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The need to stabilize global climate Climate change will be the greatest threat to humanity and global ecosystems in the coming years, and there is a pressing need to understand and communicate the impacts of warming, across the perspectives of the natural and social sciences. Hoegh-Guldberg et al. review the climate change–impact literature, expanding on the recent report of the Intergovernmental Panel on Climate Change. They provide evidence of the impacts of warming at 1°, 1.5°, and 2°C—and higher—for the physical system, ecosystems, agriculture, and human livelihoods. The benefits of limiting climate change to no more than 1.5°C above preindustrial levels would outweigh the costs. Science, this issue p. eaaw6974 BACKGROUND The United Nations Framework Convention on Climate Change (UNFCCC) was established in 1992 to pursue the “stabilization of greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interferences with the climate system.” Since 1992, five major climate change assessment cycles have been completed by the UN Intergovernmental Panel on Climate Change (IPCC). These reports identified rapidly growing climate-related impacts and risks, including more intense storms, collapsing ecosystems, and record heatwaves, among many others. Once thought to be tolerable, increases in global mean surface temperature (GMST) of 2.0°C or higher than the pre-industrial period look increasingly unmanageable and hence dangerous to natural and human systems. The Paris Climate Agreement is the most recent attempt to establish international cooperation over climate change. This agreement, ratified or acceded to by 185 countries, was designed to bring nations together voluntarily to take ambitious action on mitigating climate change, while also developing adaptation options and strategies as well as guaranteeing the means of implementation (e.g., climate finance). The Agreement is aimed at “holding the increase in the global average temperature to well below 2.0°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.” Many unanswered questions regarding a 1.5°C target surround the feasibility, costs, and inherent risks to natural and human systems. Consequently, countries invited the IPCC to prepare a Special Report on “the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty.” The Special Report was completed and approved by the 48th Session of the IPCC in October 2018. ADVANCES Multiple lines of evidence indicate that the next 0.5°C above today (which will take GMST from 1.0°C to 1.5°C above the pre-industrial period) will involve greater risks per unit temperature than those seen in the last 0.5°C increase. This principle of “accelerating risk” is also likely to drive proportionally and possibly exponentially higher risk levels in the transition from 1.5°C to 2.0°C above the pre-industrial period. We argue that this is a consequence of impacts accelerating as a function of distance from the optimal temperature for an organism or an ecosystem process. Coral reefs, for example, often appear healthy right up until the onset of mass coral bleaching and mortality, which can then destroy a reef within a few months. This also explains the observation of “tipping points” where the condition of a group of organisms or an ecosystem can appear “healthy” right up to the point of collapse, suggesting caution in extrapolating from measures of ecosystem condition to predict the future. Information of this nature needs to be combined with an appreciation of organisms’ distance from their optimal temperature. Finally, we explore elements of the costs and benefits associated with acting in response to climate change, and come to the preliminary conclusion that restraining average global temperature to 1.5°C above the pre-industrial period would be much less costly than the damage due to inaction on global climate change. OUTLOOK As an IPCC expert group, we were asked to assess the impact of recent climate change (1.0°C, 2017) and the likely impact over the next 0.5° to 1.0°C of additional global warming. At the beginning of this exercise, many of us were concerned that the task would be hindered by a lack of expert literature available for 1.5°C and 2.0°C warmer worlds. Although this was the case at the time of the Paris Agreement, it has not been our experience 4 years later. With an accelerating amount of peer-reviewed scientific literature since the IPCC Special Report Global Warming of 1.5°C, it is very clear that there is an even more compelling case for deepening commitment and actions for stabilizing GMST at 1.5°C above the pre-industrial period. Climate change and nonlinear responses. (A) Reef-building corals can bleach, losing (B) dinoflagellate symbionts (~10 mm across) and dying, thus exhibiting (C) a nonlinear response to impacts/risks from climate change. H (high) and VH (very high) are the confidence for transition from one impact/risk level to another: white, no climate change–related impacts; yellow, some detectable climate change impacts/risks; red, severe and widespread impacts/risks; purple, very high impacts/risks with significant irreversibility or persistence. CREDITS: [(A) AND (B)] O. HOEGH-GULDBERG; (C) THE IPCC SPECIAL REPORT GLOBAL WARMING OF 1.5°C, WITH PERMISSION FROM IPCC Increased concentrations of atmospheric greenhouse gases have led to a global mean surface temperature 1.0°C higher than during the pre-industrial period. We expand on the recent IPCC Special Report on global warming of 1.5°C and review the additional risks associated with higher levels of warming, each having major implications for multiple geographies, climates, and ecosystems. Limiting warming to 1.5°C rather than 2.0°C would be required to maintain substantial proportions of ecosystems and would have clear benefits for human health and economies. These conclusions are relevant for people everywhere, particularly in low- and middle-income countries, where the escalation of climate-related risks may prevent the achievement of the United Nations Sustainable Development Goals.