Global floating kelp forests have limited protection despite intensifying marine heatwave threats

Domain: earth_science
Published: 2025-04-03
Editorial outcome: major_revision

Abstract
Kelp forests are one of the earth’s most productive ecosystems and are at great risk from climate change, yet little is known regarding their current conservation status and global future threats. Here, by combining a global remote sensing dataset of floating kelp forests with climate data and projections, we find that exposure to projected marine heatwaves will increase ~6 to ~16 times in the long term (2081–2100) compared to contemporary (2001–2020) exposure. While exposure will intensify across all regions, some southern hemisphere areas which have lower exposure to contemporary and projected marine heatwaves may provide climate refugia for floating kelp forests. Under these escalating threats, less than 3% of global floating kelp forests are currently within highly restrictive marine protected areas (MPAs), the most effective MPAs for protecting biodiversity. Our findings emphasize the urgent need to increase the global protection of floating kelp forests and set bolder climate adaptation goals.

Introduction
Marine protected areas (MPAs) are a cornerstone of marine conservation1. Promoted by international agreements, such as the Convention on Biological Diversity (CBD) Aichi Target 112, the area of marine ecosystems under some form of protection has increased since the turn of the century3. Because climate change is a major long-term threat to biodiversity4–6, the newly agreed Global Biodiversity Framework at COP157 calls for effectively protecting 30% of the oceans by 2030. A central component of the post-2020 target is increasing the representation of different habitats under effective protection while adapting to climate change. Although many studies report the protection of critical habitat-forming species, such as corals, seagrass, and mangroves3, other essential marine habitats, such as kelp forests, remain largely neglected8 (but see refs. 9,10). Comprehensive maps on kelp forest distribution, threats associated with climate change, extreme events, and protection status are urgently needed to guide ongoing local and global protection efforts.

Kelp forests dominate >30% of the world’s rocky reefs and are among the most productive ecosystems on earth—comparable to terrestrial forests11–14. However, marine heatwaves (MHWs) and anthropogenic activities threaten kelp forests15–18 and their capacity to provide ecosystem services worth billions of dollars19–22. The IPCC Sixth Assessment Report identified kelp forests as the second most at-risk marine ecosystem from MHWs6, after coral reefs, which is concerning given that MHWs are projected to become more frequent and severe in the next decades23. In addition, kelp forests are facing other stressors (e.g., pollution, overgrazing) that can reduce their ability to recover from heatwaves. For example, northern California has lost >90% of its kelp forests due to the combined effects of severe marine heatwaves and overgrazing by sea urchins24,25. Climate adaptation strategies—including MPAs—are urgently needed to halt and reverse this loss16,26,27. While MPAs cannot directly counter the impacts of climate change that can surpass a species’ physiological tolerance28, MPAs can mitigate non-climatic stressors like overfishing and habitat destruction, which can enhance ecosystem resilience29,30, supporting ecological functioning and providing societal benefits31–33.

Well-managed and highly restrictive MPAs—no-take marine reserves where all fishing activities are prohibited—are the most effective type of MPA for rebuilding fished populations31, supporting the stability of kelp forest ecosystems34 and, in some documented cases, providing resilience to MHW impacts35–38. For example, in regions where urchin predators are protected from fishing and where trophic cascades are a driver of food-web dynamics, MPAs can facilitate the recovery of higher-trophic-level organisms, which helps control kelp grazer populations and prevent overgrazing of kelp30,39,40. This mechanism has been found to support resistance to and recovery from MHWs in a network of 39 MPAs in southern California30. In addition, MPAs can provide climate resilience for kelp forests ecosystems through other mechanisms35–37,41–43. For example, a recent global analysis found that fish communities were more stable to MHW in highly restrictive MPAs than unprotected sites37, and abalone populations in two MPAs in Mexico were more resilient to a hypoxia event and MHWs through increased body size and reproductive output36,42.

Monitoring subtidal kelp populations over large spatial and temporal scales can be challenging. However, the largest species (i.e., Macrocystis pyrifera, Nereocystis leutkeana, Ecklonia maxima) can be mapped by remote sensing because they create extensive canopies that float on the water surface. Recent advances in satellite imaging of surface-canopy-forming kelp species provide an opportunity to map the distribution of kelp forest habitats, quantify the threats posed by MHWs, and assess their protection status44. Floating kelp forests are a globally distributed foundation species that co-exist with other sub-canopy kelp species that structure one of Earth’s most biodiverse ecosystems12. These forests can cover thousands of hectares (e.g., 28,500 hectares in the Southern California Bight ecoregion10) and sustain hundreds to thousands of species, some of which are economically and culturally significant. For example, in the Channel Islands in Southern California, studies found 716 species associated with giant kelp forest ecosystems45 and in Patagonia, Chile and Argentina, similar studies found between 150 and 250 species46–48. In addition, in the northeast Pacific Ocean, 17 species of sub-canopy kelp coexist with floating kelp forests (www.algaebase.org/) from Alaska to Baja California, Mexico. Since remote sensing is the only available method to detect kelp forest ecosystems comprehensively (i.e., using standarized methods at large temporal and spatial scales), maps of floating kelps are good indicators of the broader ecosystem and associated biodiversity. These data can also inform other climate-adaptation strategies such as identifying and protecting climate refugia49,50—areas less impacted by or more resilient to climate change—for kelp forests. Effectively protecting climate refugia for kelp forest ecosystems is a priority for conservation51 because, in these areas, biodiversity can persist49 and may enhance the resilience of other kelp forests, depending on local and regional connectivity and life history traits, by maintaining a source of recovery for impacted kelp habitats26.

Here, we compile a comprehensive global map of floating kelp forest habitats (henceforth “kelp forests”) and leverage these datasets to project the global exposure of kelp forests to MHWs and assess their protection status within MPAs. To develop the global kelp forest map, we assemble existing regional and national remote-sensing datasets from Landsat observations (1984–present), supplemented with Sentinel-2 satellite imagery (2015–201952) (Supplementary Table 1; see methods). To project threats to kelp forests from climate change, we estimate future cumulative annual MHW intensities from an ensemble of sea surface temperature (SST) from 11 Earth System models, using three climate scenarios generated under the IPCC Shared Socio-Economic Pathways (SPPs)53 (see methods). We then quantify the global protection status and the representation of kelp forests at both country and biogeographic levels (i.e., realm, ecoregions54) within MPAs categorized as highly, moderately, or less protected based on restrictions to extractive activities obtained from Protected Seas55 (see methods). Our findings reveal increasing threats to all floating kelp forests from future MHWs, although some southern hemisphere forests may act as climate refuges. We also found that kelp forests remain largely unprotected within restrictive MPAs, the most effective type of MPA, which are poorly represented globally. These findings emphasize the urgent need to increase the global protection and effective representation of kelp forests and, given the scale of the threat posed by future MHWs, for bolder climate adaptation goals for kelp forests.

Results
Global distribution of kelp forests
We found floating kelp forest habitats in only 12 nations distributed across 6 biogeographic realms and 32 ecoregions, mostly in mid-latitudes in the Pacific, Atlantic, and Indian Oceans (Fig. 1a). Most of the kelp forests are located in five ecoregions, with 23.7% in Malvinas/Falklands, 20.9% in Channels and Fjords of Southern Chile, 12.8% in Southern California Bight, 10.3% in Kerguelen Islands, and 9.2% in Northern California; while 17 ecoregions combined account for only 1% of the distribution of kelp forests (Supplementary Fig. 1).

In the northern hemisphere, kelp forests can be found at their highest latitudes, overall, in the USA (~61.4°N), extending southward to their warm-distribution limit in Mexico (~27°N). In the southern hemisphere, kelp forests can be found at their lowest latitudes, overall, in Peru (~13.6°S), extending southwards to their cool-distribution limit in Chile (~56°S). Other warm-distribution limits of kelp forests in the southern hemisphere are located in Argentina, Namibia, South Africa, Australia, and New Zealand.

Contemporary and Future exposure of kelp forests to marine heatwaves
Projected future MHWs for kelp forests increase for each realm, ecoregion, climate scenario, and time (Figs. 1b and 2 and Supplementary Figs. 2 and 3). In the near term (2021-2040), kelp forests are projected to be subject to >2 times higher exposure to cumulative MHW intensities compared to contemporary exposure, with similar values across climate scenarios (Supplementary Table 2-5). Projections suggest that these magnitudes will continue to intensify, and under SSP5-8.5, kelp forests could be subject to >6 to >16 times higher cumulative MHW intensities in the mid (2041–2060) and long term (2081–2100), respectively, compared to contemporary exposure (Supplementary Table 6). These magnitudes are ~2 to ~3 times higher than corresponding projections under SSP1-2.6 and SSP2-4.5, respectively. In the long term, even under SSP1-2.6 and SSP2-4.5, magnitudes are ~5.6 and ~9.6 times higher than contemporary exposure, respectively. Note that these estimations where derived from the mean cumulative MHW intensities (n = 2156 pixels) for each climate scenario and time frame, and then divided by the corresponding mean contemporary values.

The Arctic and the Temperate North Pacific realms are projected to be the most exposed to future MHWs under all climate scenarios, while Temperate South America and Temperate South Africa will be the least exposed (Fig. 1b), matching the general spatial patterns in contemporary exposure. Overall, the pattern is very similar across SSP scenarios, with the northern hemisphere experiencing nearly twice the exposure to future MHWs than the southern hemisphere (Fig. 2b). However, some differences emerge. We found a difference in the latitudinal pattern of exposure between the northern and southern hemisphere. Specifically, projections suggest a latitudinal pattern of increasing exposure to future MHWs from lower to higher latitudes in the northern hemisphere, whereas in the southern hemisphere, this pattern is reversed (Fig. 2b). For example, in the mid and long term and under all future scenarios for the northern hemisphere, the Eastern Bering Sea and the Gulf of Alaska are projected to become the most exposed ecoregions, while the southern California Bight becomes the least exposed (Fig. 2a and Supplementary Fig. 3), albeit with elevated levels of MHW exposure relative to the present. In contrast, in the southern hemisphere lower latitude ecoregions such as Cape Howe and Humboldtian are projected to be the most exposed to future MHWs while remote islands in high latitudes and ecoregions such as the Channels and Fjords of Southern Chile will be the least exposed.

Global protection status of kelp forests
Globally, more than 33.1% of floating kelp forest habitats are protected by MPAs, of which 13.7% are highly protected (the most effective type of MPA), 4.6% are moderately protected, and 14.8% are in less-protected MPAs (Figs. 3a, b and 4a). However, most of the effective protection for kelp forests is in remote islands in the Southern Ocean realm (24,319.8 ha), and when excluding these areas, only 2.8% (5,870.9 ha) of the global kelp forests are highly protected from fishing activities (Fig. 3c). At the country level, France has placed all their floating kelp forests within highly protected MPAs (Fig. 4a, b) and is the only country that meets the current 30% effective representation target7. New Zealand, South Africa, Canada, Australia, and the USA have at least 10% of their kelp forests highly protected (Fig. 4a, b). However, this protection is in overseas territories in remote islands for all of France (23,007.1 ha, there is no floating kelp forests in mainland France) and much of New Zealand (145.1 ha), South Africa (285.1 ha), and Australia (78.7 ha). Australia has only 2.7% (23.0 ha), New Zealand 2.0% (14.1 ha), and South Africa (400.6 ha) 8.9% of their continental kelp forests highly protected. Mexico and the UK have provided effective protection for less than 2% of their kelp forests, Chile less than 0.02%, and Peru, Argentina, and Namibia none.

Of the world’s biogeographic realms, the Southern Ocean realm has 99.9% of its kelp forests within highly protected MPAs (which represents 11% of the global distribution, see Figs. 3c and 4), while all other realms have less than 10%. However, at least 10% of kelp forests are protected in some form of MPA in all realms, except for the Arctic, where the area of surface-canopy forming kelp is minimal and no kelp forests are protected under any category (Fig. 4c). At the ecoregional level, only 9 ecoregions have met the old 10% effective representation targets2 for kelp forests within highly protected MPAs, all in remote islands except for the Northern California ecoregion (Supplementary Fig. 4). Overall, 47.2% of ecoregions have less than 10% of their kelp forests protected, regardless of the MPA type. Only one nation, one realm, and 25% of ecoregions (all remote islands) meet the new 30% target for effective representation7 for kelp forests.

Ecoregional future marine heatwave threats and protection status
Kelp forests within the ecoregions that are most threatened by projected MHWs and currently have low levels of effective protection (highly protected) include the Bering Sea (none protected), the Gulf of Alaska (0.6%), the North American Fjordlands (2.5%), the Puget Trough (0.09%), and the Oregon to Vancouver ecoregions (2.4%) (Fig. 5a, b and Supplementary Figs. 5 and 6). Northern California is the only ecoregion projected to be highly threatened by MHWs where at least 10% of kelp forests are inside highly protected MPAs. In contrast, eight ecoregions that have all their kelp forests inside highly protected MPAs will face low to intermediate threats from projected MHWs under the SSP2-4.5 scenario. These ecoregions are all located in remote islands of the Southern Ocean realm. When combining highly and moderately protected MPAs, the Patagonian Shelf and North Patagonian Gulfs ecoregions have at least 30% of their kelp forests protected and low exposure to MHWs (Fig. 5b).

Methods
Mapping kelp forests
We compiled existing published regional and national datasets of surface-canopy forming kelp derived using remote sensing observations (Supplementary Table 1). We accessed quality-controlled estimates of kelp canopy derived from individually classified scenes observed by up to four Landsat sensors: Landsat 5 Thematic Mapper (1984–2011), Landsat 7 Enhanced Thematic Mapper+ (1999–present), Landsat 8 Operational Land Imager (2013–present), and Landsat 9 Operational Land Imager-2 (2021-present). The applicable Landsat observations have pixel resolutions of 30 × 30 m and repeat times of 16 days (8 days since 1999 in most years because two Landsat sensors were operational). Classification of floating kelp canopy was derived by applying a globally robust random forest classifier to individual Landsat scenes76. The compiled datasets include minor differences in methodologies and time periods, but they all cover approximately over 30 years (1984 onwards) (Supplementary Table 1). Kelp maps were created by compositing observations of kelp presence across this time series. The maps include most of the USA (California, Oregon, parts of Washington, and parts of Alaska) and all of Mexico, Peru, and Argentina (available at https://kelpwatch.org/)76, most of the United Kingdom77 (Malvinas/Falkland Islands), and most of Australia78 (Tasmania). We included the Malvinas/Falkland Islands as part of the United Kingdom territory, although we acknowledge that Argentina has ongoing legal claims for their sovereignty.

For areas where the Landsat maps are not available, we supplemented the Landsat time series using available maps derived using empirical thresholding of Sentintel-2 satellite imagery. For the empirical Sentinel maps, kelp area was calculated using band-difference52 or band-ratio79 relationships and released for global and South African extents, respectively. The method for the empirical map applied to global waters averages all the available images from the Sentinel-2 satellite sensor from 26 June 2015 to 23 June 2019 to create a cloud-free mosaic. It then applies band-difference thresholds to identify pixels likely containing floating kelp canopy and a land mask using global digital elevation models (ALOS and SRTM), discarding topography with elevation > 0 m. This dataset was validated across 14 in situ sites in South America that cover a variety of coastlines and ecoregions, and with existing data at 151 locations that cover four continents52. To ameliorate some potential detection caveats, we excluded pixels that fell within a 30-m buffer relative to the coastline because the global map does not distinguish between intertidal green algae and floating kelp forests and estuaries can also be a source of false positives. See Supplementary Table 1 for the coastlines used to apply the 30-m buffer. All kelp datasets were converted from coordinates to shapefiles with ArcMap 10.8 using the World Geodetic System 1984 (WGS84). Our final floating kelp habitat map includes any pixel the satellite detected kelp in the time series and represents the known presence of floating kelp habitat in the time series. We included all observations to avoid arbitrarily choosing a period to map the distribution of floating kelp forests because these ecosystems are highly dynamic76 and we were interested in detecting potential kelp habitat. For example, in some places kelps may be expanding their distribution (cold-edges)58,59, while in others places kelps may be in alternative stable states dominated by urchin barrens24 (degraded kelp ecosystems) or by more heat tolerant subcanopy kelp species (competing with floating kelps) near warm-edges80. These alternative stable states can shift, even after decades81,82.

Exposure of kelp forests to contemporary and future marine heatwaves
We estimated the expected threat of climate change to kelp forests by calculating historical and projected cumulative annual MHW intensities for each kelp pixel using a baseline climatology of 1983–2012. Marine heatwaves are periods during which temperature exceeds the 90th percentile of temperatures seasonally during a baseline period and last for at least five consecutive days83. To quantify the magnitude of present-day MHWs, we used the NOAA 0.25°-resolution Optimum Interpolation Sea-Surface Temperatures (OISST)84 dataset (1982-present). Note that cumulative MHW intensities are an indicator of exposure23,26, but they are not a measure of the vulnerability of species or ecosystems to MHWs.

We also considered MHW characteristics using SST outputs from each of 11 Coupled Model Intercomparison Project Phase 6 (CMIP6; Supplementary Table 6) Earth System models (ESMs) re-gridded to 0.25° resolution using bilinear interpolation in CDO (Climate Data Operators). For each ESM, we selected the Historical run to represent the recent past (1983–2014), and selected three future (2015–2100) climate scenarios generated under the IPCC Shared Socio-Economic Pathways (SPPs)53: SSP1-2.6, SSP2-4.5, SSP5-8.5. SSP1-2.6 represents an optimistic scenario with a peak in radiative forcing at ~3 Wm−2 by 2100 (approximating a future with 1.8°C of warming relative to the pre-industrial temperatures, in line with the Paris Agreement). SSP2-4.5 represents an intermediate mitigation scenario with radiative forcing stabilized at ~4.5 Wm−2 by 2100 (approximating implementation of current climate policies, resulting in 2.7°C of warming by 2100). SSP5-8.5 represents an extreme counterfactual climate scenario with a continued rapid increase in greenhouse gas emissions resulting in radiative forcing reaching 8.5 Wm-2 (and 4.4°C of warming) by 2100 and rising after that. We bias-corrected the SST dataset from each ESM relative to the corresponding ensemble mean of the Historical runs using the delta method (see 85). This method ensures that projections for each ESM blends smoothly to the end of the Historical runs for the reference period 1983–2014. We then determined which grid cells overlayed with kelp forests, and when the kelp cell had no corresponding SST data for the ESM models (because ESMs have relatively coarse resolution), we filled the cell using the inverse-distance-weighted mean of surrounding cells.

We then used the R package heatwaveR86 to estimate historical (1983–2020) and projected (2021–2100) cumulative annual MHW intensity (°C days) for each pixel. Note that although we used OISST data to quantify contemporary MHW intensities, we used corresponding data from each ESM’s historical run for the period 1983–2014 when quantifying projected MHW intensities. This was necessary because using ESM data in the baseline period for projections instead of the OISST data ensures like-for-like comparisons (i.e., modeled data vs modeled data), avoiding issues associated with variation in inter-ESM skill in representing daily variability in SST. We used annual cumulative intensities because they are a good indicator of the exposure of kelp forests to warm anomalies23,26. We then estimated the median cumulative annual MHW intensity for each grid cell for the contemporary (2001–2020) period and across the 11 ESMs for the near- (2021–2040), mid- (2041–2060), and long-term (2081-2100) for each SSP and grid cell. Finally, we summarized trends in MHWs at the level of biogeographic realms and ecoregions54 by conducting a spatial overlay (following the same approach as in the next sections).

Marine protected areas: level of fishing restriction
We obtained the spatial boundaries of MPAs using two different sources of information for the countries that have surface-canopy forming kelp forests. First, we downloaded MPA boundaries from official country-level agencies (Supplementary Table 1). We undertook extensive searches to ensure that we used the most updated official information, as global datasets can be less comprehensive at the country-level. We then categorized each MPA based on the level of restrictions to extractive activities (recreational, subsistance, and commercial fishing). We used the Level of Fishing Protection (LFP) score obtained from Protected Seas55 (https://protectedseas.net/). This database scores MPAs based on fishing restrictions on a scale of 1–5 scale (1 = Least restricted: no known restriction to the removal of life, 2 = Less restricted: at least one species- or gear-specific restrictions apply, 3 = Moderately restricted: several species -or gear-specific restrictions apply, 4 = Heavily restricted: marine life removal is mostly prohibited with a few exceptions, 5 = Most restricted: marine life removal is prohibited). Protected Seas further divides the scores into categories: an LFP score of 1–2 is categorized as less protected, 3 as moderately protected, and 4–5 as highly protected areas, the most effective type of MPA55. Finally, we reviewed both country-level and Protected Seas datasets and, when needed, consulted country-level experts to ensure that all MPAs were included. We did not include other area-based measures not categorized as an MPA in the national dataset (e.g., fishery management areas). For a few MPAs (34 of 817) that had no LFP score, we reviewed existing information on restrictions on the removal of life and assigned a LFP score of less protected, moderately protected, or highly protected. We did not include other regulatory activities that MPAs can manage (e.g., mining, dredging, anchoring) or indicators of management efficiency (e.g., enforcement capacity, budget capacity, implementing management plan)1 because such datasets are not comprehensively available for all countries.

Global kelp distribution and protection
To estimate the amount of kelp within each level of protection, we performed a spatial intersection of MPA types (LFP classification; 817 spatial features) and the global kelp forest distribution (428,400 spatial features). Spatial intersection is a computationally expensive operation, so avoiding trivial calculations can significantly improve performance. We therefore developed and implemented a nested, parallelized, and hierarchical intersection algorithm. The approach is “nested” because spatial layers are split based on national jurisdiction before performing the spatial intersection. The approach is “parallelized” because the country-level intersection operations can be performed across parallel computer cores. Finally, the approach is “hierarchical” because, even within a country, not all kelp forests may lie within an MPA and not all MPAs may contain kelp. We first use a simple and less computationally expensive spatial join to identify kelp forests and MPAs that do not overlap with each other and exclude them from the expensive intersection calculation. Kelp forests excluded in this step are categorized as “not protected”. Finally, we perform the spatial intersection between the kelp forests and MPAs that overlap. We then repeated this approach at the biogeographic realms and ecoregions as outlined by ref. 54. For all operations, we used unprojected coordinates (EPSG code 4326) that uses WGS84 datum and a spherical geometry engine (s2)87 via the sf package88 in R. Parallelization was done using the furrr and future89 package in R. We validated geometries throughout the pipeline using st_make_valid in sf; any invalid geometries were removed.

Knowing the location and amount of kelp protected, we proceeded to calculate the total extent of kelp by country, biogeographic realm, and ecoregion, and by MPA category and LFP score. We also determined how much kelp was outside any protection. All spatial analyses were performed in R version 4.3.1 (2023-06-16)90 using a x86_64-apple-darwin20 platform running macOS Ventura 13.4.1 and using the sf package v1.088,91 with GEOS 3.11.0, GDAL 3.5.3, and PROJ 9.1.0.

Ecoregional marine heatwave threats under SSP2.4-5 and kelp representation
Our final analysis assessed the relationship between the threat posed by projected future MHWs to kelp forests and the amount (% area) protected in each ecoregion. We conducted this analysis at the ecoregional scale because, ideally, networks of MPAs should be established to protect the underlying biophysical processes that maintain species distribution and composition26. Areas with low values of projected future MHW intensities are potential climate refugia for kelp forests. For simplicity, our measure of threat is focused only on the average cumulative MHW intensity under one SSP for each timeframe. We used SSP2-4.5 as an intermediate climate scenario that reflects less extreme outcomes and has been proposed to inform climate adaptation and policy65,66. Because the patterns of threat for each ecoregion are similar across time frames (i.e., magnitude is the largest difference across times), we focus in the main text on the mid-term and include results of the other times in the Supplementary information. We report results most conservatively for highly protected kelp, and then also for highly and moderately protected kelp combined. We did not include less protected MPAs in this analysis because this type of MPA provides minimal to no protection to marine ecosystems from extractive activities1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This review was jointly done by an ECR and mid-career researcher. This study combined maps of floating kelp forests, environmental data on future marine heatwaves under different climate scenarios, and MPA spatial data to identify the amount of kelp forests most vulnerable to MHWs and areas that are currently protected. This is important information in light of the 30X30 goals. Overall, we enjoyed reading the paper and found the use of maps and figures informative and the code and data availability suitable. The topic of kelp and MHWs is timely and the research topic should be of interest to a broad readership. However, we do have some concerns with the conceptual underpinnings of the research question, and the presentation of the results, which should be addressed.\nMain comments\nThe authors emphasize the use of MPAs as the “most effective” conservation measure for safeguarding kelp, stating they can provide “climate resilience” or “enhance their adaptability”. However, there is very limited evidence that MPAs increase the resilience of kelp forests to warming temperatures or increase how they can adapt to extreme events. The only documented case of MPAs buffering MHW impacts is the example given on line 270, which may be the exception and not the rule. MPAs are not always effective at mitigating MHW impacts because one of the main mechanisms which MHWs impact kelp forests is through direct mortality of kelp by surpassing temperature thresholds (Filbee-Dexter et al. 2021, Thomsen et al. 2021, Smale 2020). MPAs can do little to nothing against this lethal stress. MPAs may help kelps better survive non-lethal warming temperatures by reducing other stressors, yet in situ evidence of this mechanism is scarce. This was overviewed by Bates et al.’s ‘protection paradox’ study. In general, MPAs can potentially prevent kelp loss in areas where the MHW causes kelp loss due to indirect drivers that can be managed to some extent by top-down control. For example, in some regions where they are overgrazed by sea urchins (Tasmania, North California). Yet, even in areas where this link is best established such as New Zealand in the Leigh Reserve, there is no overall relationship between kelp cover and protection level at broader scales across multiple reserves in NZL (and this does not even include MHW stress). Further, MPAs do not help rebuild all urchin predators. Kelp forests in the North Pacific are impacted by otter loss, and MPAs are not a typical tool to rebuild otter populations. Same can be said for large groundfish in the North Atlantic. The limitation of MPAs against this heatwave driver should be better explored and addressed much earlier in the study.\nThe study implies that MHWs impact equally along a kelp species range. Yet, MHW impacts are likely more severe for kelp forests at the canopy species’ warm range edge, compared to cool range edges. This is mentioned on line 260 as a bit of a throwaway discussion comment - but this is essential to determine vulnerability. The presentation of the vulnerability results treat warm and cool range edges as if they are equally vulnerable to MHWs. For example, the study identifies the Bering Sea, the Gulf of Alaska, the North American Fjordlands and the Puget Trough as vulnerable areas for kelp forests. Yet, the kelp species in these Arctic and sub-Arctic areas are predominantly at the cool range edge of their thermal tolerance. This means that increased sea temperatures of a few degrees from a MHW likely will not cause direct mortality and could even make temperatures temporarily more suitable for these species. It is more likely that kelps in California and northern Chile are more vulnerable.\nThe authors discuss that their analysis excludes many kelp species (line 288) and only includes surface-forming kelps. The fact that this paper shows the distribution and lack of protection for only three species of kelp out of >100 Laminarian kelp should be emphasised. Kelp forests made by subsurface species will overlap this distribution and are found along 30% of the worlds coasts. This is not a global analysis - but an analysis of three widespread canopy kelp species. The title of this paper therefore cannot include the world ‘global’ as it is misleading.\nFinally, the paper often refers to the amount of protection in percentages, it would be more effective to also mention the area/ scale of unprotected kelp forests here. The authors show kelp area per ecoregion, but that can be better communicated in text.\nSee more specific comments/ suggestions by line.\nLine 55: Replace “for all forests” with “across all regions”.\nLine 55: The use of the word 'emerge' seems misguided here, we suggest the end of this sentence could read \"...some kelp forests in the southern hemisphere may be protected within climate refugia\".\nLine 71: Add “and extreme events” after “threats associated with climate change”.\nLine 84: Change “trophic-level” to “trophic level organisms”.\nLine 87: For scale/ context, the authors should consider providing the average canopy extent for each of these species in a known area.\nLine 89: Reference needed after “threats posed by MHWs”.\nLine 90: Reference needed after “protection status”.\nLine 90: Identifying climate refugia alone is not a climate adaptation strategy. Instead, that information needs to be considered or applied in the context of conservation management as indicated below, there is need to reframe this sentence.\nLine 93: After “can persist” change the sentence to read “...and the resilience of other kelp forests may be enhanced, depending on local and regional connectivity and species life-history traits, by maintaining a source of recovery for impacted kelp habitats.”\nLine 117: Change “while” to “while the remaining”.\nLine 118: Change “forests” to “forests, combined”.\nLine 119: Add “, overall,” after “highest latitudes” to keep consistent with the following.\nLines 121-122: Remove “at their warm distribution limit” and change “to their highest latitudes in” to “southwards to their cool-distribution limit in”.\nLine 123: Change “include” to “are located in”.\nLine 144: Add “realms” after “Pacific”.\nLines 150-151: Change the sentence to read “Specifically, projections suggest a latitudinal pattern of increasing exposure to future MHWs from lower to higher latitudes in the northern hemisphere, whereas…”\nFigure 1: Why does the Arctic region (green) have a large coloured portion that does not match the corresponding black line showing kelp distribution?\nLine 174: Remove the plural from “MPAs”.\nLine 181: So even though France meets the current 30% target, does this mean that all their kelp forests are located on remote islands and there are no protection measures on the mainland?\nLine 183: Here, it would be more effective to also mention the scale of continental kelp forests that are unprotected in these countries since it will be a very large area. Only reporting on percentages takes away from this point.\nLine 193: Add “Overall” before “only”.\nLine 281: Change “threated” to “threatened”.\nLine 240: Change “country” to “nation” to keep consistent with the above.\nLine 262: provide some reasons or justification why it is challenging.\nLines 267-269: This point needs to be made clearer earlier in the paper for example, around lines 80-84.\nLine 271: After “less biodiverse ecosystems” outline what species these systems were previously dominated by, compared to how they look now after the MHW.\nLine 272: Remove “MPAs have prevented” and “have provided”. Replace “collapse” with “collapse was prevented”.\nLine 273: add was ‘shown’\nLine 275: Replace “other” with “supplementary”.\nLines 278-279: Consider “other anthropogenic impacts” both in the ocean and on land. What manageable stressors are/ can be identified? This calls on local conservation managers to have a solid understanding of compounding stressors, aside from warming and extreme events.\nLine 288: The point that this analysis only includes surface canopy kelp is underemphasised throughout the manuscript. While we understand the complexities and limitations of gathering reliable global datasets, could the authors expand on the extent of the surface-canopy kelp distribution relative to known sub-canopy kelp distributions for key areas, for example, Australia, New Zealand, Chile, South Africa, USA etc?\nLine 329: Replace “secure” with “safeguard”.\nLine 388: After “by 2100” outline what the relative warming would be, as above.\nLine 414: What are some examples of these extractive activities relative to each of the levels?\nLine 422-423: What are the country-level criteria for fishing restrictions? The authors also mention that they do not include other types of spatial closures and area-based measures that are not MPAs, by assumption this varies from country to country therefore. Some insight should be provided, for example, does this include cultural or seasonal-based closures? MPAs are not all created equal, and this is an important point to be clear on.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "This work focuses on exposure of global floating kelp forests to marine heatwaves and analyze the spatial distribution of kelp forests with respect to the marine protected areas. Although interesting results are presented, major concerns exist, epically in terms of the soundness of methodology and results.\nResult completeness. As only the floating (surface canopy) kelp forests can be detected from satellite image, and only 3 of the largest kelp species form floating canopies out of over 120 kelp species, it is not accurate to say that the presented results from satellite observations here are “global kelp forests”, as subsurface kelp forests have not been reasonably accounted for.\nResult reliability. Classification of floating kelp canopy was derived by applying a globally robust random forest classifier, and the global map was only validated using ground observations at 14 sites in South America. It is largely unknown how the classifier works elsewhere and if the classification accuracy is good enough to support the analysis presented here.\nResults consistency. As Sentinel-2 satellite imagery (2015-2019) was adopted to supplement Landsat observations (1984-98 present), how to guarantee the consistency (or comparability) between results from different satellite data, especially in terms of different band configuration and algorithms used to kelp forest?\nConclusion soundness. A key conclusion in Abstract is that “…exposure to projected marine heatwaves will increase ~8 times compared to contemporary (2001-2020) exposure for intermediate climate scenarios”. But I didn’t find the corresponding statement in the main text, especially in terms of “the 8 times”. How was it defined and calculated?\nTitle is not correct and logical. The fact is that intensifying marine heatwaves and anthropogenic activities may threaten global kelp forests, but kelp forests cannot be threatened by protection behavior even if the protection is limited.\nIn captions of Figure 2 it read SSP-1.26, SSP-2.45, SSP-5.85, whereas in the figure it read different as SSP1-2.6, SSP2-4.5, SSP5-8.5. This issue also exists in the methods parts.", "reviewer_4": "This ms uses remotely sensed data to quantify the distribution of canopy-forming kelp across the world’s oceans before overlaying that with MPA designations under different levels of fisheries restrictions. They then calculate cumulative MHW statistics for contemporary times and under different climate forcing scenarios for near and far timescales. They show that kelp regional differences in the susceptibility to MHWs into the future, but also that because the vast majority of kelp forests are not highly protected there will be reduced resilience to MHWs. Given the important role that kelp forests play in supporting elevated levels of diversity as well as providing extensive ecosystem services the impacts of increased MHWs and the lack of protection will have wide ranging consequences. I think this is a timely paper which brings together extensive datasets to undertake the analyses presented. I think that in places the text needs to be toned down and caveated (see specific comments below), but the conclusions are generally sound and the call for better protection for kelp forests is welcomed.\nSpecific comments\nLine 50 I don’t think there is evidence that climate change poses the greatest risk and your analysis suggests some area may be more at risk from others. I would rephrase.\nLine 51I would rephrase the last part of this sentence as there is quite a lot of literature on the future threats to kelp and at least in a regional context and understanding of the current conservation status (or lack thereof). Suggest rephrasing\nAbstract – Suggest adding floating to all discussion of kelp in the abstract as it suggests as written that the outputs are extrapolated to all kelps, which isn’t the case and also not possible with this analysis.\nLine 58 “the most effective conservation measure for providing climate resilience” I find this phrase a little difficult throughout. Yes highly restrictive marine protected areas that stop habitat loss and ensure intact predators in kelp systems controlled by grazers are likely to increase climate resilience, but if the chief driver of kelp loss is say invasive species/ eutrophication (as it is in some parts of the world) this type of MPA would add little to make kelp forests for resilient to climate change. I would suggest using more nuanced language.\nLines 71-72 I suggest rephrasing this passage as we generally have a good global understanding of kelp forests distributions and an understanding of responses to climate change and MHWs from in-situ studies and experiments. I agree at regional scales kelp forest extent can be much more poorly resolved.\nLine 74 Productivity is more similar to broadleaf forests than rainforests or coral reefs. See Pessarrodona et al (2022) Science Advances eabn2465 – Fig 1a\nLine 76-77 I am not convinced by this line “Kelp forests are among the marine ecosystems at greatest risk from MHWs”. Surely this depends on where the MHW occurs in the species range and I would suggest that there is equal evidence that other species and ecosystems (e.g. coral reefs) are equally or more at risk. I suggest toning the language.\nLine 78 I am not suggesting that MHWs haven't contributed to the loss of kelp in Tasmania, but the key driver is expansion of urchins (as a result of decadal scale warming) and over fishing of large rock lobste. Suggest altering the language or removing Tasmania as an example of MHW driven kelp loss.\nLine 106 I know that you have defined above that by kelp forests you mean canopy-forming forests, but in places throughout the ms I think it would be useful to emphasise that the ms is only referring to canopy-forming forests and I think this is one of those instances.\nLine 136 Should be Fib 1b and 2 referred to here\nLine 145 Suggest adding temperate South Africa here as they look very similar to temperate South America.\nFigure 2b I suggest using the same labelling system for the SSP as in the legend i.e. the – after SSP. Also I think that the SSPs are incorrectly labelled. At present SSP – 1.26 has the most extreme cumulative MHWs. Finally I suggest changing the colouring for contemporary and (currently labelled) SSP – 5.85.\nLine 173 Suggest emphasising floating kelp forests here\nLine 186 It might be worth some discussion in the ms about whether these are real or paper HPMAs, particularly in the Southern Ocean, given their isolation (maybe some discussion of this in the section beginning line 304). Also what does this 99% protection represent in terms of global kelp? If 99% of small amount of kelp forest this doesn't really mean anything. Might be worth including this information for context.\nFigure 3 may be worth also including the island/islands that are protectorates of France rather than just stating France. Maybe consider the same for UK protectorates although these do come out in the text.\nUsing level of fishing protection is only a good measure of protection if grazing is a key pressure and more intact fish stocks reduce this or that destructive fishing gears are removed which reduces habitat loss. Where grazing by urchins may not be a key driver these HPMPAs may not increase resilience. Is there confidence that grazing is a key driver across all these regions? It isn’t in all kelp forests and if there isn’t confidence then I think that this needs caveating in the ms – maybe in the methods, but potentially also in the main ms (maybe around lines 245-246?).\nThe identification of refugia is made in the abstract and line 277, but I am somewhat sceptical that areas of lower MHW impact will able to act as a refugia given the distance between the most and least impacted areas. The identified refugia are in the Southern Ocean while the most impacted are in the Pacific Arctic. Macrocystis has greater dispersal ability than many kelps, but this seems to be stretching the resupply from refugia further than perhaps it should go. I believe this needs some discussion and toning down.\nLines 292-295 It is stated that there will be some overlap with floating and subsurface kelp and therefore the outcomes may be similar for kelp in these regions. I agree, but I think it is worth stating that whole ocean basins e.g. NE Pacific and N Atlantic with kelp present are not represented at all as they don’t possess floating kelp.\nLine 366 It isn’t clear to me what the filters and masks are in Supplementary Table 1\nLine 368-369 Given the evidence for regional losses of kelp over the last 20-30 years I am a little sceptical of using Landsat images back to 1984. What if all the detections were early in the dataset and there have none in the later years? Was the potential for range reductions in kelp forests over the time-series considered? If it wasn’t I suggest that some sort of temporal analysis of this should take place before using data back to 1984.\nLine 373 I suggest that you make the length and years of the baseline period clear here. It is mentioned below, but it would be best to mention when first stated.\nLine 398 It isn’t clear to me why a different base line would be used for the reference period and the climatology. When not use the same period?\nLines 416-419 I think it would be helpful what the different categories mean in terms of restrictions and LFP."}, "author_response": "RESPONSE TO REVIEWERS\nReviewer 1 and 2\nGeneral Comment Reviewer 1\nThis review was jointly done by an ECR and mid-career researcher. This study combined maps\nof floating kelp forests, environmental data on future marine heatwaves under different climate\nscenarios, and MPA spatial data to identify the amount of kelp forests most vulnerable to MHWs\nand areas that are currently protected. This is important information in light of the 30X30 goals.\nOverall, we enjoyed reading the paper and found the use of maps and figures informative and\nthe code and data availability suitable. The topic of kelp and MHWs is timely and the research\ntopic should be of interest to a broad readership. However, we do have some concerns with the\nconceptual underpinnings of the research question, and the presentation of the results, which\nshould be addressed.\nGeneral Comment Reviewer 2\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse General Comment Reviewer 1 and 2\nThank you so much for your valuable review, which has improved our manuscript.\nResponse General Comment\nWe appreciate the reviewer’s encouraging comments and that they enjoyed reading our\nmanuscript and found our work important, timely and relevant.\nMain comment 1\nThe authors emphasize the use of MPAs as the “most effective” conservation measure for\nsafeguarding kelp, stating they can provide “climate resilience” or “enhance their adaptability”.\nHowever, there is very limited evidence that MPAs increase the resilience of kelp forests to\nwarming temperatures or increase how they can adapt to extreme events. The only documented\ncase of MPAs buffering MHW impacts is the example given on line 270, which may be the\nexception and not the rule. MPAs are not always effective at mitigating MHW impacts because\none of the main mechanisms which MHWs impact kelp forests is through direct mortality of kelp\nby surpassing temperature thresholds (Filbee-Dexter et al. 2021, Thomsen et al. 2021, Smale\n2020). MPAs can do little to nothing against this lethal stress. MPAs may help kelps better\nsurvive non-lethal warming temperatures by reducing other stressors, yet in situ evidence of this\nmechanism is scarce. This was overviewed by Bates et al.’s ‘protection paradox’ study. In\ngeneral, MPAs can potentially prevent kelp loss in areas where the MHW causes kelp loss due\nto indirect drivers that can be managed to some extent by top-down control. For example, in\nsome regions where they are overgrazed by sea urchins (Tasmania, North California). Yet, even\nin areas where this link is best established such as New Zealand in the Leigh Reserve, there is\n1\nno overall relationship between kelp cover and protection level at broader scales across multiple\nreserves in NZL (and this does not even include MHW stress). Further, MPAs do not help\nrebuild all urchin predators. Kelp forests in the North Pacific are impacted by otter loss, and\nMPAs are not a typical tool to rebuild otter populations. Same can be said for large groundfish in\nthe North Atlantic. The limitation of MPAs against this heatwave driver should be better explored\nand addressed much earlier in the study.\nResponse main comment 1\nDone. We thank the reviewers for commenting on our statement that MPAs are the “most\neffective” conservation measure to adapt to climate change. We have modified the\nabstract to ensure we clarify that highly restrictive MPAs are the most effective MPAs.\nOrange text is added/modified text, applies throughout.\nNow Lines 59-61: “less than 3% of global kelp forests are currently within highly\nrestrictive marine protected areas (MPAs), the most effective MPAs for protecting\nbiodiversity.”\nDone: We apologize if our framing was missing further detail. We agree (discussed in\nparagraph 3 of the discussion) with the reviewers that MPAs have limited capacity to\ndirectly mitigate climate change impacts that surpass physiological thresholds. We have\nadded text to clarify when MPAs may promote resilience. For example, MPAs can support\nthe recovery of key species for ecosystem functioning and stability, which can enhance\nthe capacity of ecosystems to resist and recover from MHWs by reducing other stressors\nbesides temperature. To address the reviewer's comments, in the introduction, we have\npointed out limitations and expanded the benefits that MPAs can provide to kelp forest\necosystems, beyond climate resilience. We also reference studies reporting evidence\nthat the trophic cascade mechanism can promote resilience when urchin predators are\nprotected from fishing in MPAs, from different geographies (Filbee-Dexter et al. 2024,\nKumagai et al. 2024, Ling et al. 2009, Peleg et al. 2023). In addition, we have added more\nexamples that empirically found that MPAs facilitate resilience among kelp forest\necosystems through other mechanisms (Micheli et al. 2012, Munguia-Vega et al. 2015,\nZiegler et al. 2023, Benedetti-Cecchi et al. 2024, Olguin-Jacobson et al. 2024) and\nprovided details of the only study that comprehensively evaluated whether a network of\nMPAs provides resilience to MHWs through trophic cascades (Kumagai et al. 2024).\nThis additional review references and provide a more comprehensive overview of this\ntopic and we thank the reviewers for raising this issue.\nNow Lines 86-101 in the Introduction: “While MPAs cannot directly counter the impacts\nof climate change that can surpass a species’ physiological tolerance27, MPAs can\nmitigate non-climatic stressors like overfishing and habitat destruction, which can\nenhance ecosystem resilience27, supporting ecological functioning and providing\nsocietal benefits28-30.\nWell-managed and highly restrictive MPAs —no-take marine reserves, where all fishing\nactivities are prohibited— are the most effective type of MPA for rebuilding fished\n2\npopulations28, supporting the stability of kelp forest ecosystems31, and, in some\ndocumented cases, providing their resilience to MHW impacts32-35. For example, in\nregions where urchin predators are protected from fishing and where trophic cascades\nare a driver of food-web dynamics, MPAs can facilitate the recovery of higher-trophic-\nlevel organisms, which helps control kelp grazer populations and prevent overgrazing of\nkelp36-38. This mechanism has been found to support resistance to and recovery from\nMHWs for kelp forests in a network of 39 MPAs in southern California38. In addition, MPAs\ncan facilitate climate resilience for kelp forest ecosystems through other mechanisms32-\n34,39, 40. For example, a recent global analysis found that fish communities were more\nstable in the face of MHWs in highly restrictive MPAs than unprotected sites33, and\nabalone populations in two MPAs in Mexico were more resilient to hypoxia and MHWs\nbecause of their larger body size and greater reproductive output33,40.”\nWe want to clarify that our work evaluates the protection of kelp forests as a habitat that\nstructures an ecosystem and not as a single species. This aligns with general\nconservation principles of habitat representation as biodiversity indicators (Margules &\nPressley 2000), which links with the global targets of protecting 30% of habitats by 2030.\nThis is clearly stated in the opening paragraph (Lines 69-70), and in the discussion (Lines\n276-281), where, when discussing threats and protection, we refer to “kelp forest\necosystems”. We have added more detail in other sections to avoid confusion, including\nthe word “representation” in the title. This is addressed in following response (see\nanswer Main Comment 3, Page 5).\nTitle\n“Global floating kelp forests have limited representation in marine protected areas\ndespite rising threats from marine heatwaves”\nIntroduction\nNow Lines 125-126: “global map of floating kelp forest habitats (henceforth “kelp\nforests”)”\nResults\nNow Lines 143: “We found floating kelp forest habitats in …”\nNow Lines 156: “The exposure of kelp forest habitats to …”\nNow Lines 207: “Globally, more than 33.1% of floating kelp forest habitats are”\nDiscussion\nNow Lines 274: “global map of the protection status of floating kelp forest habitats”\nMain comment 2\nThe study implies that MHWs impact equally along a kelp species range. Yet, MHW impacts are\nlikely more severe for kelp forests at the canopy species’ warm range edge, compared to cool\nrange edges. This is mentioned on line 260 as a bit of a throwaway discussion comment - but\nthis is essential to determine vulnerability. The presentation of the vulnerability results treat\n3\nwarm and cool range edges as if they are equally vulnerable to MHWs. For example, the study\nidentifies the Bering Sea, the Gulf of Alaska, the North American Fjordlands and the Puget\nTrough as vulnerable areas for kelp forests. Yet, the kelp species in these Arctic and sub-Arctic\nareas are predominantly at the cool range edge of their thermal tolerance. This means that\nincreased sea temperatures of a few degrees from a MHW likely will not cause direct mortality\nand could even make temperatures temporarily more suitable for these species. It is more likely\nthat kelps in California and northern Chile are more vulnerable.\nResponse Main comment 2\nDone. We agree with the reviewer's comment that MHWs will not impact equally along a\nspecies range. We thank the reviewer for this comment, which allowed us to expand the\ntext to clarify these important points further. We want to clarify that our work does not\nassess the vulnerability of kelps to future MHWs but exposure, which is a measure of\npotential threat rather than species vulnerability. Assessing vulnerability would require a\ndifferent study that considers the susceptibility of kelp populations to MHW threats\nconsidering biotic and abiotic factors that drive species’ local adaptation and range shifts\n(see recent review in Nature Reviews Earth & Environment, Lawler et al. 2024). This is\nimportant because, for example, studies found that some of the most resilient kelp forests\nin the California Current following MHWs are in the species warm-distribution limit in Baja\nCalifornia (see Cavanaugh et al. 2019 and Bell et al. 2023). These kelp forests have adapted\nto higher temperatures and large anomalies (temperatures peak to >25°C during MHWs).\nWe have modified the discussion paragraph to better frame and explain the important\npoints raised by the reviewer.\nNow Lines 296-306 in the Discussion: Note that our study assessed the exposure of kelp\nforests to projected MHWs, not their vulnerability to MHWs. While kelp forests near their\ncurrent warm distribution limit will likely be the most affected and therefore subject to\nrange contractions15,56,59,60, populations living further from their thermal tolerance limit\nmay be less threatened, and novel climates may favor kelp expansion56,57. However, cold-\nrange populations subject to extreme MHWs could also be threatened, as they may be\nless adapted to extreme temperatures61. Moreover, the persistence of kelp forest\necosystems is not driven solely by available substratum and suitable temperatures but\nalso by biotic interactions62, and not all components of these ecosystems will\nnecessarily respond in the same way. Predicting whether MPAs can provide resilience to\nkelp forest ecosystems under such extreme and persistent changes is therefore\nchallenging on multiple fronts.\nWe have also reduced specific discussion of the cold-edge populations to ensure that\ncomments are more general for North America (the continent with the highest projected\nMHW threat), including for warm-edge distribution limits.\nNow Lines 319-323 in the Discussion: “Because the magnitude of future warming may\ncause the loss of kelp forests in some regions, MPAs will likely not be enough on their\nown to support the persistence of kelp forests. In these cases, supplementary climate-\n4\nadaptation strategies will be necessary, particularly for areas of high exposure to future\nMHWs, such as regions in North America and especially areas near-warm distribution\nlimits”.\nIn addition, to avoid confusion, we have added the following detail regarding our\nanalyses.\nNow Line 436-438 in the methods: “Note that cumulative MHW intensities are an indicator\nof exposure22,25, but they are not a measure of the vulnerability of species or ecosystems\nto MHWs.”\nMain comment 3\nThe authors discuss that their analysis excludes many kelp species (line 288) and only includes\nsurface-forming kelps. The fact that this paper shows the distribution and lack of protection for\nonly three species of kelp out of >100 Laminarian kelp should be emphasised. Kelp forests\nmade by subsurface species will overlap this distribution and are found along 30% of the worlds\ncoasts. This is not a global analysis - but an analysis of three widespread canopy kelp species.\nThe title of this paper therefore cannot include the world ‘global’ as it is misleading.\nResponse Main comment 3\nDone. We agree with the reviewer that our work focus on the detection of floating kelp\nforests globally, which are composed of a few species. However, these species are the\nonly ones that can currently be comprehensively mapped (i.e., at large spatial and\ntemporal scales), and they are globally distributed across all continents. Floating kelps\nco-exist with many other kelp species and structure one of Earth’s most biodiverse\necosystems. Therefore, maps of the presence of floating kelp forest canopies is an\nexcellent indicator of the presence of floating kelp forest ecosystems that include many\nother kelp species, and hundreds to thousands of species of seaweeds, fish, and\ninvertebrates, none of which can be mapped remotely.\nWe thank the reviewer's comments, which allowed us to clarify that our work is not\nsingle-species focused but ecosystem-focused (as mentioned in early comments) and\nbetter explain why floating kelp forest maps are excellent indicators of the presence of\nkelp forest ecosystems and associated species. We also changed the title to clarify that\nwe are refering to globally distributed floating kelp.\nNow Lines 105-118 in the Introduction: “Recent advances in satellite imaging of surface-\ncanopy-forming kelp species provide an opportunity to map the distribution of kelp\nforest habitats31, quantify the threats posed by MHWs, and assess their protection status.\nFloating kelp forests are a globally distributed foundation species that co-exist with other\nsub-canopy kelp species that structure one of Earth’s most biodiverse ecosystems12.\nThese forests can cover thousands of hectares (e.g., 28,500 hectares in the Southern\nCalifornia Bight ecoregion42) and sustain hundreds to thousands of species, some of\nwhich are economically and culturally significant. For example, in the Channel Islands in\necosystems43 and in Patagonia, Chile and Argentina, similar studies found between 150-\n250 species44-46. In addition, in the northeast Pacific Ocean, 17 species of sub-canopy\nkelp coexist with floating kelp forests (www.algaebase.org/) from Alaska to Baja\nCalifornia, Mexico. Since remote sensing is the only available method to detect kelp\nforest ecosystems comprehensively (i.e., using standarized methods at large temporal\nand spatial scales), maps of floating kelps are good indicators of the broader ecosystem\nand associated biodiversity.\nMain comment 4\nFinally, the paper often refers to the amount of protection in percentages, it would be more\neffective to also mention the area/ scale of unprotected kelp forests here. The authors show kelp\narea per ecoregion, but that can be better communicated in text.\nResponse Main comment 4\nDone. We have added some more detail of the coverage of kelp in the text.\nResults:\nNow Lines 210-211: “the Southern Ocean realm (24,319.8 ha), and when excluding these\nareas, only 2.8% (5,870.9 ha) of the global kelp forests are highly protected…”\nNow Lines 215-219: “However, this protection is in their overseas territories in remote\nislands for all of France (23,007.1 ha, there is no floating kelp forests in mainland France)\nand much of New Zealand (145.1 ha), South Africa (285.1 ha), and Australia (78.7 ha).\nAustralia has only 2.7% (23.0 ha), New Zealand 2.0% (14.1 ha), and South Africa 8.9%\n(400.6 ha) of their continental kelp forests highly protected.”\nSpecific comments\nSee more specific comments/ suggestions by line.\nLine 55: Replace “for all forests” with “across all regions”.\nLine 55: The use of the word 'emerge' seems misguided here, we suggest the end of this\nsentence could read \"...some kelp forests in the southern hemisphere may be protected within\nclimate refugia\".\nLine 71: Add “and extreme events” after “threats associated with climate change”.\nLine 84: Change “trophic-level” to “trophic level organisms”.\nDone\nLine 87: For scale/ context, the authors should consider providing the average canopy extent for\neach of these species in a known area.\nallow distinguishing among species. We have provided one example that is published for\ngiant kelp in the Southern California Bight ecoregion, as an example.\nNow Lines 109-110 in Introduction: “These forests can cover thousands of hectares (e.g.,\n28,500 hectares in the Southern California Bight ecoregion42)”\nLine 89: Reference needed after “threats posed by MHWs”.\nLine 90: Reference needed after “protection status”.\nDone. Moved (Cavanaugh et al. 2021) after “protection status”. This reference is a review\nof the use of remote sensing for floating kelp forest mapping, threats and management .\nNow Line 107.\nLine 90: Identifying climate refugia alone is not a climate adaptation strategy. Instead, that\ninformation needs to be considered or applied in the context of conservation management as\nindicated below, there is need to reframe this sentence.\nDone. Now Line 119: “strategies such as identifying and protecting climate refugia…”\nLine 93: After “can persist” change the sentence to read “...and the resilience of other kelp\nforests may be enhanced, depending on local and regional connectivity and species life-history\ntraits, by maintaining a source of recovery for impacted kelp habitats.”\nDone. Now Lines 121-123: “biodiversity can persist47 and may enhance the resilience of\nother kelp forests, depending on local and regional connectivity and live history-traits…”\nLine 117: Change “while” to “while the remaining”.\nDone\nLine 118: Change “forests” to “forests, combined”.\nDone. Now Lines 147-148: “while the remaining 17 ecoregions combined account for only\n1% of the distribution of kelp forests (Supplementary Fig. 1).”\nLine 119: Add “, overall,” after “highest latitudes” to keep consistent with the following.\nLines 121-122: Remove “at their warm distribution limit” and change “to their highest latitudes\nin” to “southwards to their cool-distribution limit in”.\nLine 123: Change “include” to “are located in”.\nLine 144: Add “realms” after “Pacific”.\nLines 150-151: Change the sentence to read “Specifically, projections suggest a latitudinal\npattern of increasing exposure to future MHWs from lower to higher latitudes in the northern\nhemisphere, whereas…”\nDone\nFigure 1: Why does the Arctic region (green) have a large coloured portion that does not match\nthe corresponding black line showing kelp distribution?\nThere is no detected floating kelp in parts of that region (like others, see South Africa)\nDone\nLine 181: So even though France meets the current 30% target, does this mean that all their\nkelp forests are located on remote islands and there are no protection measures on the\nmainland?\nDone. These means that all their floating kelp forest are protected and in remote islands.\nWe have further clarified in Results, Now Lines 215-217: “However, this protection is in\ntheir overseas territories in remote islands for all of France (23,007.1 ha, there is no\nfloating kelp forests in mainland France)”\nLine 183: Here, it would be more effective to also mention the scale of continental kelp forests\nthat are unprotected in these countries since it will be a very large area. Only reporting on\npercentages takes away from this point.\nDone. See previous response for Main Comment 4 (Page 6).\nLine 193: Add “Overall” before “only”.\nWe use the word overall in the previous sentence, it seems repetitive to use it again.\nLine 281: Change “threated” to “threatened”.\nDone. Although we think you mean line 218?\nLine 240: Change “country” to “nation” to keep consistent with the above.\nDone\nLine 262: provide some reasons or justification why it is challenging.:\nDone. See previous response to main comment 2 in page 4. Now Lines 301-306.\nLines 267-269: This point needs to be made clearer earlier in the paper for example, around\nlines 80-84.\nDone. Addressed in previous comments (Page 2). Now Lines 86-101 in the Introduction.\nLine 271: After “less biodiverse ecosystems” outline what species these systems were\npreviously dominated by, compared to how they look now after the MHW.\nDone. Now Lines 315-317: Studies in the Channel islands have shown that urchin barrens\nreduced sessile invertebrate diversity by 40 % and almost completely removed the\ncanopy fish assemblage43.\nLine 272: Remove “MPAs have prevented” and “have provided”. Replace “collapse” with\n“collapse was prevented”.\nDone. We have changed the sentence. Now Lines 317-319:\nMHWs by facilitating recovery of overfished predators that control urchin populations29,30\nLine 273: add was ‘shown’\nThe reviewer does not detail where we need to add “shown” in the sentence, and from\nour reading it is not clear where this would fit.\nLine 275: Replace “other” with “supplementary”.\nDone.\nLines 278-279: Consider “other anthropogenic impacts” both in the ocean and on land. What\nmanageable stressors are/ can be identified? This calls on local conservation managers to have\na solid understanding of compounding stressors, aside from warming and extreme events.\nDone. Thank you for the thought; we have added in parenthesis an example.\nNow Lines 325-326: “and managing other anthropogenic impacts (e.g., land-based\npollution) not mitigated by MPAs”.\nLine 288: The point that this analysis only includes surface canopy kelp is underemphasised\nthroughout the manuscript. While we understand the complexities and limitations of gathering\nreliable global datasets, could the authors expand on the extent of the surface-canopy kelp\ndistribution relative to known sub-canopy kelp distributions for key areas, for example, Australia,\nNew Zealand, Chile, South Africa, USA etc?\nDone. We have expended both the introduction and discussion and specified that the\nfloating kelp maps are indicators of the distribution of kelp forest ecosystems and\nprovided cases where our maps may underrepresent kelp forest ecosystems. See\nprevious response Main Comment 3, page 5.\nAnd Discussion Now Lines 335-349: “It is important to note that our analysis maps\nfloating kelp forests, thus our method will not detect other kelp forest ecosystems when\nthey do not co-occur with floating kelps. There are > 120 laminarian kelp species (many\nare intertidal), of which three of the largest kelp species form extensive floating canopies\nthat can be detected by remote sensing, including the globally distributed M. pyrifera.\nOur estimates likely represent overall kelp forest distribution and ecosystem protection\nin regions where floating kelps co-exist with other sub-canopy kelp species (e.g., the\nwest coast of North America, South America, and remote southern hemisphere islands,\namong others). However, some other nations and regions not included here have\nextensive subsurface kelp forest ecosystems which do not co-occur with floating kelp\nspecies (e.g., the east coast of North America, northern Europe, parts of Australia,\namong others). Given the limitations in detecting subsurface canopy kelp forests, they\nare likely less-well represented here than those detected by remote sensing. This is a\nsubstantial gap for kelp conservation and an avenue for novel technologies and\nresearch67 to address associated needs for these kelp ecosystems that do not overlap\nwith floating kelp, which also support diverse and productive ecosystems13,68 and human\nlivelihoods21.”\nDone\nLine 388: After “by 2100” outline what the relative warming would be, as above.\nDone. Now Lines 451-452: “radiative forcing reaching 8.5 W m-2 (and 4.4ºC of warming)\n…”\nLine 414: What are some examples of these extractive activities relative to each of the levels?\nDone\nNow Lines 478-479: We then categorized each MPA based on the level of restrictions to\nextractive activities (recreational, subsistance, and commercial fishing).\nAnd Now Lines 481-485: “This database scores MPAs based on fishing restrictions on a\nscale of 1–5 scale (1 = Least restricted: no known restriction to the removal of life, 2 =\nLess restricted: at least one species- or gear-specific restriction apply, 3 = Moderately\nrestricted: several species- or gear-specific restrictions apply, 4 = Heavily restricted:\nmarine life removal is mostly prohibited with a few exceptions, 5 = Most restricted:\nmarine life removal is prohibited).”\nLines 422-423: What are the country-level criteria for fishing restrictions? The authors also\nmention that they do not include other types of spatial closures and area-based measures that\nare not MPAs, by assumption this varies from country to country therefore. Some insight should\nbe provided, for example, does this include cultural or seasonal-based closures? MPAs are not\nall created equal, and this is an important point to be clear on.\nDone. We have explained that we followed the same approach as Protected Seas for those\nmissing MPAs from their dataset.\nSee Now Lines 490-492: “For a few MPAs (34 of 817) that had no LFP score, we reviewed\nexisting information on restrictions on the removal of life and assigned a LFP score\nfollowing the same approach described earlier.”\nFor the non-MPAs, we have better framed the text to avoid confusion. We download MPA\nshapefiles from national agencies' datasets (see Lines 476 and therefore, the category is\nbased on what nations report as an MPA. For that instance, we did not include other\npolygons not categorized as MPAs in the country database.\nNow Lines 489-490: “We did not include other area-based measures not categorized as an\nMPA in the national dataset (e.g., fishery management areas).”\nGeneral Comment\nThis work focuses on exposure of global floating kelp forests to marine heatwaves and analyze\nthe spatial distribution of kelp forests with respect to the marine protected areas. Although\ninteresting results are presented, major concerns exist, epically in terms of the soundness of\nmethodology and results.\nResponse General Comment\nWe thank the reviewer for finding our results interesting and for the concerns they\nraised, which allowed us to improve the manuscript and clarify parts of the text.\nComment 1\nResult completeness. As only the floating (surface canopy) kelp forests can be detected from\nsatellite image, and only 3 of the largest kelp species form floating canopies out of over 120 kelp\nspecies, it is not accurate to say that the presented results from satellite observations here are\n“global kelp forests”, as subsurface kelp forests have not been reasonably accounted for.\nResponse Comment 1\nDone: This point was also mentioned by Reviewer 1, and we apologize for the confusion\nand for not explaining these points better. We have now made changes to specify that we\nrefer to floating kelp forests, and that the presence of floating kelp habitat, which co-\nexists with sub-canopy kelps and 100s-1000s of other species (seaweeds, fish,\ninvertebrates) is an indicator of the presence of kelp forest ecosystems, not only a\nspecies. Now Lines 105-118 Introduction. We have also expanded the discussion about\nwhich areas our maps better represent kelp forest ecosystems and what the gaps are.\nNow Lines 335-349 . See Response to Reviewer 1 in page 5 and 9.\nComment 2\nClassification of floating kelp canopy was derived by applying a globally robust random forest\nclassifier, and the global map was only validated using ground observations at 14 sites in South\nAmerica. It is largely unknown how the classifier works elsewhere and if the classification\naccuracy is good enough to support the analysis presented here.\nResponse Comment 2\nDone. We thank the reviewer for their comment, which allowed us to provide more detail\non the datasets used, thereby avoiding future confusion. We apologise as we realized the\nmethods poorly explained the kelp maps datasets and missed important details\nregarding the validation of the global map. The methods sections was restructured many\ntimes during internal revisions, and now we realized some of the text could be improved.\nWe have carefully revised this section and improved the text to be more accurate and to\nremove confusion associated with global classifiers, availability of regional maps, and\nlimitations of the global map. In addition, we now clarify how we gathered the data to\nintegrate the global map.\nregionally validated remote-sensed maps to detect floating kelp forests. These maps\nhave been published or uploaded to Kelp Watch following the methods of Bell et al.\n(2023) (kelpwatch.org) and used similar approaches based on Landsat sensors. For\nthose regions where regional maps were unavailable, we used an existing published\nglobal map (Mora-Sota et al. 2020) for floating kelps. We apologize for omitting this detail\nin the original manuscript, but the authors of this global map first trained their data\nacross eight countries across four continents with existing kelp datasets, then validated\ntheir mapping in situ across 14 sites from Chile to the Falkland Islands that span 20° of\nlatitude. Finally, they validated their map with previous surveys or observed data at 157\nlocations in South and North America, New Zealand, Southern Australia and Tasmania,\nSouth Georgia, and sub-Antarctic islands (Mora-Soto et al. 2020).\nNow Lines in 414-419 Methods: This dataset was validated across 14 in situ sites in\nSouth America that cover a variety of coastlines and ecoregions, and with existing data\nat 151 locations that cover four continents50. To ameliorate some potential detection\ncaveats, we excluded pixels that fell within a 30-m buffer relative to the coastline because\nthe global map does not distinguish between intertidal green algae and floating kelp\nforests and estuaries can also be a source of false positives. See Supplementary Table 1\nfor the coastlines used to apply the 30-m buffer. “\nComment 3\nResults consistency. As Sentinel-2 satellite imagery (2015-2019) was adopted to supplement\nLandsat observations (1984-98 present), how to guarantee the consistency (or comparability)\nbetween results from different satellite data, especially in terms of different band configuration\nand algorithms used to kelp forest?\nResponse Comment 3\nBoth datasets (Sentinel-2 and Landsat) have similar spectral coverage, and since kelp\ncanopy has such a different spectral signature than water (especially in the near\ninfrared), the classification algorithms should produce comparable results for a\npresence-absence classification. Most importantly, both datasets have been\ncomprehensively validated (see response to above comment). The difference in temporal\ncoverage of the datasets may lead to regional differences and we have acknowledged\nand clarified this in the text. Specifically, the Sentinel-2 dataset produces a composite\nover a shorter time period and so may represent an underestimate of potential kelp\nhabitat as compared to Landsat. However, this would only affect our absolute estimates\nof kelp areas, it should not affect the percent of kelp forests inside/outside MPAs within a\nregion, as the same dataset was used for the entire region and the percentage is relative\nto the coverage.\nunderestimate floating kelp habitat for those regions where regional maps are not yet\navailable (e.g., Canada, Chile, New Zealand) because the global map covers a shorter\ntime period than the regional Landsat data and so may miss kelp habitat that was not\npresent between 2015-201950”\nComment 4\nConclusion soundness. A key conclusion in Abstract is that “…exposure to projected marine\nheatwaves will increase ~8 times compared to contemporary (2001-2020) exposure for\nintermediate climate scenarios”. But I didn’t find the corresponding statement in the main text,\nespecially in terms of “the 8 times”. How was it defined and calculated?\nResponse Comment 4\nDone. We have better explained how we obtained those values in the Results. And\nupdated in the abstract, to include the range of scenarios.\nNow Lines 173-177 in Results: “In the long term, even under SSP1-2.6 and SSP2-4.5,\nthese magnitudes are ~5.6 and ~9.6 times higher than contemporary exposure,\nrespectively. Note that these estimates where derived from the mean cumulative MHW\nintensities (n = 2156 pixels) for each climate scenario and time frame, and then divided\nby the corresponding mean contemporary values.”\nAnd Lines 55-56 in Abstract: projected marine heatwaves will increase >6 to >16 times in\nthe long term (2081-2100) compared to contemporary (2001-2020) exposure.\nComment 5\nTitle is not correct and logical. The fact is that intensifying marine heatwaves and anthropogenic\nactivities may threaten global kelp forests, but kelp forests cannot be threatened by protection\nbehavior even if the protection is limited.\nResponse Comment 5\nDone. We have changed the title to reflect the reviewers thought.\nNew Title: “Global floating kelp forests have limited representation in marine protected\nareas despite rising threats from marine heatwaves”\nComment 5\nIn captions of Figure 2 it read SSP-1.26, SSP-2.45, SSP-5.85, whereas in the figure it read\ndifferent as SSP1-2.6, SSP2-4.5, SSP5-8.5. This issue also exists in the methods parts.\nResponse Comment 5\nThank you so much, Reviewer 3 also pointed out this error and we have addressed.\nGeneral Comment\nThis ms uses remotely sensed data to quantify the distribution of canopy-forming kelp across\nthe world’s oceans before overlaying that with MPA designations under different levels of\nfisheries restrictions. They then calculate cumulative MHW statistics for contemporary times and\nunder different climate forcing scenarios for near and far timescales. They show that kelp\nregional differences in the susceptibility to MHWs into the future, but also that because the vast\nmajority of kelp forests are not highly protected there will be reduced resilience to MHWs. Given\nthe important role that kelp forests play in supporting elevated levels of diversity as well as\nproviding extensive ecosystem services the impacts of increased MHWs and the lack of\nprotection will have wide ranging consequences. I think this is a timely paper which brings\ntogether extensive datasets to undertake the analyses presented. I think that in places the text\nneeds to be toned down and caveated (see specific comments below), but the conclusions are\ngenerally sound and the call for better protection for kelp forests is welcomed.\nResponse General Comment\nThank you so much for finding our work timely, complete and that the conclusions are\nsound and relevant for kelp forest conservation\nSpecific comments\nLine 50 I don’t think there is evidence that climate change poses the greatest risk and your\nanalysis suggests some area may be more at risk from others. I would rephrase.\nDone. We have slightly modified the abstract and introduction to ensure its clear. The\nstatement is based on the IPPC Sixth Assessment Report that identified kelp forests as\nthe second most at risk ecosystem from MHWs, after corals.\nNow Lines 52 in Abstract: “ecosystems and are at great risk from climate change”\nNow Lines 79-80 in Introduction: “The IPPC Sixth Assessment Report identified kelp\nforests as the second most at-risk marine ecosystems from MHWs6… “\nLine 51I would rephrase the last part of this sentence as there is quite a lot of literature on the\nfuture threats to kelp and at least in a regional context and understanding of the current\nconservation status (or lack thereof). Suggest rephrasing\nDone. We have added “global” before “future threats and conservation status” to ensure\nits clear that we refer to their threats and conservation globally, not regionally.\nAbstract – Suggest adding floating to all discussion of kelp in the abstract as it suggests as\nwritten that the outputs are extrapolated to all kelps, which isn’t the case and also not possible\nwith this analysis.\nDone\nLine 58 “the most effective conservation measure for providing climate resilience” I find this\n14\nphrase a little difficult throughout. Yes highly restrictive marine protected areas that stop habitat\nloss and ensure intact predators in kelp systems controlled by grazers are likely to increase\nclimate resilience, but if the chief driver of kelp loss is say invasive species/ eutrophication (as it\nis in some parts of the world) this type of MPA would add little to make kelp forests for resilient\nto climate change. I would suggest using more nuanced language.\nDone. Thank you. This point was also raised by reviewer 1. Given the limited space in the\nabstract to explain when MPAs may provide resilience, we have changed this sentence to\nread more general regarding the efficiency of fully protected MPAs for biodiversity\nprotection. See Response to Reviewer 1, page 2.\nLines 71-72 I suggest rephrasing this passage as we generally have a good global\nunderstanding of kelp forests distributions and an understanding of responses to climate change\nand MHWs from in-situ studies and experiments. I agree at regional scales kelp forest extent\ncan be much more poorly resolved.\nDone. Thank you for your comment, which allowed us to clarify our point. Here, we mean\n“spatial data”. That means accurate and comprehensive maps of kelp distribution, from\nwhich we can then conduct spatial analyses of threats, protection, and so on.\nNow Lines 43: “Comprehensive maps on kelp forest distribution….”\nLine 74 Productivity is more similar to broadleaf forests than rainforests or coral reefs. See\nPessarrodona et al (2022) Science Advances eabn2465 – Fig 1a.\nDone, we have changed the sentence to say” comparable to terrestrial forests” and cited\npessarrodona et al. 2022. Now Line 77.\nLine 76-77 I am not convinced by this line “Kelp forests are among the marine ecosystems at\ngreatest risk from MHWs”. Surely this depends on where the MHW occurs in the species range\nand I would suggest that there is equal evidence that other species and ecosystems (e.g. coral\nreefs) are equally or more at risk. I suggest toning the language.\nDone. See previous comment in page 14. Here we cite the six IPPC report.\nSee Now Lines xx\nLine 78 I am not suggesting that MHWs haven't contributed to the loss of kelp in Tasmania, but\nthe key driver is expansion of urchins (as a result of decadal scale warming) and over fishing of\nlarge rock lobste. Suggest altering the language or removing Tasmania as an example of MHW\ndriven kelp loss.\nDone. We have modified the text and excluded Tasmania from the example, as suggested\nby the reviewer. Now Lines 83-84: “For example, northern California has lost >90% of its\nkelp forests due to the combined effects of severe marine heatwaves and overgrazing by\nsea urchins23,24.\nLine 106 I know that you have defined above that by kelp forests you mean canopy-forming\n15\nforests, but in places throughout the ms I think it would be useful to emphasise that the ms is\nonly referring to canopy-forming forests and I think this is one of those instances.\nDone\nLine 136 Should be Fib 1b and 2 referred to here\nDone\nLine 145 Suggest adding temperate South Africa here as they look very similar to temperate\nSouth America.\nDone\nFigure 2b I suggest using the same labelling system for the SSP as in the legend i.e. the – after\nSSP. Also I think that the SSPs are incorrectly labelled. At present SSP – 1.26 has the most\nextreme cumulative MHWs. Finally I suggest changing the colouring for contemporary and\n(currently labelled) SSP – 5.85.\nDone. Thank you for catching the mislabeling for the SSP in the legend text, we have now\nchanged it to be the same as in the figure (e.g., SSP1-2.6). In addition, we have updated\nFigure 2b and changed the mislabeled SSP1-2.6 and SSP5-8.5. We use IPPCC standard\ncolors for labelling SSPs, therefore we prefer to stick with these colors.\nLine 173 Suggest emphasising floating kelp forests here\nDone\nLine 186 It might be worth some discussion in the ms about whether these are real or paper\nHPMAs, particularly in the Southern Ocean, given their isolation (maybe some discussion of this\nin the section beginning line 304). Also what does this 99% protection represent in terms of\nglobal kelp? If 99% of small amount of kelp forest this doesn't really mean anything. Might be\nworth including this information for context.\nDone. The discussion about the Southern Ocean realm (not the Southern Ocean) being\nisolated and likely less threatened by human activities (Paper parks are places not well\nmanaged or regulated, not isolated or less threatened) is touched on and referenced in\nthe first paragraph of the Discussion (Lines 276-278). However, we add a bit more detail\nregarding the effectiveness of MPAs.\nNow Lines 364-367 in the discussion: Therefore, including such information will likely\ndecrease the coverage of kelp forests within MPAs with high levels of effective protection\nbecause many lack effective governance, enforcement, or community involvement70.”\nWe have added this text to clarify how much that 99% represents globally.\nNow Lines 223-224 in Results: “has 99.9% of its kelp forests within highly protected\nMPAs (which represents 11% of the global distribution, see Fig. 3c, Fig. 4).”\nAlso see: Figure 3c, (protection decreases from 13.7% to 2.8% when excluding these\nareas), described in Results (Lines 209-212) and discussion (Lines 276-282). In addition,\nkelp.\nFigure 3 may be worth also including the island/islands that are protectorates of France rather\nthan just stating France. Maybe consider the same for UK protectorates although these do\ncome out in the text.\nDone. We have added text.\nLegend Fig 3, Lines 236: France (Kerguelen and Crozet Islands)\nLegend Fig 4, Lines 247-249: “Note that mainland France and the United Kingdom have\nno floating kelp forests in their mainland and are all in their overseas territories in the\nSouthern Ocean realm.”\nUsing level of fishing protection is only a good measure of protection if grazing is a key pressure\nand more intact fish stocks reduce this or that destructive fishing gears are removed which\nreduces habitat loss. Where grazing by urchins may not be a key driver these HPMPAs may not\nincrease resilience. Is there confidence that grazing is a key driver across all these regions? It\nisn’t in all kelp forests and if there isn’t confidence then I think that this needs caveating in the\nms – maybe in the methods, but potentially also in the main ms (maybe around lines 245-246?).\nDone. Great point, that has been brought up by Reviewer 1 and has helped us clarify and\nimprove the manuscript. Please see response to Reviewer 1, page 2, and Lines now 86-\n101.\nThe identification of refugia is made in the abstract and line 277, but I am somewhat sceptical\nthat areas of lower MHW impact will able to act as a refugia given the distance between the\nmost and least impacted areas. The identified refugia are in the Southern Ocean while the most\nimpacted are in the Pacific Arctic. Macrocystis has greater dispersal ability than many kelps, but\nthis seems to be stretching the resupply from refugia further than perhaps it should go. I believe\nthis needs some discussion and toning down.\nDone. We are not suggesting that these areas will be climate refugia to support recovery\nof global floating kelp populations which are 1000-10,000 kms apart. Our analyses\nidentify areas less exposed to future MHWs, and thus may provide climate refugia for\nthose regions. Following comments by Reviewer 1, we have modified parts of the\nabstract and introduction to clarify.\nNow Lines 56-59 in Abstract: “While exposure will intensify across all regions, some\nsouthern hemisphere areas which have lower exposure to contemporary and projected\nmarine heatwaves may provide climate refugia for floating kelp forests.”\nAnd Lines 122-124 in Introduction: “may enhance the resilience of other kelp forests,\ndepending on local and regional connectivity and live history-traits, by maintaining a\nsource of recovery for impacted kelp habitats24.”\ntherefore the outcomes may be similar for kelp in these regions. I agree, but I think it is worth\nstating that whole ocean basins e.g. NE Pacific and N Atlantic with kelp present are not\nrepresented at all as they don’t possess floating kelp.\nDone. We agree with the reviewer and as suggested by Reviewer 1, we have made this\npoint clear. See response to reviewer 1 comments in page 5 and 9. Now in Lines 105-118\nin the Introduction, and Lines 335-349 in the Discussion.\nLine 366 It isn’t clear to me what the filters and masks are in Supplementary Table 1\nDone. Now Line 419: “for the coastlines used to apply the 30-m buffer.”\nLine 368-369 Given the evidence for regional losses of kelp over the last 20-30 years I am a\nlittle sceptical of using Landsat images back to 1984. What if all the detections were early in the\ndataset and there have none in the later years? Was the potential for range reductions in kelp\nforests over the time-series considered? If it wasn’t I suggest that some sort of temporal\nanalysis of this should take place before using data back to 1984.\nDone. We have better clarified why we used all available data in the Methods.\nNow lines 421-429: Our final floating kelp habitat map includes any pixel for which the\nsatellite detected kelp in the time series and represents the known presence of floating\nkelp in the timeseries. We included all observations to avoid arbitrarily choosing a period\nto map the distribution of floating kelp forests because these ecosystems are highly\ndynamic74 and we were interested in detecting potential kelp habitat. For example, in\nsome places kelps may be expanding their distribution (cold-edges)56,57, while in others\nplaces kelps may be in alternative stable states dominated by urchin barrens23 (degraded\nkelp ecosystems) or by more heat tolerant subcanopy kelp species (competing with\nfloating kelps) near warm-edges78. These alternative stable states can shift, even after\ndecades79,80.\nLine 373 I suggest that you make the length and years of the baseline period clear here. It is\nmentioned below, but it would be best to mention when first stated.\nDone. Now Lines 432-433 in Methods: “projected cumulative annual MHW intensities for\neach kelp pixel using a baseline climatology of 1983–2012.”\nLine 398 It isn’t clear to me why a different base line would be used for the reference period and\nthe climatology. When not use the same period?\nWe have revised the wording in the Methods to make this clearer. We use only OISST\n(observational) temperatures to assess contemporary marine heatwaves. We use only\nCMIP6 ESM data to assess future marine heatwaves. The latter is necessary because\nESMs do not necessarily faithfully represent daily variability, so projected MHWs should\nbe assessed on a like-for-like basis against data from ESM historical runs.\npast (1983–2014), and selected three future (2015–2100)…”\nAnd Now Lines 452-455: “We bias-corrected the SST dataset from each ESM relative to\nthe corresponding ensemble mean of the Historical runs using the delta method (see83).\nThis method ensures that projections for each ESM blend smoothly to the end of the\nHistorical runs for the reference period 1983–2014.”\nLines 416-419 I think it would be helpful what the different categories mean in terms of\nrestrictions and LFP.\nDone. Reviewer 1 also suggested and we have now address. See Page 10, and Now Line\n478-485 .\n19"}, {"round": 2, "reviewer_comments": {"reviewer_1": "the authors have addressed all comments suitably", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "All my concerns have been addressed."}, "author_response": ""}]

Decision letter