

Drought index
The near-surface soil moisture deficit (drought) index is calculated using the total soil water content from 0 to 8 m depth.

Wet days index
The Wet days index, as used in the ETCCDI, is defined as the annual total of days with precipitation exceeding 1 mm. Eight ensemble members were utilized to analyze the index. The total fraction of areas with wet days is calculated as the percentage difference between the specific period and the reference period, divided by the reference period.

Area means
The Global mean values are calculated over the latitude range from 60°S to 90°N, excluding Antarctica. The land and ocean mean values are calculated by applying land and ocean masks, respectively.

Percentage change in precipitation
The percentage change in precipitation is calculated for each ensemble member relative to the reference period (2001-2030).

Significance test
The significance level was determined using a Kolmogorov-Smirnov two-sample test.

Regional acronyms
Using the regional domains defined in Figure SPM.3 of the Intergovernmental Panel on Climate Change Sixth Assessment Report Working Group I (IPCC AR6 WGI) Summary for Policymakers (SPM) report69 (Supplementary Fig. 10), we calculated the area-averaged heavy precipitation (Fig. 4) and drought (Supplementary Fig. 8) for each region. These regions are represented by hexagons of equal size, positioned in their approximate geographical locations. The abbreviations used for each region are as follows:
North America: NWN (North-Western North America), NEN (North-Eastern North America), WNA (Western North America), CNA (Central North America), ENA (Eastern North America); Central America: NCA (Northern Central America), SCA (Southern Central America), CAR (Caribbean); South America: NWS (North-Western South America), NSA (Northern South America), NES (North-Eastern South America), SAM (South American Monsoon), SWS (South-Western South America), SES (South-Eastern South America), SSA (Southern South America); Europe: GIC (Greenland/Iceland), NEU (Northern Europe), WCE (Western and Central Europe), EEU (Eastern Europe), MED (Mediterranean); Africa: MED (Mediterranean), SAH (Sahara), WAF (Western Africa), CAF (Central Africa), NEAF (North Eastern Africa), SEAF (South Eastern Africa), WSAF (West Southern Africa), ESAF (East Southern Africa), MDG (Madagascar); Asia: RAR (Russian Arctic), WSB (West Siberia), ESB (East Siberia), RFE (Russian Far East), WCA (West Central Asia), ECA (East Central Asia), TIB (Tibetan Plateau), EAS (East Asia), ARP (Arabian Peninsula), SAS (South Asia), SEA (South East Asia);
Australasia: NAU (Northern Australia), CAU (Central Australia), EAU (Eastern Australia), SAU (Southern Australia), NZ (New Zealand); Ocean: ARO (ARctic Ocean), ARS (Arabian Sea), BOB (Bay of Bengal), EAO (Equatorial Atlantic Ocean), EIO (Equatorial Indic Ocean), EPO (Equatorial Pacific Ocean), NAO (N. Atlantic Ocean), NPO (N. Pacific Ocean), SAO (S. Atlantic Ocean), SIO (South Indic Ocean), SOO (Southern Ocean), SPO (S. Pacific Ocean), Small Islands: CAR (Caribbean).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review for “Negative CO2 emissions to mitigate extremes in land hydrological”\nUsing a single climate model CESM2, this study conducts idealized simulations based on both Zero and Negative emission\nscenarios for 400 years from year 2000 to 2400. This study show that terrestrial precipitation increases with greenhouse\nwarming, and the trend continues even with zero or negative emissions. Under negative emissions, physiological feedback\nfrom terrestrial vegetation plays a significant role, primarily due to enhanced transpiration with reduced CO₂. Despite an\noverall increase in precipitation, negative emissions scenarios result in less severe hydrological extremes compared to zero\nemissions.\nSuch simulations are ideal for investigating long-term processes and feedbacks, especially those involving slow-response\ncomponents of the climate system like vegetation dynamics, deep-ocean circulation, and carbon cycle feedbacks.\nHowever, I believe the framing of the study as relevant to climate mitigation and adaptation is not fully substantiated. The key\nfindings, while methodologically robust, pertain to processes and impacts that manifest on centennial timescales (post-\n2250), which are far removed from the actionable horizons of 10–50 years typically addressed in mitigation and adaptation\ncontexts. This disconnect limits the immediate applicability of the study's results to policy and planning efforts. Also, if one\nkeeps the Paris agreement in mind this does not fall in the mitigation and adaptation contexts.\nI have few major comments listed below:\n1. The abstract and main text emphasize the implications for climate mitigation and adaptation, yet the results focus primarily\non long-term trends [on several century timescales] in precipitation and vegetation feedbacks. To better align with the stated\nobjectives, the authors should explicitly discuss any near-term impacts (if present) or clearly acknowledge that the findings\nare more relevant for understanding long-term Earth system dynamics.\n2. While the study demonstrates that negative emissions reduce hydrological extremes globally, it does not offer actionable\ninsights for the critical mitigation/adaptation period (10–50 years). Again, reframing the discussion to highlight its contribution\nto understanding Earth system behavior under extended scenarios would enhance clarity.\n3. The abstract should avoid overstating the relevance of the findings to mitigation and adaptation. Instead, it could focus on\nthe mechanistic contributions and their implications for modeling long-term climate responses. For instance, the\nphysiological feedback [through transpiration] driven by vegetation could be framed as a key process that needs further\ninvestigation in models, rather than as an actionable insight for mitigation efforts.\n4. The authors could frame the study as a contribution to understanding fundamental Earth system feedbacks rather than\npositioning it as directly relevant to mitigation and adaptation. This would align better with technical journals that emphasize\nadvancing theoretical and model-based understanding, such as Earth’s Future, AGU advances, Journal of Climate,\nEnvironmental Research Letters, Geophysical Research Letters.\n5. In addition, as the authors mentioned in the “Discussion” section that this study utilizes a single Earth System Model and\nconsiders idealised simulations among few more limitations listed; suggest that this study is better framed as a theoretical or\nmechanistic study rather than a mitigation/adaptation context. Highlighting the study’s role in advancing conceptual\nunderstanding of long-term Earth system feedbacks—particularly vegetation and precipitation dynamics—would ensure that\nits contributions are accurately represented.\nConsequently, I do not recommend this study for publication in Nature Communication.\nMinor comments:\nL44: Here, I request the authors to also mention the large scale climate intervention methods (Climate Engineering or\nGeoengineering) as well.\nL47: At this point please cite Matthews and Caldeira 2008.\nStabilizing climate requires near-zero emissions, H. Damon Matthews and Ken Caldeira,\nhttps://doi.org/10.1029/2007GL032388\nL67: It is apt to avoid writing “water vapour holding capacity”. Please rephrase here.\nFigure 1: What does ocean temperature mean? Does it mean SAT over the oceans? Did the authors land\nmasked and ocean masked while estimating the ocean and land temperatures?\nFigure 1: Please show precipitation change in %.\nL139-141 & Figure 2a: The values shown here are global mean values. Does the negative values mean that there is an\nimport of moisture from adjacent grids which leads to decreased precipitation. Please elaborate and clarify.\nFigures: I request the authors to improve the figure quality. They are giving a hard time. Already in Figures 1 and 2, there are\nseveral line plots hence a high quality figure would be good.\nL142: “as” is not appropriate. I believe the authors mean “when there Sia precipitation rise”\nL144: please add “increase in land precipitation” during RS only otherwise during 2200 there is a decrease.\nL242: “0.02 mm/day” is pretty small value. A % change would be useful and helpful.", "reviewer_2": "This paper examines the terrestrial precipitation changes under zero or negative CO2 emissions in the future, and also\nreveals that under the negative emissions scenario, physiological feedback in terrestrial vegetation plays an important role\nin precipitation changes. The results are interesting and my concerns and questions could be found below.\nAbstract was too descriptive without any numbers for clarification. The peak CO2 concentration? The amount of terrestrial\nprecipitation changes compared to current state? the exact percentage of contributions that made by physiological\nfeedback? Limited information was provided by the current abstract.\nLine 73-83, more information should be provided to clarify how the authors did this study, including how they design the\nmodel scenarios (with exact number, such as CO2 concentration). I understand the length may be limited in the main text,\nhowever, it is helpful to include some details to improve the readability.\nThis paper tried to examine if the climate system reversed under zero or negative emission scenarios, and the model was\nrun for another 200 years. However, for climate system, the time scale is likely to be centuries or millennium. I am wondering\nif 200 years are enough for examine the reversibility of climate system.\nThe changes in terrestrial precipitation were the main focus of this study, however, the descriptions of SAT changes could\nbe found anywhere with so many details. Somehow it may be helpful to know how SAT change along CO2 changes to\nadvance the understanding of precipitation changes, but it was too much.\n“The rebounded warming observed in ZE and NE can be attributed to the AMOC change” (Line 103), any data support? Or\njust a guess?\nThe stomatal conductance is decreasing with rising CO2, however, LAI is increasing with rising CO2, as previous studies\nsaid. However, there is no validation about the simulation of these mechanisms in the climate model runs (If they have been\ndone with the model used here, please point to them). If not, there needs to be some validation. There are ample datasets of\nLAI, ET, transpiration available from ground and satellite measurements available.\nLine 156-164, the changes in LAI and stomatal conductance are in opposite direction (i.e. LAI increases while stomatal\nconductance decreases under rising CO2). Vegetation evaporation and transpiration all increases, which means such\nincreases are mainly attributed to LAI changes? Any evidence supporting this result? I am also curious that if there is a\nturning point between LAI- and stomatal conductance-induced ET changes?\nMost of the current discussions were about caveats of this study. More in-depth discussions of the advance the current study\ncould offer beyond the literature should be provided here. Any ideas about the results of this study compared to the results of\nother concertation-based scenarios? Especially the contributions of physiological feedback to precipitation changes, which\nwere investigated by many prior studies.\nPrevious studies showed that temperature changes under positive emissions were kind of asymmetric with that under\nnegative emissions. How about precipitation? Any ideas?\nAll the figures were shown in low quality, and the results could not be found clearly from those figures.\nMore details should be provided in the methods for clarification."}, "author_response": "Major Comments\nComment 1:\n1. The abstract and main text emphasize the implications for climate mitigation and adaptation, yet the\nresults focus primarily on long-term trends [on several century timescales] in precipitation and vegetation\nfeedbacks. To better align with the stated objectives, the authors should explicitly discuss any near-term\nimpacts (if present) or clearly acknowledge that the findings are more relevant for understanding long-term\nEarth system dynamics.\nResponse:\ninform the development of international climate agreements beyond the Paris Agreement. Specifically, we\nhighlighted that even if zero emissions are achieved, precipitation will continue to increase, and extreme\nevents may intensify unless atmospheric CO₂ levels are actively reduced through negative emissions. How-\never, as the reviewer rightly pointed out, our study is indeed more relevant to the long-term understanding\nof Earth system responses rather than near-term impacts. To reflect this clearly, we have revised both the\nAbstract and the Discussion to more clearly emphasize the study’s relevance to long-term Earth system\ndynamics rather than near-term policy applications. Below, we provide the revised versions of each section,\nwith key changes highlighted in bold (or shown using track changes in the submitted manuscript).\nRevised Abstract (excerpt):\nThese findings suggest that more aggressive climate mitigation strategies are needed not only to curb im-\nmediate global warming but also to reduce hydrological extremes and enhance water availability under\nlong-term climate change.\nRevised Discussion 1 (excerpt):\nThe CO concentration was increased to nearly twice the current levels, representing an idealized scenario\n2\ndesigned to investigate long-term Earth system feedbacks. While this framework provides theoretical\ninsight, it limits the direct applicability of the results to near-term climate policy. This underscores the\nneed for complementary experiments based on more realistic mitigation pathways, such as achieving\nzero or negative emissions by 2050.\nRevised Discussion 2 (excerpt):\nWhile current mitigation efforts often prioritize addressing immediate climate impacts, our findings sug-\ngest that early and sustained action can also deliver substantial long-term benefits for Earth system sus-\ntainability. This underlines the importance of forward-looking, comprehensive research to inform and\nguide effective climate response strategies.\nComment 2:\n2. While the study demonstrates that negative emissions reduce hydrological extremes globally, it does not\noffer actionable insights for the critical mitigation/adaptation period (10–50 years). Again, reframing the\ndiscussion to highlight its contribution to understanding Earth system behavior under extended scenarios\nwould enhance clarity.\nAs mentioned in our response to the previous comment, we have revised the abstract and discussion\nsections to emphasize our study's contribution to understanding Earth system responses under extended sce-\nnarios rather than providing immediate actionable insights for the critical mitigation/adaptation period (10–\n50 years).\nComment 3:\n3. The abstract should avoid overstating the relevance of the findings to mitigation and adaptation. Instead,\nit could focus on the mechanistic contributions and their implications for modeling long-term climate re-\nsponses. For instance, the physiological feedback [through transpiration] driven by vegetation could be\nframed as a key process that needs further investigation in models, rather than as an actionable insight for\nmitigation efforts.\nResponse:\nThank you for your valuable comment. We agree that physiological feedback is indeed a critical process,\nespecially as it significantly influences land precipitation under scenarios where temperature and precipita-\ntion increase without additional CO₂ forcing. This highlights the importance of accurately representing such\nphysiological processes in Earth system models to improve our understanding of long-term precipitation\nresponses. Following the reviewer’s suggestion, we have revised the abstract and discussion to explicitly\nemphasize the mechanistic insights, underscoring the necessity for their improved representation in Earth\nsystem models to enhance our theoretical understanding of climate system responses.\nComment 4:\n4. The authors could frame the study as a contribution to understanding fundamental Earth system feed-\nbacks rather than positioning it as directly relevant to mitigation and adaptation. This would align better\nwith technical journals that emphasize advancing theoretical and model-based understanding, such as\nEarth’s Future, AGU advances, Journal of Climate,\nResponse:\nSince our results offer a new understanding of vegetation-climate feedbacks under zero and negative CO₂\nemissions scenarios, we believe it also fits to Nature Communications. These insights are not only\n3\nfundamental to Earth system science but also carry important implications for future climate trajectories\nand policy discussions.\nComment 5:\n5. In addition, as the authors mentioned in the “Discussion” section that this study utilizes a single Earth\nSystem Model and considers idealised simulations among few more limitations listed; suggest that this\nstudy is better framed as a theoretical or mechanistic study rather than a mitigation/adaptation context.\nHighlighting the study’s role in advancing conceptual understanding of long-term Earth system feed-\nbacks—particularly vegetation and precipitation dynamics—would ensure that its contributions are accu-\nrately represented.\nResponse:\nThank you for this valuable suggestion. Following the reviewer’s suggestion, we have removed references\nto mitigation and adaptation from the discussion section and instead emphasized our study’s role in advanc-\ning the conceptual understanding of long-term Earth system feedbacks, particularly focusing on vegetation\nand precipitation dynamics.\nRevised Discussion 1 (excerpt):\nThis study advances our understanding of precipitation responses to CO₂ removal by highlighting two\nkey distinctions from previous work. First, most earlier studies have used concentration-driven CO₂ sce-\nnarios focused on temperature and precipitation hysteresis25,28, without accounting for CO₂ outgassing\nfrom land and ocean as concentrations decline. In contrast, our emissions-driven experiment includes a\nfully interactive carbon cycle, capturing this outgassing and providing a more realistic representation of\nphysiological feedbacks. Second, while previous studies emphasized reduced transpiration due to sto-\nmatal closure under positive CO₂ emissions55, our results reveal the role of physiological processes in\ndriving precipitation changes under the negative emissions scenario. These findings underscore the\nimportance of vegetation in shaping hydrological outcomes under extended mitigation scenarios and high-\nlight the value of emissions-based experiments in capturing long-term Earth system feedbacks.\nRevised Discussion 2 (excerpt):\nHowever, these findings highlight the importance of CO physiological effects, which could be crucial to\n2\nunderstand the reversibility of Earth systems.\nComment 1:\nL44: Here, I request the authors to also mention the large scale climate intervention methods (Climate\nEngineering or Geoengineering) as well.\nResponse:\nThank you for the comment. We have added the suggested reference to large-scale climate intervention\nmethods with the following sentence:\n“along with considering large-scale climate intervention methods, such as climate engineering or geoen-\ngineering (Lawrence et al., 2018).”\nComment 2:\nL47: At this point please cite Matthews and Caldeira 2008. Stabilizing climate requires near-zero emissions,\nH. Damon Matthews and Ken Caldeira, https://doi.org/10.1029/2007GL032388\nResponse:\nThank you for the recommended reference. After reviewing the paper, we agree that it is highly relevant to\nthis part of the manuscript and have added the citation to Matthews and Caldeira (2008) accordingly.\nComment 3:\nL67: It is apt to avoid writing “water vapour holding capacity”. Please rephrase here.\nResponse:\nThank you for the helpful comment. As the reviewer pointed out, the term “water vapour holding capacity”\nis ambiguous, so we have revised it to the more precise phrase “the saturation water vapor pressure.”. The\nupdated sentence now reads:\n“thereby enhancing atmospheric moisture content by increasing the saturation water vapor pressure of\nair.”\nFigure 1: What does ocean temperature mean? Does it mean SAT over the oceans? Did the authors land\nmasked and ocean masked while estimating the ocean and land temperatures?\nResponse:\nThank you for raising this important clarification. In Fig. 1, “ocean temperature” refers to the surface air\ntemperature (SAT) over ocean areas only, while “land temperature” indicates SAT over land areas. These\nvalues were calculated separately by applying land and ocean masks to the global SAT data, allowing us to\ndistinguish temperature changes over land and ocean regions. We have clarified this point in the caption\nand method sections:\n“Mean values are computed over the latitude range from 60°S to 90°N using land-sea masking.”\nComment 5:\nFigure 1: Please show precipitation change in %.\nResponse:\nThank you for the valuable comment. We have revised Figure 1 to show precipitation change in percentage\n(%) instead of mm/day, as this better illustrates the relative changes. Additionally, we have included a de-\ntailed description of the calculation method in the Methods section. We believe this revision improves the\nclarity and interpretability of the precipitation changes presented.\nComment 6:\nL139-141 & Figure 2a: The values shown here are global mean values. Does the negative values mean that\nthere is an import of moisture from adjacent grids which leads to decreased precipitation. Please elaborate\nand clarify.\nResponse:\nWe’ve clarified this point in the revised manuscript by adding the following explanation:\n“In ZE, moisture flux convergence mostly shows negative values during the restoring period (RS, 2300-\n2399), indicating weakening of moisture transport from the adjacent oceans except towards the late 2300s\n(purple line in Supplementary Fig. 3a), contributing to a decrease in precipitation.”\nFigures: I request the authors to improve the figure quality. They are giving a hard time. Already in Figures\n1 and 2, there are several line plots hence a high quality figure would be good.\nResponse:\nThank you for the feedback. We have regenerated all figures, including Figures 1 and 2, at high resolution\n(3600 × 3600 pixels) in PNG format to improve clarity and readability, especially given the number of line\nplots. These high-quality images were provided during submission; however, it appears that some degrada-\ntion may have occurred during the insertion of figures into the Word file and transforming into PDF file.\nWe have confirmed this formatting issue and now additionally provide vector format (PDF) versions of all\nfigures to ensure optimal quality. In addition, as the reviewer suggested, we revised Fig. 2 to improve visual\nclarity. Specifically, Fig. 2a-b now shows only the negative emissions (NE) scenario, and the corresponding\nresults for the zero emissions (ZE) scenario have been moved to the Supplementary Information (Supple-\nmentary Fig. 3). We hope these changes improve both the visual quality and interpretability of the figures.\nComment 8:\nL142: “as” is not appropriate. I believe the authors mean “when there Sia precipitation rise”\nResponse:\nWe have revised the sentence accordingly.\nComment 9:\nL144: please add “increase in land precipitation” during RS only otherwise during 2200 there is a decrease.\nResponse:\nThank you for the helpful comment. The precipitation decreases in the early 2200s. To clarify this, we\nexplicitly specified the time period by adding “during RS” to indicate that the increase in land precipitation\noccurs only during the RS period.\nL242: “0.02 mm/day” is pretty small value. A % change would be useful and helpful.\nResponse:\nThank you for the suggestion. We removed the value “0.02 mm/day” and instead added a clarifying sentence\nto better convey its significance: “representing only 35% of the increase seen in RS.” This provides better\ncontext for interpreting the magnitude of the change.\nThis paper examines the terrestrial precipitation changes under zero or negative CO2 emissions in the future,\nand also reveals that under the negative emissions scenario, physiological feedback in terrestrial vegetation\nplays an important role in precipitation changes. The results are interesting and my concerns and questions\ncould be found below.\nComments\nComment 1:\nAbstract was too descriptive without any numbers for clarification. The peak CO2 concentration? The\namount of terrestrial precipitation changes compared to current state? the exact percentage of contributions\nthat made by physiological feedback? Limited information was provided by the current abstract.\nResponse:\nThank you for the helpful comment. As the reviewer suggested, we have revised the abstract to include the\npeak CO₂ concentration and clarified the magnitude of land precipitation change, stating that it nearly dou-\nbles compared to the peak CO period. We provide the revised abstract, with key changes highlighted in\n2\nbold.\nRevised Abstract:\nGlobal warming has profound effects on the terrestrial hydrological cycle, leading to alterations in regional\nextreme weather patterns. While terrestrial precipitation responses under continued greenhouse gas emis-\nsions are well established, the responses of terrestrial precipitation and vegetation feedbacks under climate\nmitigation scenarios remain uncertain. Here, we investigate terrestrial precipitation changes under ideal-\nized negative and zero CO2 emissions scenarios. Terrestrial precipitation increases by approximately 1.1%\nat the peak CO2 concentration (~725 ppm), but shows even greater increases of about 1.9% and 2.6%\nunder substantially lower atmospheric CO2 concentrations following zero (~600 ppm) and negative\n(~428 ppm) CO2 emissions, respectively. In particular, physiological feedbacks from terrestrial vegetation\nplay a pivotal role in driving this precipitation increase under the negative emissions scenario, primarily\nthrough enhanced transpiration in response to reduced CO2 levels. Furthermore, despite a substantial in-\ncrease in terrestrial precipitation, extreme precipitation events and droughts become less severe globally\nunder the negative emissions scenario—even compared to the zero emissions scenario. These findings\nsuggest that more aggressive climate mitigation strategies are needed not only to curb immediate global\n9\nwarming but also to reduce hydrological extremes and enhance water availability under long-term climate\nchange.\nComment 2:\nLine 73-83, more information should be provided to clarify how the authors did this study, including how\nthey design the model scenarios (with exact number, such as CO2 concentration). I understand the length\nmay be limited in the main text, however, it is helpful to include some details to improve the readability.\nResponse:\nThank you so much for your comment. Initially, we kept the experimental description in the main text brief\nto avoid overwhelming readers with technical details, and provided full explanations in the Methods section.\nHowever, as you pointed out, this may have made it difficult to grasp the overall experimental design. To\nimprove clarity, we have now added the specific CO₂ concentrations and corresponding time periods di-\nrectly in the main text, so readers can better understand the experimental setup briefly, with key changes\nhighlighted in bold:\nRevised Introduction (excerpt):\nTo examine climate reversibility and hysteresis, we conducted two idealized CO emission-driven experi-\n2\nments under the positive, negative, and zero emission scenarios (Methods and Supplementary Fig. 1). The\nfirst scenario, referred to as Z (zero emissions scenario), achieves zero emissions by the year 2125 by first\nE\nincreasing emissions linearly from the year 2001 to 2050 and subsequently decreasing them linearly to\nreach zero in 2124 (Supplementary Fig. 1a). The rate of increase until 2050 closely follows that of the\nSSP5-8.5 scenario. In this scenario, atmospheric CO concentrations peak at 728 ppm in 2124, approx\n2\nimately 1.9 times the initial level of 383 ppm in 2000. The second scenario, referred to as N (negative\nE\nemissions scenario), follows the same emission trajectory as Z until 2124 but then continues with negative\nE\nemissions, reducing atmospheric CO concentrations return to the present-day level of 383 ppm by 2197\n2\n(Supplementary Fig. 1b). Additionally, we performed a long-term integration experiment under zero CO\n2\nemissions until 2400 for both scenarios, extending for over 200 years, to determine whether climate system\ncan revert to its current state, exploring climate reversibility. These experiments allow us to investigate the\nresponse of the global hydrological cycle under zero and negative emissions scenarios, as well evaluating\nthe potential for reversibility of these changes.\nComment 3:\nthe model was run for another 200 years. However, for climate system, the time scale is likely to be centuries\nor millennium. I am wondering if 200 years are enough for examine the reversibility of climate system.\nResponse:\nAs the reviewer pointed out, the 200-yr simulation might be not sufficient for evaluating the irreversibility\nof several climate components. Nevertheless, since the main focus of our study is not climate irreversibility,\nwe simply define irreversibility from this human perspective, focusing on climate responses within a couple\nof centuries.\nComment 4:\nThe changes in terrestrial precipitation were the main focus of this study, however, the descriptions of SAT\nchanges could be found anywhere with so many details. Somehow it may be helpful to know how SAT\nchange along CO2 changes to advance the understanding of precipitation changes, but it was too much.\nResponse:\nThank you for the thoughtful comment. We agree that the primary focus of this study is on changes in\nterrestrial precipitation, and we have made a conscious effort to center the discussion accordingly. While\nwe retain some description of surface air temperature (SAT) to provide physical context—given its role as\na key driver of precipitation—we acknowledge the reviewer’s concern regarding the level of detail. To\naddress this, we have slightly reduced the length of SAT-related explanations while preserving the key\npoints necessary to support our interpretation, for the following reasons:\nFirst, surface temperature is a fundamental driver of precipitation, and understanding its evolution is essen\ntial to interpreting the hydrological response. For example, as shown in Fig. 3c, SAT changes are closely\nlinked to precipitation trends. Without this context, it would be difficult to fully understand the role of\nphysiological feedbacks in the precipitation response.\nSecond, we believe it is important to clarify SAT trends before focusing on precipitation changes. This\nprovides a necessary foundation for distinguishing vegetation-driven effects from temperature-driven\nchanges in the subsequent discussion.\nWe believe the revised version maintains clarity while keeping the primary focus on precipitation.\n“The rebounded warming observed in ZE and NE can be attributed to the AMOC change” (Line 103), any\ndata support? Or just a guess?\nResponse:\nThank you for the comment. The rebounded warming is supported by previous modeling studies (An et al.,\n2021; Oh et al., 2022; Schwinger et al., 2022; Wu et al., 2011; Wu et al., 2015), which show that surface\ntemperature responses under CO₂ emission and removal scenarios are closely related to changes in the\nAMOC. These studies demonstrate that a recovery of the AMOC enhances northward heat transport, lead\ning to warming in the Northern Hemisphere. In particular, Schwinger et al. (2022) highlights the role of\nAMOC decline and recovery in amplitude and spatial pattern of temperature changes. Therefore, the re\nbounded warming observed in Z and N can be attributed to AMOC changes, as documented in these\nE E\nprevious modeling studies. We cited these references in the manuscript.\nComment 6:\nThe stomatal conductance is decreasing with rising CO2, however, LAI is increasing with rising CO2, as\nprevious studies said. However, there is no validation about the simulation of these mechanisms in the\nclimate model runs (If they have been done with the model used here, please point to them). If not, there\nneeds to be some validation. There are ample datasets of LAI, ET, transpiration available from ground and\nsatellite measurements available.\nResponse:\nThank you for raising this important point regarding model validation. We agree that it is essential to assess\nwhether the model realistically captures the physiological responses to rising CO₂, such as changes in sto\nmatal conductance and LAI. Below, we summarize previous studies that used this model and evaluated\nthese processes.\n1. Representation of CO and vegetation interactions in CLM5\ntial pressure, enhancing carbon assimilation and leading to higher LAI. Simultaneously, stomatal conduct\nance declines as leaf-surface CO concentration increases, consistent with the Medlyn stomatal conductance\n2\nmodel (CLM5 documentation, pp. 104–105; Eq. 9.1).\n12\n2. Validation against forcing datasets\nLawrence et al. (2019) showed that CLM5 represents well in simulating LAI and evapotranspiration (ET)\nwhen evaluated against forcing datasets using the International Land Model Benchmarking (ILAMBv2)\npackage (1901–2014). The model also captures the observed relationships between precipitation and both\nLAI and ET more accurately than earlier versions (i.e., CLM4 and CLM4.5).\n3. CO driven LAI response\net al., 2025).\n4. Stomatal conductance response under the high-emission scenario\nUnder the high-emission SSP5-8.5 scenario, CLM5 simulates decreased stomatal conductance and ET,\nalong with increased LAI—consistent with expectations under the rising CO concentrations (Yuan et al.,\n2\n2024).\nIn summary, CLM5 incorporates well-established physiological mechanisms and has been validated against\na wide range of observational datasets. These features make it a reliable tool for understanding the role and\nfeedbacks of vegetation under CO mitigation scenarios. Accordingly, these validation results have been\n2\nadded to the “Model configuration” subsection of the Methods.\nRevised Model configuration (excerpt):\nCLM5 has been evaluated as part of an ensemble of simulations—including prescribed and prognostic\nvegetation states, multiple forcing datasets, and previous model versions (CLM4 and CLM4.5)—using the\nInternational Land Model Benchmarking (ILAMBv2) packagee46. It simulates LAI and evapotranspiration\nat a level comparable to dataset and shows improved performance in capturing observed relationships\nbetween precipitation, LAI, and evapotranspiration. The CO fertilization effect on LAI has been demon\n2\nstrated in CLM5 through perturbed physics ensemble simulations63 and historical single-forcing experi\nments64. In addition, under the SSP5-8.5 high-emission scenario, CLM5 simulates decreased stomatal con\nductance and evapotranspiration, along with increased LAI, consistent with expectations under elevated\nCO concentrations55. These studies indicate that the model is capable of reasonably representing between\n2\nvegetation and climate feedbacks.\nComment 7:\nwhile stomatal conductance decreases under rising CO2). Vegetation evaporation and transpiration all in\ncreases, which means such increases are mainly attributed to LAI changes? Any evidence supporting this\nresult? I am also curious that if there is a turning point between LAI- and stomatal conductance-induced\nET changes?\nResponse:\nThank you for this insightful question. While both increased precipitation and enhanced LAI contribute to\nhigher vegetation evaporation and transpiration, we argue that physiological forcing associated with the\nstomatal conductance plays a critical role. During the zero emission period, when atmospheric CO₂ con\ncentrations and stomatal conductance stabilize, LAI and precipitation continue to rise, leading to increased\nvegetation evaporation and transpiration. This is distinct from the typical reduction in transpiration pro\njected under elevated CO₂ concentrations, where decreased stomatal conductance dominates.\nIncreased vegetation evaporation and transpiration, in turn, reinforce precipitation through the physiologi\ncal feedbacks, further amplifying LAI. This positive feedback highlights the importance of physiological\nprocesses in hydrological responses under the zero emissions scenarios.\nTo further support this process, we conducted an additional experiment under similar climatic conditions,\nin which LAI was higher but total precipitation was lower. The reduction in transpiration suggests that\nstomatal regulation, rather than LAI alone, is a key driver of vegetation water flux and its feedback on\nprecipitation.\nHowever, vegetation feedbacks represent a complex and debated mechanism in shaping hydrological out\ncomes. Elevated CO₂ generally reduces stomatal conductance and increases plant water use efficiency, po\ntentially leading to decreased transpiration and surface drying (Swann et al., 2016). At the same time, CO₂\nfertilization promotes increases in LAI (Norby & Zak, 2011), which can enhance evapotranspiration and\npartly offset stomatal suppression (Lemordant et al., 2018). The net impact on evapotranspiration remains\nuncertain and likely varies by region, vegetation type, and climate conditions (Fatichi et al., 2016;\nForzieri et al., 2017; Kooperman et al., 2018; Skinner et al., 2018).\nEnhanced LAI and reduced stomatal conductance can produce nonlinear and either competing or mutually\namplifying effects on hydroclimate (McDermid et al., 2021). However, in many regions, the net result still\npoints to reduced transpiration (Chadwick et al., 2019; Skinner et al., 2018). This interplay suggests a po\ntential threshold (turning point) beyond which further greening allows LAI-driven evapotranspiration in\ncreases to overcome stomatal limitations, possibly reversing the sign of evapotranspiration change. These\n14\ndiscrepancies across studies underscore substantial uncertainty in vegetation–climate feedbacks. Acknowl\nedging this uncertainty, future research should focus on better constraining the relative contributions of LAI\nand stomatal regulation to evapotranspiration in order to improve projections of hydrological cycles.\nWe have summarized these points and included them in the revised Discussion.\nRevised Discussion (excerpt):\nVegetation feedbacks remain a complex and regionally variable mechanism in shaping hydrological re\nsponses. Elevated CO typically reduces stomatal conductance and evapotranspiration by increasing plant\n2\nwater use efficiency52, while simultaneously enhancing LAI via CO fertilization57. The net impact on evap\n2\ntranspiration remains uncertain, as it depends on the relative strength of these opposing processes, which\nvaries across climate conditions and vegetation types38,58,59. Some studies suggest the existence of a physi\nological threshold beyond which LAI-driven increases in transpiration can outweigh stomatal limitations\nand reverse the direction of evapotranspiration change38,60. These discrepancies underscore the need to\nbetter constrain the contributions of LAI versus stomatal closure in land surface models to improve future\nprojections of the hydrological cycle.\nComment 8:\nMost of the current discussions were about caveats of this study. More in-depth discussions of the advance\nthe current study could offer beyond the literature should be provided here. Any ideas about the results of\nthis study compared to the results of other concertation-based scenarios? Especially the contributions of\nphysiological feedback to precipitation changes, which were investigated by many prior studies.\nResponse:\nThank you for this thoughtful comment. In response, we have expanded the discussion to better highlight\nthe novel contributions of our study relative to existing literature.\nMost previous studies using concentration-driven CO₂ scenarios have focused on the hysteresis behavior\nof temperature and carbon cycle responses (Kim et al., 2022; Yeh et al., 2021), while the reversibility of\nland precipitation and the role of vegetation feedbacks have been less explored. In contrast, our emissions\ndriven experiment, which includes a fully interactive carbon cycle, offers new insights into how physiolog\nical processes influence precipitation during zero emissions scenarios.\nImportantly, concentration-driven scenarios do not account for CO outgassing from the ocean and land\n2\nonce atmospheric CO levels begin to decline. As a result, they may underestimate the strength and\n2\n15\npersistence of physiological feedbacks. In our study, the dynamic carbon cycle allows for this outgassing,\nwhich may further amplify the physiological effects through CO concentrations changes. For example,\n2\ndown and then held constant. Under such conditions, stomatal conductance remains steady because partial\nCO pressure does not change, leading to relatively muted changes in land precipitation. In contrast, our\n2\nemissions-driven experiment reveals that when CO is actively reduced, the associated decrease in stomatal\n2\nlimitation triggers strong physiological responses, enhancing transpiration and contributing significantly to\nland precipitation increases.\nThus, our study provides a clearer mechanistic understanding of how vegetation-mediated feedbacks oper\nate under negative emissions scenarios. It also underscores the importance of using emissions-based simu\nlations with a fully coupled carbon cycle to accurately capture the long-term evolution of precipitation and\nassociated hydrological extremes. We have revised the discussion section to include these points and the\nrelevant citations.\nRevised Discussion (excerpt):\nThis study advances our understanding of precipitation responses to CO removal by highlighting two key\n2\ndistinctions from previous work. First, most earlier studies used concentration-driven CO scenarios fo\n2\ncused on temperature and precipitation hysteresis25,28, without accounting for CO outgassing from land\n2\nand ocean as concentrations decline. In contrast, our emissions-driven experiment includes a fully interac\ntive carbon cycle, capturing this outgassing and providing a more realistic representation of physiological\nfeedbacks. Second, while previous study emphasized reduced transpiration due to stomatal closure under\npositive CO emissions55, our results reveal the role of physiological processes in driving precipitation\n2\nchanges under the negative emissions scenario. These findings underscore the importance of vegetation in\nshaping hydrological outcomes under extended mitigation scenarios and highlight the value of emissions\nbased experiments in capturing long-term Earth system feedbacks.\nComment 9:\nPrevious studies showed that temperature changes under positive emissions were kind of asymmetric with\nthat under negative emissions. How about precipitation? Any ideas?\nResponse:\nThank you for this important question. Similar to temperature, precipitation also exhibits asymmetric be\nhavior under positive and negative CO emissions scenarios. From a global mean perspective, temperature\n2\n16\nincreases enhance the atmosphere’s water vapor, leading to increased precipitation. Therefore, the asym\nmetry in precipitation is closely related to that in temperature.\nThis asymmetry has been previously documented in concentration-driven experiments (Kim et al., 2022),\nand we also observe similar behavior in our emissions-driven simulation. In Figure 3c of our study, the\nasymmetric temperature response to CO forcing is evident in the divergent paths of temperature under\n2\npositive versus negative emissions—an area defined as “hysteresis area” in Kim et al. (2022). Similarly, the\nprecipitation asymmetry shows in the difference in color at the same CO concentration (Fig. 3c), indicating\n2\nprecipitation changes is depending on the emissions trajectory.\nHowever, because our experiment follows an emissions-driven path with an asymmetric CO₂ trajectory,\nquantifying the precise degree of precipitation asymmetry is inherently more complex. Nevertheless, the\novershoot in precipitation exhibits a nonlinear response to CO₂ forcing, which is distinct from the temper\nature response discussed in the manuscript.\nComment 10:\nAll the figures were shown in low quality, and the results could not be found clearly from those figures.\nResponse:\nThank you for pointing this out. We have revised all figures to improve their resolution and clarity. High\nquality versions (3600 × 3600 pixels in PNG format) were provided during submission; however, it appears\nthat some degradation occurred during the insertion of figures into the Word file. To address this, we now\nalso provide all figures in vector format (PDF) to ensure optimal quality and legibility. We hope these\nchanges improve both the visual presentation and interpretability of the results.\nComment 11:\nMore details should be provided in the methods\nResponse:\nThank you for the comment. In response, we have added additional details to the Methods section to clarify\nhow diagnostics were calculated. Specifically, we now describe how area means and percentage changes in\nprecipitation were computed, including the use of land–sea masks and the reference period (2001–2030).\nWe hope this improves the transparency and reproducibility of our analysis.\n17"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review for “Negative CO2 emissions to mitigate extremes in land hydrological”\nI thank the authors for their thoughtful and comprehensive revisions. The manuscript is now substantially\nimproved, particularly in how it frames the study’s contribution to the long-term understanding of Earth\nsystem feedbacks. I appreciate the responsiveness of the authors in clarifying the study's scope, addressing\nconcerns around policy relevance, and improving the alignment between the scientific content and its\nframing.\nI have just two remaining suggestions:\nPlease revise the final sentence of the abstract to “While near-term mitigation is essential to curb immediate warming, these\nfindings suggest that sustained negative emissions are required to achieve long-term reductions in hydrological extremes\nand to enhance terrestrial water availability.” This revision better reflects the temporal scope of the study and avoids\nambiguity.\nPlease change the title to “Negative CO₂ emissions to mitigate long-term extremes in the land hydrological cycle”\nI recommend this study for publication after addressing the above minor revisions.", "reviewer_2": "This study takes idealized simulations from CESM2 to investigate negative CO2 emissionsto mitigate extremes in land\nhydrological cycle. Through the first round of review, I still think the those idealized simulations, especailly only from one\nearth system model, may not have enough and robust implications in the field. Also, the physiolgoical feedbacks are\nmentioned many times throughout the text without exact analyses or simulations. The physiological feedbacks are only\naccounted for quantitatively through factorial experiments to separate the land/ocean and atmospheric processes. Thus,\nthere are no actual results supported the physioloigcal feedbacks, which the authors think may have important contributions\nto influence precipitation under negative CO2 emissions.\nAnother concern I have is that precipitation experiences greater increases under negative CO2 emissions (in the abstract)\nthan zero emissions, why? Furthermore, the authors conclude that more aggressive climate mitigation strageties are\nneeded. I do not fully follow this suggestion as more precipitation is predicted under negative CO2 emission than zero ones.\nThird, the results show that ET and transpiration both show symmetrical variations with CO2 increases and decreases.\nHowever, temperature and precipitation changes are asymmetrical with CO2 increases compared with CO2 decreases.\nThen I am wondering how such asymmetry is represented here since the authors concluded that ET or tranpiration largely\ncontributed to precipitation changes?\nA minor comment here is that, in Line 200, \"NE\", \"E\" may be needed to be subscribed here.", "reviewer_3": "Per the editor’s request, I am not providing a full review but rather commenting on the exchange between the authors and\nreviewer #2, and focusing on how plant physiological responses to CO2 influence the hydrological response to negative\nCO2 emissions (i.e. reviewer #2 comments 6-7)\nI agree with reviewer #2 that it is important for the authors to comment on how much we should trust the modeled stomatal\nconductance and LAI responses. I think the authors have adequately responded to this comment by summarizing model\nvalidation of stomatal conductance and LAI in CESM/CLM. And more importantly, they discuss how the relative influence of\nstomatal conductance vs. LAI changes on ET remains highly uncertain. This uncertainty transcends land surface modeling\n— we can compare modeled trends in ET and LAI to observations, but there are just very few observational constraints on\nhow LAI and stomatal conductance respond to separate changes in temperature, precipitation, and CO2, and it’s necessary\nto disentangle those drivers to model responses to zero or negative emissions scenarios when CO2 and temperature adjust\nover different timescales.\nReviewer 2’s comment about explaining the relative importance of LAI and stomatal conductance strikes me as rather minor.\nI found the author’s response muddled and hard to follow, but think I agree with what the authors are trying to say. LAI\nresponds slowly to multiple factors, while stomatal conductance responds quickly and primarily is a function of CO2\nconcentration. After 2200, CO2 concentration stops changing so LAI explains increases after that point. But the full time\nevolution of transpiration is driven by the interplay of stomatal conductance and LAI, which are both of first-order importance.\nI don’t think the authors have fully fleshed out the mechanism through which plant responses contribute to the overshoot\neffect, but I see that as OK given the scope of this paper. Their main contributions are identifying land precipitation changes\nafter negative and zero emissions (which I do find novel) and suggesting that plant physiological processes are an important\ndriver of lags / overshoot dynamics in the land precipitation response. Fully disentangling the mechanism will require further\nstudies, likely including analysis of more models, new simulations, and field experiments."}, "author_response": "We appreciate Reviewer #1’s encouraging and constructive comments. The reviewers’ comments were fully\nincorporated into the revised manuscript.\nMinor Comments\nComment 1:\nPlease revise the final sentence of the abstract to “While near-term mitigation is essential to curb immediate\nwarming, these findings suggest that sustained negative emissions are required to achieve long-term\nreductions in hydrological extremes and to enhance terrestrial water availability.” This revision better\nreflects the temporal scope of the study and avoids ambiguity.\nResponse:\nFollowing your recommendation, we have revised this sentence in the Abstract to:\n“While near-term mitigation is essential to curb immediate warming, these findings suggest that sustained\nnegative emissions are required to achieve long-term reductions in hydrological extremes and to enhance\nterrestrial water availability.”\n.\nComment 2:\nPlease change the title to “Negative CO₂ emissions to mitigate long-term extremes in the land hydrological\ncycle”\nI recommend this study for publication after addressing the above minor revisions.\nResponse:\ntitle to “Negative CO₂ emissions for long-term mitigation of extremes in the land hydrological cycle”.\nMajor Comments:\nComment 1:\nThis study takes idealized simulations from CESM2 to investigate negative CO2 emissions to mitigate\nextremes in land hydrological cycle. Through the first round of review, I still think the those idealized\nsimulations, especailly only from one earth system model, may not have enough and robust implications in\nthe field. Also, the physiolgoical feedbacks are mentioned many times throughout the text without exact\nanalyses or simulations. The physiological feedbacks are only accounted for quantitatively through\nfactorial experiments to separate the land/ocean and atmospheric processes. Thus, there are no actual\nresults supported the physioloigcal feedbacks, which the authors think may have important contributions\nto influence precipitation under negative CO2 emissions.\nResponse:\nWe appreciate the reviewer’s continued concern regarding the robustness of our conclusions and the\ntreatment of physiological feedbacks. As acknowledged in the Discussion (lines 350–357), our study is\nbased on simulations from a single Earth system model (i.e., CESM2), which we recognize as a limitation.\nHowever, long-term, emissions-driven simulations with fully coupled carbon–climate systems and\ninteractive vegetation are extremely computationally demanding. While not explicitly stated in the\nmanuscript, the full simulation spanned approximately two years of supercomputing time and produced\nover 1,400 TB of data. Conducting similar simulations with multiple models would require several\nadditional years and an estimated cost of over 400,000 USD, which exceeds the capacity of a typical\nresearch group.\nMoreover, idealized scenario design is necessary under such constraints, especially in studies aiming to\nunderstand long-term system responses. For instance, idealized pathways are used in the Couple Climate–\nCarbon Cycle Model Intercomparison Project (C4MIP) and adopted in the carbon–climate reversibility\nexperiments in the Carbon Dioxide Removal Model Intercomparison Project1,2, where CO₂ concentrations\nare ramped up by 1% per year and then symmetrically decreased. These simplified scenarios are specifically\ndesigned to test climate system reversibility and to diagnose the mechanisms behind it. Thus, despite\nlimitations associated with using a single model and idealized forcing, this approach remains essential for\nadvancing our understanding of coupled physiological processes—particularly the role of vegetation.\nTo further isolate the physiological contribution, we conducted the N experiment, in which the\nE-LAND\natmospheric CO₂ concentration is fixed at 430 ppm while the vegetation component receives 725 ppm. This\nexperimental setup allows us to examine direct physiological responses under identical climate conditions\n(lines 556–566). As shown in the Results (lines 252–265, Fig. 3), when vegetation is exposed to higher CO₂,\ntranspiration and land precipitation markedly decrease—consistent with stomatal closure—compared to the\nN scenario. This provides direct evidence of physiological forcing, consistent with previous studies3,4\nE\nBeyond these studies, numerous CMIP6-based investigations—spanning dozens of papers—have similarly\nemployed the 1pctCO₂-bgc experiment, commonly referred to as a physiological simulation, to isolate and\nquantify CO₂ physiological effects5-7. As in our N experiment, this study demonstrates that the\nE-LAND\nincrease in land precipitation under N conditions cannot be explained without accounting for physiological\nE\nforcing (Fig. 3). Specifically, during the zero emissions under the N scenario, transpiration increases and\nE\nsubsequently drives additional precipitation, a process that does not operate under the positive CO₂\nemissions due to reduced transpiration from stomatal closure (Fig. 2a–b). Although our experiment setup\ndiffers from 1pctCO₂-bgc configuration, it reveals a consistent mechanism: under future negative-emissions\nconditions, physiological feedbacks can substantially enhance land precipitation.\nAnother concern I have is that precipitation experiences greater increases under negative CO2 emissions\n(in the abstract) than zero emissions, why? Furthermore, the authors conclude that more aggressive climate\nmitigation strageties are needed. I do not fully follow this suggestion as more precipitation is predicted\nunder negative CO2 emission than zero ones.\nResponse:\nThank you for pointing out the need for further clarification. The main finding of our study is that the\nincrease in land precipitation under the N scenario, compared to Z , is largely driven by physiological\nE E\nfeedbacks—particularly transpiration—and this mechanism is key to our interpretation.\nIt is important to recognize that increases in land precipitation have both beneficial and adverse implications.\nOn one hand, more land precipitation can improve water availability; on the other, it may also intensify\nextreme events such as heavy rainfall. In our study, we aim to show that the enhanced land precipitation\nunder N results in net benefits compared to Z . Although total land precipitation is higher in N , heavy\nE E E\nprecipitation (Fig. 4b) and drought (Supplementary Fig. 8b) are substantially reduced (lines 268–305). This\noutcome reflects a shift toward more frequent and spatially uniform rainfall (Supplementary Fig. 9),\nimproving overall water availability and reducing hydrological extremes.\nIn contrast, under the Z scenario, although emissions stabilize, heavy precipitation continues to intensify\nE\nin several regions, and droughts remain persistent (Fig. 4a and Supplementary Fig. 8a), relative to the period\nof maximum CO concentrations.\nin the abstract, we noted why more aggressive climate mitigation (i.e., negative emissions) is necessary:\nAbstract (lines 42–46)\n“Furthermore, despite a substantial increase in terrestrial precipitation, extreme precipitation events and\ndroughts become less severe globally under the negative emissions scenario—even compared to the zero\nemissions scenario. While near-term mitigation is essential to curb immediate warming, these findings\nsuggest that sustained negative emissions are required to achieve long-term reductions in hydrological\nextremes and to enhance terrestrial water availability.”\nIn the discussion, we also elaborated on why negative emissions are necessary:\nDiscussion (lines 342–349)\n“Our findings further indicate substantial regional implications. While future climate impacts may worsen\neven after achieving zero emissions, substantial negative emissions could effectively mitigate several\nclimate conditions, including hydrological extremes. Reductions in heavy precipitation are most significant\nin densely populated regions such as Eastern North America, South America, Central Africa, and Asia.\nAdditionally, droughts are expected to decrease in Europe, Central America, and Western Asia. These\nregional patterns differ significantly from zero and negative emission scenarios, highlighting that the\nextreme events are not simply proportional to total precipitation.”\nDiscussion (lines 377–383)\n“This study highlights the critical role of physiological feedback in enhancing terrestrial precipitation\nunder a negative emissions scenario, demonstrating its potential to improve the hydrological cycle and\nreduce the severity of extreme precipitation events. These findings underscore the need to move beyond net\nzero targets toward negative emissions strategies—an increasingly important consideration for future\nclimate policy. While current mitigation efforts often prioritize addressing immediate climate impacts, our\n4\nfindings suggest that early and sustained action can also deliver substantial long-term benefits for Earth\nsystem sustainability.”\nTherefore, our recommendation for stronger climate mitigation is not based solely on the increase in total\nprecipitation, but on the hydrological benefits of reduced extremes and improved water cycle stability—\nfactors critical for long–term climate resilience and ecosystem health.\nComment 3:\nThird, the results show that ET and transpiration both show symmetrical variations with CO2 increases\nand decreases. However, temperature and precipitation changes are asymmetrical with CO2 increases\ncompared with CO2 decreases. Then I am wondering how such asymmetry is represented here since the\nauthors concluded that ET or transpiration largely contributed to precipitation changes?\nResponse:\nWe appreciate the reviewer’s insightful question regarding the apparent inconsistency between symmetric\ntranspiration and asymmetric precipitation responses. The key distinction is that the relative contribution of\ntranspiration differs by period. During RS in N (blue shading in Fig. 2b), anomalous ET plays a central\nE\nrole in land precipitation changes rather than during CO ramp-up and ramp-down phases. During RS, ET\n2\naccounts for approximately 75% of the land precipitation changes (green and orange lines in Fig.2a), while\nthe moisture flux convergence (MFC) remains small (purple line in Fig.2a) (lines 155–163).\nBy contrast, during the CO ramp-up and -down phases, the asymmetry in precipitation (orange line) arises\n2\nmainly from anomalous MFC, which continues to decline after the 2020s (purple line in Fig. 2a). This\nreduction explains the sharp decrease in land precipitation during the CO ramp-down phase (e.g., 2120–\n2\n2200), compared with the increase phase. Thus, the precipitation asymmetry stems from asymmetric MFC\ndespite symmetric ET during the ramp-up and ramp-down phases.\nWe also recognize that the original sentence in the Discussion may have been confusing. To clarify our\nargument, we revised it as follows:\nDiscussions (lines 327–330)\nFrom:\n“Our findings demonstrate that the land precipitation response to negative emissions is asymmetric to that\nunder positive emissions, yet distinct from the asymmetry observed in the global temperature response.”\nTo:\n“Our findings demonstrate that the increased land precipitation under negative emissions is distinct from\nthat under positive emissions, with vegetation playing an important role.”\nMinor comment:\nA minor comment here is that, in Line 200, \"NE\", \"E\" may be needed to be subscribed here.\nResponse:\nWe thank the reviewer for pointing this out. We have revised the notation, so that “E” appears as subscripts.\nComments:\nPer the editor’s request, I am not providing a full review but rather commenting on the exchange between\nthe authors and reviewer #2, and focusing on how plant physiological responses to CO2 influence the\nhydrological response to negative CO2 emissions (i.e. reviewer #2 comments 6-7)\nI agree with reviewer #2 that it is important for the authors to comment on how much we should trust the\nmodeled stomatal conductance and LAI responses. I think the authors have adequately responded to this\ncomment by summarizing model validation of stomatal conductance and LAI in CESM/CLM. And more\nimportantly, they discuss how the relative influence of stomatal conductance vs. LAI changes on ET remains\nhighly uncertain. This uncertainty transcends land surface modeling — we can compare modeled trends in\nET and LAI to observations, but there are just very few observational constraints on how LAI and stomatal\nconductance respond to separate changes in temperature, precipitation, and CO2, and it’s necessary to\ndisentangle those drivers to model responses to zero or negative emissions scenarios when CO2 and\ntemperature adjust over different timescales.\nReviewer 2’s comment about explaining the relative importance of LAI and stomatal conductance strikes\nme as rather minor. I found the author’s response muddled and hard to follow, but think I agree with what\nthe authors are trying to say. LAI responds slowly to multiple factors, while stomatal conductance responds\nquickly and primarily is a function of CO2 concentration. After 2200, CO2 concentration stops changing\nso LAI explains increases after that point. But the full time evolution of transpiration is driven by the\ninterplay of stomatal conductance and LAI, which are both of first-order importance. I don’t think the\nauthors have fully fleshed out the mechanism through which plant responses contribute to the overshoot\neffect, but I see that as OK given the scope of this paper. Their main contributions are identifying land\nprecipitation changes after negative and zero emissions (which I do find novel) and suggesting that plant\nphysiological processes are an important driver of lags / overshoot dynamics in the land precipitation\nresponse. Fully disentangling the mechanism will require further studies, likely including analysis of more\nmodels, new simulations, and field experiments.\nResponse:\nWe thank the reviewer for the constructive remarks and for recognizing the novelty of our study in\nidentifying precipitation changes after negative- and zero-emission scenarios and in highlighting the role\nof plant physiological processes. We agree that both stomatal conductance and LAI are of first-order\nimportance for transpiration; however, their relative contributions remain uncertain because observational\nconstraints are still very limited.\nEarth system models have been improved substantially from CMIP3 to CMIP6, and we anticipate an even\nbetter representation of physiological processes in CMIP7. Although uncertainties remain—as the reviewer\nnotes—it is essential to explore climate change across zero-emission scenarios as thoroughly as possible\nwith the tools and knowledge currently available. Such efforts will provide the scientific evidence needed\nto inform mitigation policies. Our findings on physiological feedbacks are consistent with previous studies,\nas discussed in the manuscript, and further underscore the critical role of plants in shaping climate responses\nafter the achievement of zero-emission targets. Finally, as the reviewer emphasizes, fully resolving these\nmechanisms will require future multi-model comparisons, new simulations, and dedicated field\nexperiments, which we regard as an important direction for subsequent research beyond the scope of the\npresent study.\n6"}, {"round": 3, "reviewer_comments": {"reviewer_4": "The authors have mostly addressed my major concerns, particularly those regarding claims supported (or not) by results and changing the use of \"hysteresis\" to be consistent with what the plot actually shows.", "reviewer_1": "Review for “Negative CO2 emissions to mitigate extremes in land hydrological”\nUsing a single climate model CESM2, this study conducts idealized simulations based on both Zero and\nNegative emission scenarios for 400 years from year 2000 to 2400. This study show that terrestrial precip-\nitation increases with greenhouse warming, and the trend continues even with zero or negative emissions.\nUnder negative emissions, physiological feedback from terrestrial vegetation plays a significant role, pri-\nmarily due to enhanced transpiration with reduced CO₂. Despite an overall increase in precipitation, nega-\ntive emissions scenarios result in less severe hydrological extremes compared to zero emissions.\nSuch simulations are ideal for investigating long-term processes and feedbacks, especially those involving\nslow-response components of the climate system like vegetation dynamics, deep-ocean circulation, and\ncarbon cycle feedbacks.\nHowever, I believe the framing of the study as relevant to climate mitigation and adaptation is not fully\nsubstantiated. The key findings, while methodologically robust, pertain to processes and impacts that man-\nifest on centennial timescales (post-2250), which are far removed from the actionable horizons of 10–50\nyears typically addressed in mitigation and adaptation contexts. This disconnect limits the immediate appli-\ncability of the study's results to policy and planning efforts. Also, if one keeps the Paris agreement in\nmind this does not fall in the mitigation and adaptation contexts.\nI have few major comments listed below:\nMajor Comments\nComment 1:\n1. The abstract and main text emphasize the implications for climate mitigation and adaptation, yet the\nresults focus primarily on long-term trends [on several century timescales] in precipitation and vegetation\nfeedbacks. To better align with the stated objectives, the authors should explicitly discuss any near-term\nimpacts (if present) or clearly acknowledge that the findings are more relevant for understanding long-term\nEarth system dynamics."}, "author_response": "We sincerely thank the reviewer for the constructive feedback throughout the revision process. We are\npleased that our clarifications regarding the result-based claims and the revised terminology for ‘hysteresis’\nhave addressed the previous concerns. We have further reviewed the manuscript to ensure that all terms and\ninterpretations are strictly consistent with the visual evidence provided in the figures. We appreciate the\nreviewer’s valuable time and insightful suggestions, which have significantly improved the quality of our\nmanuscript."}]

Decision letter