21st Century Warming Research Articles

Severe cold-water bleaching of a deep-water reef underscores future challenges for Mesophotic Coral Ecosystems

Elevated sea surface temperatures are causing an increase in coral bleaching events worldwide, and represent an existential threat to coral reefs. Early studies of Mesophotic Coral Ecosystems (MCEs) highlighted their potential as thermal refuges for shallow-water coral species in the face of predicted 21st century warming. However, recent genetic evidence implies that limited ecological connectivity between shallow- and deep-water coral communities inhibits their effectiveness as refugia; instead MCEs host distinct endemic communities that are ecologically significant in and of themselves. In either scenario, understanding the response of MCEs to climate change is critical given their ecological significance and widespread global distribution. Such an understanding has so far eluded the community, however, because of the challenges associated with long-term field monitoring, the stochastic nature of climatic events that drive bleaching, and the paucity of deep-water observations. Here we document the first observed cold-water bleaching of a mesophotic coral reef at Clipperton Atoll, a remote Eastern Tropical Pacific (ETP) atoll with high coral cover and a well-developed MCE. The severe bleaching (>70 % partially or fully bleached coral cover at 32 m depth) was driven by an anomalously shallow thermocline, and highlights a significant and previously unreported challenge for MCEs. Prompted by these observations, we compiled published cold-water bleaching events for the ETP, and demonstrate that the timing of past cold-water bleaching events in the ETP coincides with decadal oscillations in mean zonal wind strength and thermocline depth. The latter observation suggests any future intensification of easterly winds in the Pacific could be a significant concern for its MCEs. Our observations, in combination with recent reports of warm-water bleaching of Red Sea and Indian Ocean MCEs, highlight that 21st century MCEs in the Eastern Pacific face a two-pronged challenge: warm-water bleaching from above, and cold-water bleaching from below.

Realistic representation of mixed-phase clouds increases projected climate warming

Clouds are the main source of uncertainties when projecting climate change. Mixed-phase clouds that contain ice and supercooled-liquid particles are especially hard to constrain, and climate models neither agree on their phase nor their spatial extent. This is problematic, as models that underestimate contemporary supercooled-liquid in mixed-phase clouds will underestimate future warming. Furthermore, it has recently been shown that supercooled-liquid water in mixed-phase clouds is not homogeneously-mixed, neither vertically nor horizontally. However, while there have been attempts at observationally constraining mixed-phase clouds to constrain uncertainties in future warming, all studies only use the phase of the interior of mixed-phase clouds. Here we show, using novel satellite observations that distinguish between cloud-top and interior phase in mixed-phase clouds, that mixed-phase clouds are more liquid at the cloud top globally. We use these observations to constrain the cloud top phase in addition to the interior in a global climate model, leading to +1 °C more 21st century warming in NorESM2 SSP5-8.5 climate projections. We anticipate that the difference between cloud top and interior phase in mixed-phase clouds is an important new target metric for future climate model development, because similar mixed-phase clouds related biases in future warming are likely present in many climate models.

Response of aspen to a warming climate along a latitudinal gradient in the Rocky Mountains, USA

The 21st century’s warming climate threatens aspen ( Populus tremuloides) growth in the southern Rocky Mountains (western US), endangering ecosystem diversity, functionality, and associated services. This study linked aspen growth with temperature and moisture variations, assessing how warmer climates and seasonal changes affect ring widths. Sampling 211 aspen trees from 12 latitudinally distributed sites, we compiled three regional aspen chronologies spanning 1913–2018. We investigated climate-growth associations using correlation and modeling ( climwin) techniques. Results revealed that aspen at the trailing southern edge of the Rocky Mountains are vulnerable to drought, where elevated temperatures and diminished precipitation emerge as primary factors contributing to their reduced growth. In contrast, aspens in the northern region of the gradient exhibited a positive growth response to rising temperatures, potentially linked to the alleviation of growth-limiting cold temperatures. However, while pre-2000s droughts increased growth, recent droughts elicited growth reductions in the North; suggesting that with continued warming, northern populations will increasingly face sensitivity to droughts akin to their southern counterparts. These findings emphasize the increased vulnerability of southern Rocky Mountain aspen populations to climate change-induced growth constraints, particularly in the anticipated warmer and drier conditions of the 21st century. This study is crucial to understanding aspen responses to climate fluctuations in the region.

Earth system model’s capability of predicting drought-induced crop failure

We used the Meteorological Research Institute Earth System Model to investigate land precipitation and net primary production (NPP, proxy for agricultural production here) in terms of food and water security. In the preindustrial state, the largest decrease in Southeast Asian precipitation (> 20%) occurs along with the largest El Niño and positive phase of the Atlantic multidecadal oscillation, consistent with the 1876-78 Great Drought, leading to the largest decrease in NPP (~ 20%); >2 standard deviations, extreme dryness and crop failure. In relatively arid regions (e.g., India and northeastern Brazil), the largest decreases in precipitation and NPP reach ≥ 50%, particularly 80 ~ 90% in Deccan, India. In the 21st century warming projection (Shared Socioeconomic Pathways 5-8.5, the largest CO2 emission scenario), the interannual variability of precipitation and NPP in Monsoon Asia increase under the influence of enhanced El Niño-Southern Oscillation. Although the increased precipitation variability indicates more frequent severer droughts than in the preindustrial state, this severity does not influence NPP so much: the increased NPP variability is negligible compared to the significant increase in normal NPP by CO2 fertilization effect, which indicates usually rich harvests. Compared to a recent severe crop failure (− 25% in Japan in 1993), the simulated preindustrial drought-induced NPP decreases of ≥ 50% in parts of low latitudes are extraordinarily large enough to correspond to devastating crop failures. The model indicates that great famines before the 20th century were natural in origin, and that manmade factors, such as harsh taxes and hoarding and export of grain, made the famines more devastating.

Placing 21st Century Warming in Southern California, USA in a Multi-Century Historical Context

Warming in southern California during the 21st century is unprecedented in the instrumental record. To place this warming in a multi-century historical context, we analyzed tree ring data sampled from Jeffrey pine (Pinus jeffreyi) and sugar pine (Pinus lambertiana) collected from minimally disturbed, old-growth high-elevation forests within Mt. San Jacinto State Park California, USA. Based on a calibration/verification period of 1960–2020 between earlywood radial growth and California Climate Division 6 climate data, we reconstructed annual (November–October) minimum temperature (Tmin) from 1658 to 2020. During the 61-year calibration/verification period, instrumental Tmin increased (r = 0.69, p < 0.01) and was positively associated with annual radial growth (r = 0.71, p < 0.01). Using regime shift analysis, we found that the 363-year reconstruction revealed Tmin stability until 1958 and then decreased until 1980, followed by the two warmest regimes (1981–2007, 2008–2020) on record. The last 13-year period was 0.77 °C warmer than the multi-century average with nine of the ten warmest years in the reconstruction recorded. These results suggest that 21st century warming in southern California is unique in the context of the past four centuries, indicating the rarity of exceptional warmth captured in the tree ring record.

The evolution of future Antarctic surface melt using PISM-dEBM-simple

Abstract. It is virtually certain that Antarctica's contribution to sea-level rise will increase with future warming, although competing mass balance processes hamper accurate quantification of the exact magnitudes. Today, ocean-induced melting underneath the floating ice shelves dominates mass losses, but melting at the surface will gain importance as global warming continues. Meltwater at the ice surface has crucial implications for the ice sheet's stability, as it increases the risk of hydrofracturing and ice-shelf collapse that could cause enhanced glacier outflow into the ocean. Simultaneously, positive feedbacks between ice and atmosphere can accelerate mass losses and increase the ice sheet's sensitivity to warming. However, due to long response times, it may take hundreds to thousands of years until the ice sheet fully adjusts to the environmental changes. Therefore, ice-sheet model simulations must be computationally fast and capture the relevant feedbacks, including the ones at the ice–atmosphere interface. Here we use the novel surface melt module dEBM-simple (a slightly modified version of the “simple” diurnal Energy Balance Model) coupled to the Parallel Ice Sheet Model (PISM, together referred to as PISM-dEBM-simple) to estimate the impact of 21st-century atmospheric warming on Antarctic surface melt and ice dynamics. As an enhancement compared to the widely adopted positive degree-day (PDD) scheme, dEBM-simple includes an implicit diurnal cycle and computes melt not only from the temperature, but also from the influence of solar radiation and changes in ice albedo, thus accounting for the melt–albedo feedback. We calibrate PISM-dEBM-simple to reproduce historical and present-day Antarctic surface melt rates given by the regional atmospheric climate model RACMO2.3p2 and use the calibrated model to assess the range of possible future surface melt trajectories under Shared Socioeconomic Pathway SSP5-8.5 warming projections until the year 2100. To investigate the committed impacts of the enhanced surface melting on the ice-sheet dynamics, we extend the simulations under fixed climatological conditions until the ice sheet has reached a state close to equilibrium with its environment. Our findings reveal a substantial surface-melt-induced speed-up in ice flow associated with large-scale elevation reductions in sensitive ice-sheet regions, underscoring the critical role of self-reinforcing ice-sheet–atmosphere feedbacks in future mass losses and sea-level contribution from the Antarctic Ice Sheet on centennial to millennial timescales.

Much faster warming-tracking rate of the latitudinal forest line in North America than that in Eurasia under the early 21st century's warming

Warming drives poleward expansion of forests over high latitudes. However, it remains unresolved whether the poleward forest lines can track the warming rate at continental scales. To answer this question, we compared latitudinal forest line shifts detected by remote sensing under the early 21st century's warming (i.e., from 2000 to 2019) with shifts of temperature isolines derived from a time-invariant temperature-forest line relationship. The results show that the northward forest line shift in North America (NA) can generally keep pace with changes in temperature isolines; while in Eurasia (EA) it greatly trails that of temperature changes. Therefore, the northward shift of the forest line tracking temperature warming over NA is much faster than that over EA. The slower warming rate over NA allows slow temperature acclimation. In contrast, the warming rate is much faster in EA, which makes other environmental factors (e.g., winter temperature, water stress, and active layer thickness, biotic factors, and land use and land cover change) increasingly important in limiting the northward shift of forest lines.

Biodiversity and Stoichiometric Plasticity Increase Pico‐Phytoplankton Contributions to Marine Net Primary Productivity and the Biological Pump

AbstractEarth System Models generally predict increasing upper ocean stratification from 21st century warming, which will cause a decrease in the vertical nutrient flux forcing declines in marine net primary productivity (NPP) and carbon export. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low‐latitude oligotrophic systems harboring pico‐sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. The climate forced changes in nutrient flux stoichiometry and phytoplankton community composition are thus likely to alter the ocean's biogeochemical response and feedback with the carbon‐climate system. We have added three pico‐phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico‐eukaryote populations that contribute significantly to productivity and export within the subtropical to mid‐latitude transition zone, with the western subtropical regions of each basin supporting the most P‐poor stoichiometries. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C‐rich/P‐poor stoichiometry, preferentially inhabited by pico‐eukaryotes and diatoms. Collectively, pico‐phytoplankton contribute ∼58% of global NPP and ∼46% of global particulate organic carbon export below 100 m through direct and ecosystem processing pathways. Biodiversity and cellular nutrient plasticity in marine pico‐phytoplankton combine to increase their contributions to ocean productivity and the biological carbon pump.

21st Century Warming, Site Aspect, and Reversal of Age-Related Growth Decline in Shortleaf Pine (Pinus echinata) in North Carolina, USA

We examined the influence of significant 21st century warming on the radial growth patterns of shortleaf pine growing on adjacent north/northeast- and south/southwest-facing slopes (hereafter NS and SS), in the Uwharrie Mountains of North Carolina, USA. Using two chronologies developed from old-growth trees dating to the 1700s, we compared raw radial growth rates (hereafter radial growth) associated with earlywood, latewood, and totalwood during 1935–2020. Both chronologies exhibited similar (r = 0.951, p < 0.001) age-related growth decreases through the 20th century. However, both chronologies experienced abrupt increases in radial growth with less fidelity (r = 0.86, p < 0.001), correlating with the onset of warming mean annual temperatures (r = 0.58, p < 0.01) and warming winter temperatures (r = 0.55, p < 0.05) in 2002. These results show that shortleaf pine growing on both NS and SS have experienced significant radial growth increases since the early 21st century, but that aspect affected growth rates. During 2002–2020, NS radial growth increased significantly (p < 0.05) more than SS earlywood, latewood, and totalwood, indicating that the effects of warming were greater for NS trees. We conclude that old-growth shortleaf pine trees retain climatic sensitivity to significant environmental changes associated with a warming climate and can reverse age-related growth declines.

Cloud and Surface Albedo Feedbacks Reshape 21st Century Warming in Successive Generations of An Earth System Model

AbstractThe relative importance of radiative feedbacks and emissions scenarios in controlling surface warming patterns is challenging to quantify across model generations. We analyze three variants of the Community Earth System Model (CESM) with differing equilibrium climate sensitivities under identical CMIP5 historical and high‐emissions scenarios. CESM1, our base model, exhibits Arctic‐amplified warming with the least warming in the Southern Hemisphere middle latitudes. A variant of CESM1 with enhanced extratropical shortwave cloud feedbacks shows slightly increased late‐21st century warming at all latitudes. In the next‐generation model, CESM2, global‐mean warming is also slightly greater, but the warming is zonally redistributed in a pattern mirroring cloud and surface albedo feedbacks. However, if the nominally equivalent CMIP6 scenario is applied to CESM2, the redistributed warming pattern is preserved, but global‐mean warming is significantly greater. These results demonstrate how model structural differences and scenario differences combine to produce differences in climate projections across model generations.

Responses of lake diatoms to rapid 21st century warming on the southeastern Tibetan Plateau

Many paleolimnological studies from the Northern Hemisphere have shown how diatom assemblages preserved in dated lake sediment cores have responded to anthropogenic climate warming over the past ~100 years. In contrast, diatom records from lakes in the southeastern Tibetan Plateau have typically registered minimal compositional change during this recent period of warming. Site-specific conditions can dampen the biological response to regional climate change in this topographically complex mountain landscape. In this study, we used a 210 Lead/ 137 Cesium-dated sedimentary diatom record collected in 2018 from a remote, oligotrophic, alpine lake (Cuomujiri Lake) in the southeastern Tibetan Plateau to track environmental changes during the past ~100 years. Change-point analysis on air temperature trends from the nearby Nyingchi meteorological station and from across the Tibetan Plateau (composite of 150 stations) showed exceptionally rapid and statistically significant warming since 2004 CE, although localized variations exist in this heterogeneous region. Concurrent with this pronounced temperature rise, the Cuomujiri diatom record registered an abrupt shift from a benthic to a planktonic-dominated assemblage for the first time in the record, with a clear increase in Lindavia ocellata since ca. 2004 CE. Despite substantial warming in the southeastern Tibetan Plateau over the past several decades, diatom assemblages in Cuomujiri Lake did not register a notable response until the significantly high temperatures experienced during the past ~14 years. These results from Earth’s highest plateau signify that, with sustained anthropogenic warming, exceedance of ecological thresholds will continue in lakes throughout the Tibetan Plateau and likely in other high latitude regions of the world.

Hydrologic Controls on Peat Permafrost and Carbon Processes: New Insights From Past and Future Modeling

Soil carbon (C) in permafrost peatlands is vulnerable to decomposition with thaw under a warming climate. The amount and form of C loss likely depends on the site hydrology following permafrost thaw, but antecedent conditions during peat accumulation are also likely important. We test the role of differing hydrologic conditions on rates of peat accumulation, permafrost formation, and response to warming at an Arctic tundra fen using a process-based model of peatland dynamics in wet and dry landscape settings that persist from peat initiation in the mid-Holocene through future simulations to 2100 CE and 2300 CE. Climate conditions for both the wet and dry landscape settings are driven by the same downscaled TraCE-21ka transient paleoclimate simulations and CCSM4 RCP8.5 climate drivers. The landscape setting controlled the rates of peat accumulation, permafrost formation and the response to climatic warming and permafrost thaw. The dry landscape scenario had high rates of initial peat accumulation (11.7 ± 3.4 mm decade−1) and rapid permafrost aggradation but similar total C stocks as the wet landscape scenario. The wet landscape scenario was more resilient to 21st century warming temperatures than the dry landscape scenario and showed 60% smaller C losses and 70% more new net peat C additions by 2100 CE. Differences in the modeled responses indicate the largest effect is related to the landscape setting and basin hydrology due to permafrost controls on decomposition, suggesting an important sensitivity to changing runoff patterns. These subtle hydrological effects will be difficult to capture at circumpolar scales but are important for the carbon balance of permafrost peatlands under future climate warming.

Plausible 2005–2050 emissions scenarios project between 2 °C and 3 °C of warming by 2100

Emissions scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate change research and policy. Here, we identify subsets of scenarios of the IPCC’s 5th (AR5) and forthcoming 6th (AR6) Assessment Reports, including the Shared Socioeconomic Pathway scenarios, that project 2005–2050 fossil-fuel-and-industry (FFI) CO2 emissions growth rates most consistent with observations from 2005 to 2020 and International Energy Agency (IEA) projections to 2050. These scenarios project between 2 °C and 3 °C of warming by 2100, with a median of 2.2 °C. The subset of plausible IPCC scenarios does not represent all possible trajectories of future emissions and warming. Collectively, they project continued mitigation progress and suggest the world is presently on a lower emissions trajectory than is often assumed. However, these scenarios also indicate that the world is still off track from limiting 21st-century warming to 1.5 °C or below 2 °C.

Mass loss of the Antarctic ice sheet until the year 3000 under a sustained late-21st-century climate

AbstractIce-sheet simulations of Antarctica extending to the year 3000 are analysed to investigate the long-term impacts of 21st-century warming. Climate projections are used as forcing until 2100 and afterwards no climate trend is applied. Fourteen experiments are for the ‘unabated warming’ pathway, and three are for the ‘reduced emissions’ pathway. For the unabated warming path simulations, West Antarctica suffers a much more severe ice loss than East Antarctica. In these cases, the mass loss amounts to an ensemble average of ~3.5 m sea-level equivalent (SLE) by the year 3000 and ~5.3 m for the most sensitive experiment. Four phases of mass loss occur during the collapse of the West Antarctic ice sheet. For the reduced emissions pathway, the mean mass loss is ~0.24 m SLE. By demonstrating that the consequences of the 21st century unabated warming path forcing are large and long term, the results present a different perspective to ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). Extended ABUMIP (Antarctic BUttressing Model Intercomparison Project) simulations, assuming sudden and sustained ice-shelf collapse, with and without bedrock rebound, corroborate a negative feedback for ice loss found in previous studies, where bedrock rebound acts to slow the rate of ice loss.

Hydroclimate response in Texas and Gulf of Mexico to rapid warming during the last deglacial: High-resolution speleothem proxy and monitoring evidence

Cave calcite (i.e., speleothem) proxy records that span abrupt warming events, such as those of the last deglacial, may prologue regional responses of hydroclimate to 21st century warming. Proxy reliability is advanced by an understanding of the modern hydrologic system and controls on calcite growth. Here we integrate monitoring of Cave CWN in central Texas with high-resolution imaging and δ18O analysis of Stalagmite CWN-4 to understand how regional hydroclimate changed during a rapid deglacial warming event - the transition from Heinrich Stadial 1 (HS1) to the Bølling-Allerød (BA) warm period. This event is recorded in CWN-4 as a rapid negative δ18O excursion of 1.7‰ over ∼210 years (HS1-BA δ18O excursion = ‘HBO’), followed by a 6-fold growth rate increase.Monitoring demonstrates that modern calcite grows during cool seasons, growth rates follow drip rate, and drip water and calcite δ18O reflect the δ18O of recharge. Because drip waters have interannual epikarst residence times, their δ18O values are well-mixed averages of their Gulf of Mexico (GoM) moisture source δ18O values, which rapidly decreased during Laurentide Ice Sheet (LIS) melting events. Confocal laser fluorescence microscopy imaging indicates that growth lamina are seasonal within U-series age constraints. The timing of the HBO (14.75 ± 0.08 to 14.54 ± 0.08 ka), corresponds with 1) the onset of Bølling warming in Greenland (14.64 ka), 2) AMOC reinvigoration (∼14.61–14.25 ka), 3) Meltwater Pulse 1a (14.65–14.31 ka), and 4) Meltwater Flood 3 (MWF-3) - the most intense episode of LIS meltwater discharge into the GoM (14.97–14.46 ka). The rapid increase in growth rates following the HBO indicates an abrupt transition to a wetter hydroclimate in central Texas following these regional and global events.The HBO δ18O excursion occurred monotonically, consistent with meltwater discharge being continuous. This excursion also corresponds with climate change recognized across the HS1-BA transition in marine and terrestrial records influenced by changes in North Atlantic climate, AMOC, and ITCZ migrations. We infer that central Texas deglacial hydroclimate largely followed changes in AMOC, linked via meltwater input to the GoM. Periods of fast (slow) speleothem growth coincide with warmer (cooler) North Atlantic episodes, with the exception of the HS1 to BA transition. For this transition, slow growth rates during MWF-3 may reflect protracted cooling of GoM SST and corresponding atmospheric moisture reduction. The common timing and duration of the HBO with shifts toward wetter conditions across the HS1-BA transition in many northern Hemisphere climate records supports common ocean-atmosphere teleconnections related to AMOC intensity.

Impact of merging of historical and future climate data sets on land carbon cycle projections for South America

AbstractEarth System Models (ESMs) project climate change, but they often contain biases in their estimates of contemporary climate that propagate into simulated futures. Land models translate climate projections into surface impacts, but these will be inaccurate if ESMs have substantial errors. Bias concerns are relevant for terrestrial physiological processes which often respond non‐linearly (i.e. contain threshold responses) and are therefore sensitive to absolute environmental conditions as well as changes. We bias‐correct the UK Met Office ESM, HadGEM2‐ES, against the CRU–JRA observation‐based gridded estimates of recent climate. We apply the derived bias corrections to future projections by HadGEM2‐ES for the RCP8.5 scenario of future greenhouse gas concentrations. Focusing on South America, the bias correction includes adjusting for ESM estimates that, annually, are approximately 1 degree too cold, for comparison against 21st Century warming of around 4 degrees. Locally, these values can be much higher. The ESM is also too wet on average, by approximately 1 mm·day−1, which is substantially larger than the mean predicted change. The corrected climate fields force the Joint UK Land Environment Simulator (JULES) dynamic global vegetation model to estimate land surface changes, with an emphasis on the carbon cycle. Results show land carbon sink reductions across South America, and in some locations, the net land–atmosphere CO2 flux becomes a source to the atmosphere by the end of this century. Transitions to a CO2 source is where increases in plant net primary productivity are offset by larger enhancements in soil respiration. Bias‐corrected simulations estimate the rise in South American land carbon stocks between pre‐industrial times and the end of the 2080s is ∼12 GtC lower than that without climate bias removal, demonstrating the importance of merging historical observational meteorological forcing with ESM diagnostics. We present evidence for a substantial climate‐induced role of greater soil decomposition in the fate of the Amazon carbon sink.

North Atlantic jet stream projections in the context of the past 1,250 years

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope-enabled climate model simulations and a wide array of mean annual water isotope ([Formula: see text]O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

Modeling tree radial growth in a warming climate: where, when, and how much do potential evapotranspiration models matter?

Process-based models of tree-ring width are used both for reconstructing past climates and for projecting changes in growth due to climate change. Since soil moisture observations are unavailable at appropriate spatial and temporal scales, these models generally rely on simple water budgets driven in part by temperature-based potential evapotranspiration (PET) estimates, but the choice of PET model could have large effects on simulated soil moisture, moisture stress, and radial growth. Here, I use four different PET models to drive the VS-Lite model and evaluate the extent to which they differ in both their ability to replicate observed growth variability and their simulated responses to projected 21st century warming. Across more than 1200 tree-ring width chronologies in the conterminous United States, there were no significant differences among the four PET models in their ability to replicate observed radial growth, but the models differed in their responses to 21st century warming. The temperature-driven empirical PET models (Thornthwaite and Hargreaves) simulated much larger warming-induced increases in PET and decreases in soil moisture than the more physically realistic PET models (Priestley–Taylor and Penman–Monteith). In cooler and more mesic regions with relatively minimal moisture constraints to growth, the models simulated similarly small reductions in growth with increased warming. However, in dry regions, the Thornthwaite- and Hargreaves-driven VS-Lite models simulated an increase in moisture stress roughly double that of the Priestley–Taylor and Penman–Monteith models, which also translated to larger simulated declines in radial growth under warming. While the lack of difference in the models’ ability to replicate observed radial growth variability is an encouraging sign for some applications (e.g. attributing changes in growth to specific climatic drivers), the large differences in model responses to warming suggest that caution is needed when applying the temperature-driven PET models to climatic conditions with large trends in temperature.

Health and sustainability of glaciers in High Mountain Asia

Glaciers in High Mountain Asia generate meltwater that supports the water needs of 250 million people, but current knowledge of annual accumulation and ablation is limited to sparse field measurements biased in location and glacier size. Here, we present altitudinally-resolved specific mass balances (surface, internal, and basal combined) for 5527 glaciers in High Mountain Asia for 2000–2016, derived by correcting observed glacier thinning patterns for mass redistribution due to ice flow. We find that 41% of glaciers accumulated mass over less than 20% of their area, and only 60% ± 10% of regional annual ablation was compensated by accumulation. Even without 21st century warming, 21% ± 1% of ice volume will be lost by 2100 due to current climatic-geometric imbalance, representing a reduction in glacier ablation into rivers of 28% ± 1%. The ablation of glaciers in the Himalayas and Tien Shan was mostly unsustainable and ice volume in these regions will reduce by at least 30% by 2100. The most important and vulnerable glacier-fed river basins (Amu Darya, Indus, Syr Darya, Tarim Interior) were supplied with >50% sustainable glacier ablation but will see long-term reductions in ice mass and glacier meltwater supply regardless of the Karakoram Anomaly.

Surface Melt and Runoff on Antarctic Ice Shelves at 1.5°C, 2°C, and 4°C of Future Warming

AbstractThe future surface mass balance (SMB) of Antarctic ice shelves has not been constrained with models of sufficient resolution and complexity. Here, we force the high‐resolution Modèle Atmosphérique Régional with future simulations from four CMIP models to evaluate the likely effects on the SMB of warming of 1.5°C, 2°C, and 4°C above pre‐industrial temperatures. We find non‐linear growth in melt and runoff which causes SMB to become less positive with more pronounced warming. Consequently, Antarctic ice shelves may be more likely to contribute indirectly to sea level rise via hydrofracturing‐induced collapse, which facilitates accelerated glacial discharge. Using runoff and melt as indicators of ice shelf stability, we find that several Antarctic ice shelves (Larsen C, Wilkins, Pine Island, and Shackleton) are vulnerable to disintegration at 4°C. Limiting 21st century warming to 2°C will halve the ice shelf area susceptible to hydrofracturing‐induced collapse compared to 4°C.