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# Focus on extreme events and the carbon cycle

Chuixiang Yi, Elise Pendall, Philippe Ciais

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Chuixiang Yi, Elise Pendall, Philippe Ciais. Focus on extreme events and the carbon cycle. Environmental Research Letters, 2015, 10 (7), 10.1088/1748-9326/10/7/070201. [DOI: 10.1088/1748-9326/10/7/070201](https://doi.org/10.1088/1748-9326/10/7/070201)

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EDITORIAL • OPEN ACCESS

Focus on extreme events and the carbon cycle

To cite this article: Chuixiang Yi et al 2015 *Environ. Res. Lett.* **10** 070201

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EDITORIAL

# Focus on extreme events and the carbon cycle

Chuixiang Yi<fnref n="1" />,<fnref n="2" />, Elise Pendall<fnref n="3" /> and Philippe Ciais<fnref n="4" />

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2 July 2015

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E-mail: cyi@qc.cuny.edu

Keywords: extreme events, ecological disturbance, droughts, hurricane, bark beetles, carbon cycle, climate mitigation

## Abstract

Climate physics indicates that warming climate is a likely cause of extreme weather and more frequent and intense climate events. These extreme events can disrupt terrestrial carbon dynamics dramatically by triggering ecological disturbances and potentially forcing climate-carbon feedbacks. In this paper we synthesize the findings of 26 papers that focus on collecting evidence and developing knowledge of how extreme events disturb terrestrial carbon dynamics.

## 1. Introduction

The World Meteorological Organization reported recently 'that the world experienced unprecedented high-impact climate extremes during the 2001–2010 decade, which was the warmest since the start of modern measurements in 1850 and continued an extended period of pronounced global warming' (World Meteorological Organization 2013). Even though since 2000, the warming rate is less than during the previous decade (the Hiatus period) extreme hot days have continued to increase in frequency and severity (Seneviratne et al 2014). Although global mean annual temperature is not increasing every year, global decadal mean temperature has increased every decade since 1970s. On the other hand, it is clearly evident that the frequency and intensity of extreme events of weather and climate have been increasing in this warming world (World Meteorological Organization 2011, Coumou and Rahmstorf 2012). Does global warming generate more extreme weather events? There has been no direct answer yet. However, the link between extreme weather and warming climate can be understood by the physics of climate. In principle, the ability of atmosphere to hold water is dictated by the Clausius–Clapeyron equation;

$$e_s(T) = 6.1094 \exp\left(\frac{17.625T}{T + 243.04}\right), \quad (1)$$

where, $e_s$ is saturation vapor pressure in hPa, representing the maximum amount of water vapor that air

can hold at a given temperature $T$ (°C) (Ahrens 2000). The Clausius–Clapeyron equation (1) is derived from the first and second law of thermodynamics and has been broadly verified under both laboratory and natural conditions. It predicts that the water-holding capacity of the atmosphere increases by about 7% for every 1 °C rise in temperature, which is close to satellite-based observations that recorded a rate of change in the atmospheric water vapor content 6% °C⁻<fnref n="1" /> (Kininmonth 2010).

Figure 1 illustrates conceptual links between global warming and extreme weather. Warming air has higher saturation vapor pressure, i. e. larger size of the 'room' for water to evaporate from soils and water bodies and transpire from plants. In addition to increased saturation vapor pressure in the atmosphere, warming directly increases plant evaporation demand, leading to more water emitted by vegetated surfaces to the atmosphere unless soil moisture gets too depleted. This is likely to cause extreme precipitation events as the probability of water molecules condensing increases. Meanwhile, weather systems accompanied with extreme precipitation events can become more violent as more latent heat is released. It will also take longer to recharge the atmosphere with moisture as its capacity to hold water increases. Thus, duration between rain events becomes longer and hence drought would increase. We cannot be certain that any particular extreme weather event is caused by global warming. However,

<footnote n="1">¹ School of Earth and Environmental Sciences, Queens College, City University of New York, NY 11367, USA</footnote>

<footnote n="2">² Department of Meteorology, Bert Bolin Centre for Climate Research, Stockholm University, Stockholm S-106 91, Sweden</footnote>

<footnote n="3">³ Hawkesbury Institute for the Environment, University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751, Australia</footnote>

<footnote n="4">⁴ Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, 91191 Gif sur Yvette, France</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Conceptual links between warming climate and extreme weather.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Schematic map of a potential climate-carbon feedback loop. This focus issue collects evidence on the links between extreme weather/climate and the carbon cycle.</figcaption></figure>

we are certain that global warming amplifies the risk factors for extreme weather events from the principle of thermodynamics—hot air can hold more water molecules. Global warming increases atmospheric evaporative demand, which is supported by observational data; about half of the land in warm regions (annual temperature >16 °C) has been drying with increasing temperature since the 1970s (Jung et al 2010, Yi et al 2014).

The high risk consequence of these warming-induced extreme events is that they often lead to ecological disturbances which can affect land-atmosphere exchanges of carbon and water dramatically, potentially contributing to climate-carbon feedback (Cox et al 2013), illustrated in figure 2. Most climate-related disturbance events have negative impacts on the health and carbon balance of ecosystems, especially in forests, because they kill trees, inducing a large carbon source to the atmosphere followed by a slow recovery sink (figure 3). Some ecosystems are more resistant to disturbance; for instance, deserts and grasslands are

adapted to water stress. However, mega-drought and heat-waves can cause damage to plants that results in mortality, and more frequent drought can in the long-term change ecosystem structure (Breshears et al 2005, Edburg et al 2012; figure 3).

In addition to altering the carbon balance, extreme events cause feedbacks to climate due to changed albedo and roughness, and because of altered sensible and latent heat emissions. Stand replacing fires in northern regions causes an increase in winter albedo due to higher snow cover over the short vegetation that covers the ground when the forest has burned. This has a net cooling climate effect (Randerson et al 2006). Extreme drought generally suppresses evapotranspiration and increases sensible heat (Teuling et al 2013), which acts to increase air temperature and decrease rain probability. This causes a positive feedback on heat and drought. Thus, the direction of feedback between ecological disturbance and global warming is challenging to predict, warranting the
<figure><img src="image_7.png" /></figure>

**Figure 3.** Hypothetical framework showing how ecological disturbance (red arrow) can perturb the ecosystem carbon cycle. First, vegetation is killed and its potential for C uptake via gross primary production (GPP, green line) declines. Disturbance may enhance C losses from ecosystem respiration (TER, dark blue solid line) if increased inputs of dead biomass stimulate decomposition. Alternatively, TER may be reduced as autotrophic respiration is eliminated (dark blue dashed line). Net ecosystem production (NEP, light blue line), or net C storage on land, declines rapidly but recovers more slowly, even up to decades. Eventually, gains and losses are balanced and the ecosystem returns to steady state (Odum 1969). Based on a figure in Edburg *et al* (2012), originally published in *Frontiers in Ecology and the Environment*.

investigations across a range of disturbance types and ecosystems presented in this focus issue.

The aim of this focus issue is not to verify the existence of feedbacks but to collect evidence on how these extreme events, such as heat waves, droughts, hurricanes and insect outbreaks change land-atmosphere exchanges, and to use quantitative approaches to identify these events and explore driving forces behind their ecological consequences.

## 2. Drought

Fourteen papers in this focus provide lines of evidence of how ongoing extreme droughts affect terrestrial ecosystem exchange of CO₂ with the atmosphere from tower-based flux measurements and satellite-based modeling. Net ecosystem-atmosphere exchange (NEE) of CO₂ is a small imbalance between two large CO₂ fluxes with opposite directions: uptake from the atmosphere by gross primary production (GPP) and release back to the atmosphere by terrestrial ecosystem respiration (TER). These papers demonstrate that impact of extreme droughts on these two components is different and also varies from season to season and from biome to biome.

### 2.1. Spring drought

Soil moisture availability in spring is usually higher than in summer because soil reservoirs have been recharged by winter precipitation, in many summer-dry northern regions. However, lower precipitation combined with warmer winter temperatures can lead to spring droughts. Based on eddy-flux data collected in Switzerland, Wolf *et al* (2013) found that grasslands and forests respond differently to spring droughts because these ecosystems exhibit different water use

strategies. The ratio of CO₂ uptake (GPP) to water loss (transpiration) is called water use efficiency (WUE), which can be optimized at the ecosystem level under water limited conditions as plants can regulate the size of stomatal openings (Claesson and Nycander 2013). The eddy-flux data indicate that forests increased their WUE during the spring drought, while grasslands' WUE did not change substantially during the spring drought (Wolf 2013).

The response of ecosystems to drought is different from biome to biome. For peatlands, drought conditions will not only reduce soil wetness (reducing GPP) but also lower the water table, increasing the depth of the aerobic respiration layer (increasing TER). The data collected in a south Swedish nutrient-poor peatland by eddy-flux measurements indicate that droughts can turn a temperate peatland from a carbon sink to a carbon source (Lund *et al* 2012). The effect of drought on the peatlands depends on drought duration. A prolonged drought in the early growing season resulted in reduced rates of GPP, while a short but severe drought in the middle of the growing season resulted in increased rates of TER (Lund *et al* 2012), demonstrating the contrasting effects of extreme climate events on components of the carbon cycle.

The 2010 spring drought in southwestern China was the most severe and long-lasting of the last half century. Satellite-based observations show that severe and extended spring droughts substantially reduced the enhanced vegetation index (EVI) and GPP and both did not recover from drought stress until August (Zhang *et al* 2012). This spring drought event also lowered the water levels of rivers, reservoirs and lakes, and even dried up some water bodies, which affected agricultural irrigation and hence crop production. Other satellite-based observations also show that spring drought induced by the El Niño/La Niña-Southern
Oscillation (ENSO) delayed vegetation onset time and reduced annual net primary production (NPP) in the subtropical island of Taiwan (Chang et al 2013).

## 2.2. Late-summer drought

Kolb et al (2013) found that impacts of extreme late-summer drought on net ecosystem production (NEP) of semi-arid forest in the southern west United State (SWUS) differed between disturbed and undisturbed ecosystems based on six-year eddy-covariance measurements. The late-summer drought shifted August NEP at the undisturbed site from a carbon sink to source because the reduction of GPP (70%) exceeded the reduction of TER (35%). At the burned site, late-summer drought shifted August NEP from a weak carbon source to neutral because the reduction of TER (40%) exceeded the reduction of GPP (20%). The results illustrate the sensitivity of semi-arid forest carbon sequestration to extreme drought and suggest that warming-associated drought may interact with disturbances such as fire, possibly weakening semi-arid forest carbon sinks.

The contrasting impact of drought on GPP and TER between grazed and ungrazed desert steppe was also evidenced by data collected with tower-based eddy covariance approach on the Mongolian Plateau (Shao et al 2013). This two-year dataset indicates that grazing can play a positive role in biophysical regulation of carbon fluxes in a desert steppe. The positive effects of grazing on the uptake of CO₂ in semiarid steppe were also reported by Kang et al (2013). Although both these examples were short, the analyses will be useful for grazing management. Nevertheless, additional long-term flux measurements are needed to understand the mechanisms underlying the interactions between extreme climate, ecological disturbance, and carbon cycling.

## 2.3. Long-term drought events

Babst et al (2012) did clustering analysis of tree-ring growth data at ~1000 sites across Europe that showed regionally consistent growth patterns driven by climate regimes and species-specific growth characteristics. Fifteen regional, synthetic chronologies that represent radial tree growth anomalies across Europe were reconstructed over the past ~500 years. The analysis revealed 18 extreme tree-growth events during the pre-instrumental and 2 events (1947/48 and 1976) in the instrumental period, which may be been caused by extreme late-summer drought conditions.

The complexity of interactions between drought and vegetation response is demonstrated in a 26-year study that analyzed vegetation growth based on Normalized Difference Vegetation Index (NDVI) in China (Xu et al 2012). Extreme events in temperature, precipitation and drought based on Palmer Drought Severity Index (PDSI) appear to have increased across China over the past three decades. Associated with the

increasing trend in heat waves and drought events, a decreasing trend in the extreme low NDVI events (low vegetation growth) was observed. However, different regions in China experienced contrasting temporal patterns. Although extreme drought events increased in the 2000s in both northern and southern China, extreme low NDVI events keep decreasing in southern China, while the decrease is stalled or even reversed in northern China. Apparently, drought does not impact vegetation productivity in a consistent fashion across continents; prior land use such as grazing may need to be considered to reveal detailed mechanisms.

# 3. Extreme weather

## 3.1. Hurricane

The impact of catastrophic hurricanes on life loss and property damage has received considerable attention, but environmental impacts are often neglected. Hurricanes create major ecological disturbances by killing hundreds of millions of trees and adding plant litter on the forest floor, and potentially increasing risk of forest fire conditions. The drastic ecological disturbance by hurricanes is expected to have significant impacts on regional forest carbon balances. In this focus issue, we report case-studies for hurricane impacts on carbon cycling in tropical savannas and temperate forest ecosystems (Hutley et al 2013, Vargas 2012, Fisk et al 2013).

### 3.1.1. *Hurricane-savanna-fire interactions*

Hutley et al (2013) estimated the ecological consequences of the mega-cyclone Monica that damaged or destroyed 140 million trees across the coastal and subcoastal region of north Australia in 2006. They used the moderate resolution imaging spectroradiometer (MODIS) GPP (MOD17A2) to track spatial and temporal patterns between six years preceding and six years following the cyclone Monica. They found that there was a significant decline in the region's GPP (directly proportional to its capacity to sequester carbon) relative to the pre-cyclone rates, and this reduced productivity persisted for four years postcyclone. The most important ecological consequences of hurricanes is that the hundreds of millions of trees killed or damaged will be converted into CO₂ and returned back to the atmosphere sooner by fire or later by decomposition. Hutley et al (2013) examined the impact that the massive increase in fuel load (vegetation debris) following Cyclone Monica would have on the region's fire regime. They found that fire frequency was extreme in the year following the cyclone, but fire frequency more or less returned to pre-cyclone conditions in the years that followed—an unexpected result.

### 3.1.2. *Effect of hurricanes on the soil carbon flux*

Vargas (2012) observed large pulses of soil CO₂ flux after hurricane Wilma in two-year time series of soil
carbon dioxide efflux measurements from two soil-
sensor nodes. He reported that in the year following
hurricane Wilma, annual soil carbon dioxide emis-
sions were more than 3.9 kgC m⁻² but decreased to
1.7 kgC m⁻² for the second post-hurricane year.
Vargas attributed the increase in the soil carbon flux
post-Wilma to tree damage depositing nitrogen-rich,
easily decomposable organic matter and higher basal
soil CO₂ efflux rates. Moreover, GPP was not impacted
immediately after the hurricane but during the
drought period of the first year, a depletion of non-
structural carbon reserves that likely were used for a
'rapid' recovery of canopy cover (e.g., leaf area index)
during the first 2 months following the hurricane.

3.1.3. *Carbon footprint of historical hurricanes in eastern US*

The case studies conducted by Hutley *et al* (2013) and Vargas (2012) have demonstrated that a single hurricane event can exert a large disturbance in forest carbon balance, but forests have strong resilience to recover. In the short- to medium-term, hurricanes release carbon due to leaves and litters can be easily decomposed, while over the longer term, GPP will be increased from forest re-growth. Therefore, the recovery times of the two components, GPP and TER, of forest carbon balance from hurricane disturbances to approaching the normal rates are not the same (see figure 3). This imbalance between GPP and TER might affect the ability of forests to absorb carbon in a long-term perspective. Fisk *et al* (2013) investigated it by modeling the reconstructed maps of tree mortality from hurricanes impacting the eastern US from 1851 to 2000. They found how long it takes for the forest re-growth to recover the carbon loss depends on the type of forest. Small disturbances in fast-growing regions can recover quickly, but large disturbances or slower growth rates can result in recovery that takes more than a century. Fisk *et al* (2013) concluded that on average, tropical cyclones contribute a net carbon source over latter half of the 19th century due to high storm activity and the existence of larger forests during that period. However, throughout much of the 20th century a regional carbon sink is estimated resulting from periods of forest recovery exceeding damage.

**3.2. Ice storms**

Terrestrial ecosystems in temperate to subtropical
regions are vulnerable to the invasion of extreme cold
events like ice storms. These cold events are common
in East Asia and can cause massive structural damage
to forests and often define the northern boundaries of
key tree species. Sun et al (2012) investigated the
impact of the massive 2008 Chinese ice storm on forest
ecosystems in southern China and found that while
forest structures were substantially changed by the ice
storm, forest greenness, as measured by satellite,
recovered fairly quickly overall; however, moderately

damaged forests recovered the slowest as a conse-
quence of salvage logging. These cold events can also
interrupt ecosystem potential productivity dramati-
cally. Huang *et al* (2013) observed the large reduction
of GPP from Qianyanzhou Ecological Experimental
Station in southern China as a result of ice storms in
2005 and 2008.

**3.3. Extreme snow events**

3.3.1. Extreme short snow season

Climate change is likely to shorten the duration of
snow cover in mountainous regions such as the
European Alps. In 2011, Europe experienced extre-
mely warm spells in late winter. These events did not
spare the Alps, and the Italian Aosta Valley region saw
the earliest snowmelt for 65 years and the shortest
snow season for 83 years. Galvagno et al (2013)
examined how the short snow season modified the
carbon balance of subalpine grassland in the region by
eddy-covariance measurements. They observed that
the extremely short snow season in the Alps led to a
100% increase in the annual net CO2 uptake. They
found, interestingly, this larger carbon sink was
attributable to smaller carbon dioxide emissions by
TER during the shorter winter, rather than to
enhanced plant activity and GPP during the longer
summer.

3.3.2. Summer drought worsens impacts of bitter winter
in Mongolia Plateau

'Dzud' in the Mongolian Plateau refers to a winter with extreme cold, heavy snowfall, reduced availability of forage and widespread mortality of livestock (John et al 2013). Recently, the dzuds and droughts in the Mongolian Plateau have become more serious. Particularly when a dzud is preceded by a summer drought, the combined summer drought-dzud often leads to even higher mortality of livestock (Fernández-Giménez et al 2012). John et al (2013) identified such extreme events in the Mongolian Plateau during the period of 2000-2010 using precipitation and PDSI data, and examined the vegetation response to these extreme events using anomalies of vegetation indices. They concluded that the grassland biome on the plateau was more resilient to the extreme events than the desert biome.

4. Bark beetles

Global warming and drought are associated with
expanding populations of forest insects and pathogens
(Logan et al 2003), which cause widespread tree
mortality and cascading ecosystem level disturbance
impacts (Edburg et al 2012). Severe outbreaks of bark
beetles may coincide with or follow extreme droughts
(Breshears et al 2005), as stressed trees are unable to
defend themselves from the attacking insects. Disrup-
tion to the carbon cycle by beetle-induced mortality
has been compared with the carbon losses by forest fires in Canada (Kurz et al 2008). In this focus issue, we provide four reports on climate-associated insect outbreaks and their impacts on carbon cycling in North America.

Over the last two decades, a widespread outbreak of bark beetles (Dendroctonus spp.) has affected tens of millions of hectares of conifer forests (Edburg et al 2012). In an Idaho pine forest, aboveground carbon stocks in trees were reduced by up to 50%, as shown by multispectral imaging and LIDAR (Bright et al 2012). Most of the area in this region was moderately affected by the beetles, although small areas experienced severe mortality. An inventory-based analysis over the western United States found that the amount of carbon in trees killed by beetles exceeded that lost to forest fires (Hicke et al 2013). These disturbances represent a loss of about 10% of the total carbon stock in Western US forests, comparable to the amount removed by logging activities. Although the impact of bark beetle-induced mortality is visually dramatic, their influence on carbon and water cycling may be less than anticipated. Using eddy covariance, Reed et al (2014) found that maximum carbon uptake by a Wyoming pine forest was not strongly altered despite the progression of mortality from 30% to nearly 80% in the tower footprint. They attributed the resilience of the ecosystem to enhanced light use efficiency by the surviving trees as the canopy opened.

The effects of defoliation on carbon cycling were evaluated using an ecosystem demography model that represents fine-scale canopy structure to scale up physiological processes (Medvigy et al 2012). The model indicated that net ecosystem production decreased linearly with increasing defoliation by gypsy moths, but that the carbon cycle responses interacted with drought impacts. Drought-induced tree mortality was lower in scenarios including defoliation disturbance (Medvigy et al 2012), possibly due to compensating processes such as enhanced water or nutrient availability at intermediate disturbance levels (Reed et al 2014).

## 5. Quantitative approaches

How to quantify relationships between the extreme events and their ecological consequences is a knowledge gap. In this focus issue, a few quantitative approaches have been demonstrated.

### 5.1. Tail Approach

The ecological consequences of extreme climate events can be mapped from remote sensing images with sharper changes in vegetation greenness and leaf area index, or can be identified by a large reduction in carbon uptake (GPP) with eddy-covariance measurements. Extreme events occur with small probability but with large amplitudes of changes in variables

characterizing the state of the terrestrial biosphere (Zscheischler et al 2014). These extreme events can be defined as the occurrence of certain values in the tails of the probability distribution of the anomalies of the variables, similar to extreme weather defined by IPCC (Seneviratne et al 2012). We call this method the 'tail approach'. The question is how long the tails can be cut to define extreme events.

Zscheischler et al (2014) applied this approach to the terrestrial carbon balance by defining extremes to be outside a certain threshold $q$, which is defined by a percentile (i.e. 1%,...,10%) on the absolute values of the anomalies. They then defined an extreme event by spatiotemporally contiguous points whose values are larger than $q$ (positive extremes) and smaller than $-q$ (negative extremes), respectively. Zscheischler et al (2014) reanalyzed four global MODIS-based GPP datasets over the last 30 years and found that positive (extreme excess in carbon uptake) and negative (extreme reduction in photosynthetic carbon uptake) GPP extremes occurring on 7% of the spatiotemporal domain explained 78% of the global interannual variation in GPP. Most of the negative GPP extremes (larger than positive extremes) were attributed to droughts.

The tail approach depends on the shape of the probability distribution of the anomalies, and on their severity. The identification of extreme events requires that the shapes of probability distributions are similar so that comparison and analysis cross all sites can be made. However, it is impossible to have similar distribution of probability of the anomalies on a global scale due to heterogeneous properties of vegetation from region to region. Liu et al (2013) developed a new method of using the Box-Cox transformation (Box and Cox 1964) to convert non-normally distributed dataset into normally distributed dataset. They applied the new method to biweekly time series of global normalized difference vegetation index (NDVI) from 1982 to 2006 to investigate the extreme events on a global scale. They found that extreme NDVI events were aggregated in Amazonia and in the semi-arid and semi-humid regions in low and middle latitudes, while they seldom occurred in high latitudes. This NDVI data analysis indicates that precipitation was a major control on the NDVI extremes in low and middle latitudes, providing a consistent picture as by FLUXNET data synthetic analysis (Yi et al 2010, 2014).

### 5.2. Risk approach

Risk analysis is not a new method, but applying it to the understanding of interactions between extreme events and ecological resources is challenging. van Oijen et al (2013) introduced a simple probabilistic method that can be used to assess ecosystem risks imposed by extreme climatic events. In this approach, risk is quantified as the product of two components: the probability of hazardous conditions and the
vulnerability of ecosystems to those hazardous conditions. The challenge in this approach is to determine the threshold values of hazardous climate conditions, which varies between ecosystems and can also evolve with time. They have used their method to analyze the risk of drought in European forests under recent (1971–2000) and future (2071–2100) climate conditions. Vulnerability is defined as the mean difference in primary vegetation productivity between non-drought and drought years.

### 5.3. Perfect-deficit approach

A perfect-deficit approach developed by Yi *et al* (2012) is different from the widely used, traditional method of the tails approach (extreme fluctuations relative to mean values). The nature of the perfect-deficit approach is data-based. During observational period, a perfect growth (carbon uptake) curve can be constructed by the best growth of ecosystems across the years at the site. Thus, the growth curves of each year can be compared with the perfect growth curve. The difference between the perfect-growth curve and yearly growth curve is defined as deficit. Yi *et al* (2012) derived this approach based on GPP data measured from five grassland tower sites. They found strong links between GPP deficits and drought index. This approach can be applied to any continuous dataset of ecosystem-climate interactions. Therefore, the same extreme events can be cross-verified by different independent datasets.

Wei *et al* (2014) applied the perfect deficit approach to assess the effects of climate extremes on forest carbon assimilation capacity based on FLUX-NET data and MODIS GPP datasets. They found a strong link between periods of drought and reduced capacity of trees to absorb carbon. In particular, broadleaf (evergreen and deciduous) trees were very sensitive, whereas coniferous trees were less affected. This is because most broadleaf trees are adapted to generally well watered and good nutrient conditions, whereas needles are a specific adaptation that evolved to maximize carbon uptake in water-and temperature-limited regions. Huang *et al* (2013) applied the perfect-deficit approach to explore how potential productivity of coniferous plantation forest was significantly reduced by extreme drought and ice storm. They found that temperature deficits rather than droughts were main drivers for the GPP deficits.

### 5.4. Climate mitigation

As illustrated in figure 1, more frequent extreme weather and drought events are likely to be associated with the warming climate. In this focus issue, we have collected evidence on how these warming-associated natural disturbances interrupted current terrestrial carbon dynamics. The disruption of the terrestrial carbon dynamics may trigger the potential climate-carbon feedbacks illustrated in figure 2. Le Page *et al*

(2013) used the Global Change Assessment Model (GCAM) to test what successful climate mitigation should be undertaken with the natural disturbances. They studied scenarios of stable, increasing and decreasing natural disturbance rates, with all model runs stabilizing atmospheric carbon dioxide at a level that is expected to keep global temperature increase within a certain range above pre-industrial levels by 2095. They found that in the case of increased disturbance rates, resulting terrestrial emissions would have to be balanced by further deployment of low-carbon technologies for society to achieve a given level of mitigation. Under a carbon market policy, this implies increased carbon prices and mitigation costs up to 2.5 times higher in the most extreme scenario (doubling disturbance rates). As a case study, Campbell *et al* (2012) examined the strategic effects of future mountaintop coal mining on the carbon budget of the southern Appalachian forest region by ecosystem modeling and field experiment data.

## Acknowledgements

CY is grateful to the International Meteorological Institute for supporting him as a Rossby Fellow to complete this synthesis and review.

## References

Ahrens C D 2000 *Essentials of Meteorology: An Invitation to the Atmosphere* 4th Edn (Pacific Grove, CA: Brooks/Cole)

Babst F, Carrer M, Poulter B, Urbinati C, Neuwirth B and Frank D 2012 500 years of regional forest growth variability and links to climatic extreme events in Europe *Environ. Res. Lett.* 7 045705

Box G E P and Cox D R 1964 An analysis of transformations *J. R. Stat. Soc. Ser. B* 26 211–52

Breshears D D *et al* 2005 Regional vegetation die-off in response to global-change-type drought *Proc. Natl Acad. Sci. USA* 102 15144–8

Bright B C, Hicke J A and Hudak A T 2012 Landscape-scale analysis of aboveground tree carbon stocks affected by mountain pine beetles in Idaho *Environ. Res. Lett.* 7 045702

Campbell J E, Fox J F and Acton P M 2012 Terrestrial carbon losses from mountaintop coal mining offset regional forest carbon sequestration in the 21st century *Environ. Res. Lett.* 7 045701

Chang C-T, Wang H-C and Huang C-Y 2013 Impacts of vegetation onset time on the net primary productivity in a mountainous island in Pacific Asia *Environ. Res. Lett.* 8 045030

Claesson J and Nycander J 2013 Combined effect of global warming and increased CO₂-concentration on vegetation growth in water-limited conditions *Ecological Modelling* 256 23–30

Coumou D and Rahmstorf S 2012 A decade of weather extremes *Nat. Clim. Change* 2 491–6

Cox P M, Pearson D, Booth B B, Friedlingstein P, Huntingford C, Jones C D and Luke C M 2013 Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability *Nature* 494 341–4

Edburg S L *et al* 2012 Cascading impacts of bark beetle-caused tree mortality on coupled biogeophysical and biogeochemical processes *Frontiers in Ecology and the Environment* 10 416–24

Fisk J P, Hurtt G C, Chambers J Q, Zeng H, Dolan K A and Negrón-Juárez R I 2013 The impacts of tropical cyclones on
the net carbon balance of eastern US forests (1851–2000)
*Environ. Res. Lett.* **8** 045017

Fernández-Giménez M E, Batkhishig B and Batbuyan B 2012 Cross-boundary and cross-level dynamics increase vulnerability to severe winter disasters (dzud) in Mongolia *Glob. Environ. Change* **22** 836–51

Galvagno M et al 2013 Phenology and carbon dioxide source/sink strength of a subalpine grassland in response to an exceptionally short snow season *Environ. Res. Lett.* **8** 025008

Hicke J A, Meddens A J H, Allen C D and Kolden C A 2013 Carbon stocks of trees killed by bark beetles and wildfire in the western United States *Environ. Res. Lett.* **8** 035032

Huang K, Wang S, Zhou L, Wang H, Liu Y and Yang F 2013 Effects of drought and ice rain on potential productivity of a subtropical coniferous plantation from 2003 to 2010 based on eddy covariance flux observation *Environ. Res. Lett.* **8** 035021

Hutley L B, Evans B J, Beringer J, Cook G D, Maier S W and Razon E 2013 Impacts of an extreme cyclone event on landscape-scale savanna fire, productivity and greenhouse gas emissions *Environ. Res. Lett.* **8** 045023

Jung M et al 2010 Recent decline in the global land evapotranspiration trend due to limited moisture supply *Nature* **467** 951–4

John R, Chen J, Ou-Yang Z-T, Xiao J, Becker R, Samanta A, Ganguly S, Yuan W and Batkhishig O 2013 Vegetation response to extreme climate events on the Mongolian Plateau from 2000 to 2010 *Environ. Res. Lett.* **8** 035033

Kang X M et al 2013 Effects of grazing on CO₂ balance in a semiarid steppe: field observations and modeling *Journal of Soils and Sediments* **13** 1012–23

Kininmonth W 2010 Clausius-clapeyron and the regulation of global warming *Fisica E* **26** 61–70

Kolb T, Dore S and Montes-Helu M 2013 Extreme late-summer drought causes neutral annual carbon balance in southwestern ponderosa pine forests and grasslands *Environ. Res. Lett.* **8** 015015

Kurz W A et al 2008 Mountain pine beetle and forest carbon feedback to climate change *Nature* **452** 987–90

Le Page Y et al 2013 Sensitivity of climate mitigation strategies to natural disturbances *Environ. Res. Lett.* **8** 015018

Liu G, Liu H and Yin Y 2013 Global patterns of NDVI-indicated vegetation extremes and their sensitivity to climate extremes *Environ. Res. Lett.* **8** 025009

Logan J, Regniere J and Powell J A 2003 Assessing the impacts of global warming on forest pest dynamics *Frontiers in Ecology and the Environment* **1** 130–7

Lund M, Christensen T R, Lindroth A and Schubert P 2012 Effects of drought conditions on the carbon dioxide dynamics in a temperate peatland *Environ. Res. Lett.* **7** 045704

Medvigy D, Clark K L, Skowronski N S and Schäfer K V R 2012 Simulated impacts of insect defoliation on forest carbon dynamics *Environ. Res. Lett.* **7** 045703

Odum E P 1969 The strategy of ecosystem development *Science* **164** 262–70

Reed D E, Ewers B E and Pendall E 2014 Impact of mountain pine beetle induced mortality on forest carbon and water fluxes *Environ. Res. Lett.* **9** 105004

Randerson J T et al 2006 The impact of boreal forest fire on climate warming *Science* **314** 1130–2

Seneviratne S I et al 2012 Changes in climate extremes and their impacts on the natural physical environment 109–230
Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation: A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change

Seneviratne S I et al 2014 No pause in the increase of hot temperature extremes *Nature Clim. Change* **4** 161–3

Shao C, Chen J and Li L 2013 Grazing alters the biophysical regulation of carbon fluxes in a desert steppe *Environ. Res. Lett.* **8** 025012

Sun Y, Gu L, Dickinson R E and Zhou B 2012 Forest greenness after the massive 2008 Chinese ice storm: integrated effects of natural processes and human intervention *Environ. Res. Lett.* **7** 035702

Teuling A J et al 2013 Evapotranspiration amplifies European summer drought *Geophys. Res. Lett.* **40** 2071–5

Vargas R 2012 How a hurricane disturbance influences extreme CO₂ fluxes and variance in a tropical forest *Environ. Res. Lett.* **7** 035704

van Oijen M, Beer C, Cramer W, Rammig A, Reichstein M, Rolinski S and Soussana J-F 2013 A novel probabilistic risk analysis to determine the vulnerability of ecosystems to extreme climatic events *Environ. Res. Lett.* **8** 015032

Wei S et al 2014 Data-based perfect-deficit approach to understanding climate extremes and forest carbon assimilation capacity *Environ. Res. Lett.* **9** 065002

Wolf S, Eugster W, Ammann C, Häni M, Zielis S, Hiller R, Stieger J, Imer D, Merbold L and Buchmann N 2013 Contrasting response of grassland versus forest carbon and water fluxes to spring drought in Switzerland *Environ. Res. Lett.* **8** 035007

World Meteorological Organization (WMO) 2013 The global climate 2001–2010: a decade of climate extremes *Summary report* (Geneva: World Meteorological Organization) no 1119 http://library.wmo.int/pmb_ged/wmo_1119_en.pdf

World Meteorological Organization 2011 *Weather Extremes in a Changing Climate: Hindsight on Foresight* (Geneva: World Meteorological Organization) no 1075

Xu X, Piao S, Wang X, Chen A, Ciais P and Myneni R B 2012 Spatio-temporal patterns of the area experiencing negative vegetation growth anomalies in China over the last three decades *Environ. Res. Lett.* **7** 035701

Yi C, Wei S and Hendrey G 2014 Warming climate extends dryness-controlled areas of terrestrial carbon sequestration *Sci. Rep.* **4** 5472

Yi C et al 2012 Climate extremes and grassland potential productivity *Environ. Res. Lett.* **7** 035703

Yi C et al 2010 Climate control of terrestrial carbon exchange across biomes and continents *Environ. Res. Lett.* **5** 1748–9326

Zhang L, Xiao J, Li J, Wang K, Lei L and Guo H 2012 The 2010 spring drought reduced primary productivity in southwestern China *Environ. Res. Lett.* **7** 045706

Zscheischler J et al 2014 A few extreme events dominate global interannual variability in gross primary production *Environ. Res. Lett.* **9** 035001