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Future intensification of Amazon drought resulting from climate change may cause increased fire activity, tree mortality, and emissions of carbon to the atmosphere across large areas of Amazonia. To provide a basis for addressing these issues, we examine properties of recent and future meteorological droughts in the Amazon in 35 climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that the CMIP5 climate models, as a group, simulate important properties of historical meteorological droughts in the Amazon. In addition, this group of models reproduces observed relationships between Amazon precipitation and regional sea surface temperature anomalies in the tropical Pacific and the North Atlantic oceans. Assuming the Representative Concentration Pathway 8.5 scenario for future drivers of climate change, the models project increases in the frequency and geographic extent of meteorological drought in the eastern Amazon, and the opposite in the West. For the region as a whole, the CMIP5 models suggest that the area affected by mild and severe meteorological drought will nearly double and triple, respectively, by 2100. Extremes of wetness are also projected to increase after 2040. Specifically, the frequency of periods of unusual wetness and the area affected by unusual wetness are projected to increase after 2040 in the Amazon as a whole, including in locations where annualmean precipitation is projected to decrease. Our analyses suggest that continued emissions of greenhouse gases will increase the likelihood of extreme events that have been shown to alter and degrade Amazonian forests.

Rapid Arctic warming has intensified northern wildfires and is thawing carbon-rich permafrost. Carbon emissions from permafrost thaw and Arctic wildfires, which are not fully accounted for in global emissions budgets, will greatly reduce the amount of greenhouse gases that humans can emit to remain below 1.5 °C or 2 °C. The Paris Agreement provides ongoing opportunities to increase ambition to reduce society’s greenhouse gas emissions, which will also reduce emissions from thawing permafrost. In December 2020, more than 70 countries announced more ambitious nationally determined contributions as part of their Paris Agreement commitments; however, the carbon budgets that informed these commitments were incomplete, as they do not fully account for Arctic feedbacks. There is an urgent need to incorporate the latest science on carbon emissions from permafrost thaw and northern wildfires into international consideration of how much more aggressively societal emissions must be reduced to address the global climate crisis.

Climate simulation-based scenarios are routinely used to characterize a range of plausible climate futures. Despite some recent progress on bending the emissions curve, RCP8.5, the most aggressive scenario in assumed fossil fuel use for global climate models, will continue to serve as a useful tool for quantifying physical climate risk, especially over near- to midterm policy-relevant time horizons. Not only are the emissions consistent with RCP8.5 in close agreementwith historical total cumulative CO₂ emissions (within 1%), but RCP8.5 is also the best match out to midcentury under current and stated policies with still highly plausible levels of CO₂ emissions in 2100.

A race against climate change In the event of climate change, species have to move if they are to remain in an area with the same average temperature: their chances of survival therefore depend on the ability to keep pace with a moving climate as well as on the extent of change in temperature and other climate factors. To put this pressure on species into context, a novel index designed to quantify climate change in the coming century has been developed. Its value gives the local velocity along the Earth's surface needed to maintain constant temperatures, and is derived from temperature gradients scaled by distance (°C per km) and time (°C per year). The index provides a quantitative view of the role of topography in buffering climate change as it would affect plants and animals: on the IPCC's A1B emission scenario the index has a global mean of 0.42 km per year, compared to extremes of 0.08 and 1.26 km per year for mountains forest biomes and flooded grasslands, respectively. Climate change velocity, it turns out, is large relative to species migration speeds and the sizes of protected habitats. The data suggest that, in some ecosystems, helping species to relocate more rapidly via habitat corridors or new reserves could be an important contribution to conservation. As the climate changes, species will have to move if they are to remain in an area with the same average temperature. Here, this required movement — termed the velocity of temperature change — is quantified. The results indicate management strategies for minimizing biodiversity loss from climate change and suggest that montane landscapes may effectively shelter many species into the next century. The ranges of plants and animals are moving in response to recent changes in climate 1 . As temperatures rise, ecosystems with ‘nowhere to go’, such as mountains, are considered to be more threatened 2 , 3 . However, species survival may depend as much on keeping pace with moving climates as the climate’s ultimate persistence 4 , 5 . Here we present a new index of the velocity of temperature change (km yr -1 ), derived from spatial gradients (°C km -1 ) and multimodel ensemble forecasts of rates of temperature increase (°C yr -1 ) in the twenty-first century. This index represents the instantaneous local velocity along Earth’s surface needed to maintain constant temperatures, and has a global mean of 0.42 km yr -1 (A1B emission scenario). Owing to topographic effects, the velocity of temperature change is lowest in mountainous biomes such as tropical and subtropical coniferous forests (0.08 km yr -1 ), temperate coniferous forest, and montane grasslands. Velocities are highest in flooded grasslands (1.26 km yr -1 ), mangroves and deserts. High velocities suggest that the climates of only 8% of global protected areas have residence times exceeding 100 years. Small protected areas exacerbate the problem in Mediterranean-type and temperate coniferous forest biomes. Large protected areas may mitigate the problem in desert biomes. These results indicate management strategies for minimizing biodiversity loss from climate change. Montane landscapes may effectively shelter many species into the next century. Elsewhere, reduced emissions, a much expanded network of protected areas 6 , or efforts to increase species movement may be necessary 7 .

An ocean-climate model that shows high fluxes of anthropogenic carbon dioxide into the Southern Ocean, but very low storage of anthropogenic carbon there, agrees with observation-based estimates of ocean storage of anthropogenic carbon dioxide. This low simulated storage indicates a subordinate role for deep convection in the present-day Southern Ocean. The primary mechanism transporting anthropogenic carbon out of the Southern Ocean is isopycnal transport. These results imply that if global climate change reduces the density of surface waters in the Southern Ocean, isopycnal surfaces that now outcrop may become isolated from the atmosphere, tending to diminish Southern Ocean carbon uptake.

In 2009, the U.S. Environmental Protection Agency (EPA) established the so-called “Endangerment Finding.” This defined a suite of six long-lived greenhouse gases as “air pollution.” Such air pollution was anticipated to represent a danger to the health and welfare of current and future generations. Thus, the EPA has the authority to regulate these gases under the rules of the U.S. Clean Air Act. Duffy et al. provide a comprehensive review of the scientific evidence gathered in the years since then. These findings further support and strengthen the basis of the Endangerment Finding. Thus, a compelling case has been made even more compelling with an enormous body of additional data. Science , this issue p. eaat5982 We assess scientific evidence that has emerged since the U.S. Environmental Protection Agency’s 2009 Endangerment Finding for six well-mixed greenhouse gases and find that this new evidence lends increased support to the conclusion that these gases pose a danger to public health and welfare. Newly available evidence about a wide range of observed and projected impacts strengthens the association between the risk of some of these impacts and anthropogenic climate change, indicates that some impacts or combinations of impacts have the potential to be more severe than previously understood, and identifies substantial risk of additional impacts through processes and pathways not considered in the Endangerment Finding.

Climate simulation-based scenarios are routinely used to characterize a range of plausible climate futures. Despite some recent progress on bending the emissions curve, RCP8.5, the most aggressive scenario in assumed fossil fuel use for global climate models, will continue to serve as a useful tool for quantifying physical climate risk, especially over near- to midterm policy-relevant time horizons. Not only are the emissions consistent with RCP8.5 in close agreement with historical total cumulative CO 2 emissions (within 1%), but RCP8.5 is also the best match out to midcentury under current and stated policies with still highly plausible levels of CO 2 emissions in 2100.

We use nine different observational datasets to estimate California-average temperature trends during the periods 1950-1999 and 1915-2000. Observed results are compared to trends from a suite of climate model simulations of natural internal climate variability. On the longer (86-year) timescale, increases in annual-mean surface temperature in all observational datasets are consistently distinguishable from climate noise. On the shorter (50-year) timescale, results are sensitive to the choice of observational dataset. For both timescales, the most robust results are large positive trends in mean and maximum daily temperatures in late winter/early spring, as well as increases in minimum daily temperatures from January to September. These trends are inconsistent with model-based estimates of natural internal climate variability, and thus require one or more external forcing agents to be explained. Observational datasets with adjustments for urbanization effects do not yield markedly different results from unadjusted data. Our findings suggest that the warming of Californian winters over the twentieth century is associated with human-induced changes in large-scale atmospheric circulation. We hypothesize that the lack of a detectable increase in summertime maximum temperature arises from a cooling associated with large-scale irrigation. This cooling may have, until now, counteracted summertime warming induced by increasing greenhouse gases effects.