Editing IPCC:AR6/SR15/TS (section)

=== 1.5°C and 2°C Warmer Worlds ===

'''The global climate has changed relative to the pre-industrial period, and there are multiple lines of evidence that these changes have had impacts on organisms and ecosystems, as well as on human systems and well-being (''high confidence'').''' The increase in global mean surface temperature (GMST), which reached 0.87°C in 2006–2015 relative to 1850–1900, has increased the frequency and magnitude of impacts (''high confidence''), strengthening evidence of how an increase in GMST of 1.5°C or more could impact natural and human systems (1.5°C versus 2°C). {3.3, 3.4, 3.5, 3.6, Cross-Chapter Boxes 6, 7 and 8 in this chapter}

'''Human-induced global warming has already caused multiple observed changes in the climate system (''high confidence'').''' Changes include increases in both land and ocean temperatures, as well as more frequent heatwaves in most land regions (''high confidence''). There is also ''high confidence'' that global warming has resulted in an increase in the frequency and duration of marine heatwaves. Further, there is substantial evidence that human-induced global warming has led to an increase in the frequency, intensity and/or amount of heavy precipitation events at the global scale (''medium confidence''), as well as an increased risk of drought in the Mediterranean region (''medium confidence''). {3.3.1, 3.3.2, 3.3.3, 3.3.4, Box 3.4}

'''Trends in intensity and frequency of some climate and weather extremes have been detected over time spans during which about 0.5°C of global warming occurred (''medium confidence'').''' This assessment is based on several lines of evidence, including attribution studies for changes in extremes since 1950. {3.2, 3.3.1, 3.3.2, 3.3.3, 3.3.4}

'''Several regional changes in climate are assessed to occur with global warming up to 1.5°C as compared to pre-industrial levels, including warming of extreme temperatures in many regions (''high confidence''),''' increases in frequency, intensity and/or amount of heavy precipitation in several regions (''high confidence''), and an increase in intensity or frequency of droughts in some regions (''medium confidence''). {3.3.1, 3.3.2, 3.3.3, 3.3.4, Table 3.2}

'''There is no single ‘1.5°C warmer world’ (''high confidence'').''' In addition to the overall increase in GMST, it is important to consider the size and duration of potential overshoots in temperature. Furthermore, there are questions on how the stabilization of an increase in GMST of 1.5°C can be achieved, and how policies might be able to influence the resilience of human and natural systems, and the nature of regional and subregional risks. Overshooting poses large risks for natural and human systems, especially if the temperature at peak warming is high, because some risks may be long-lasting and irreversible, such as the loss of some ecosystems (''high confidence''). The rate of change for several types of risks may also have relevance, with potentially large risks in the case of a rapid rise to overshooting temperatures, even if a decrease to 1.5°C can be achieved at the end of the 21st century or later (''medium confidence''). If overshoot is to be minimized, the remaining equivalent CO<sub>2</sub> budget available for emissions is very small, which implies that large, immediate and unprecedented global efforts to mitigate greenhouse gases are required (''high confidence''). {3.2, 3.6.2, Cross-Chapter Box 8 in this chapter}

'''Robust <sup>[[#fn:2|2]]</sup> global differences in temperature means and extremes are expected if global warming reaches 1.5°C versus 2°C above the pre-industrial levels (''high confidence'').''' For oceans, regional surface temperature means and extremes are projected to be higher at 2°C compared to 1.5°C of global warming (''high confidence''). Temperature means and extremes are also projected to be higher at 2°C compared to 1.5°C in most land regions, with increases being 2–3 times greater than the increase in GMST projected for some regions (''high confidence''). Robust increases in temperature means and extremes are also projected at 1.5°C compared to present-day values (''high confidence'') {3.3.1, 3.3.2}. There are decreases in the occurrence of cold extremes, but substantial increases in their temperature, in particular in regions with snow or ice cover (''high confidence'') {3.3.1}.

'''Climate models project robust <sup>[[#fn:2|2]]</sup> differences in regional climate between present-day and global warming up to 1.5°C <sup>[[#fn:3|3]]</sup>, and between 1.5°C and 2°C <sup>[[#fn:3|3]]</sup> ''(high confidence)'', depending on the variable and region in question''(high confidence).''Large, robust and widespread differences are expected for temperature extremes (''high confidence'').''' Regarding hot extremes, the strongest warming is expected to occur at mid-latitudes in the warm season (with increases of up to 3°C for 1.5°C of global warming, i.e., a factor of two) and at high latitudes in the cold season (with increases of up to 4.5°C at 1.5°C of global warming, i.e., a factor of three) (high confidence). The strongest warming of hot extremes is projected to occur in central and eastern North America, central and southern Europe, the Mediterranean region (including southern Europe, northern Africa and the Near East), western and central Asia, and southern Africa (''medium confidence''). The number of exceptionally hot days are expected to increase the most in the tropics, where interannual temperature variability is lowest; extreme heatwaves are thus projected to emerge earliest in these regions, and they are expected to already become widespread there at 1.5°C global warming (''high confidence''). Limiting global warming to 1.5°C instead of 2°C could result in around 420 million fewer people being frequently exposed to extreme heatwaves, and about 65 million fewer people being exposed to exceptional heatwaves, assuming constant vulnerability (''medium confidence''). {3.3.1, 3.3.2, Cross-Chapter Box 8 in this chapter}

'''Limiting global warming to 1.5°C would limit risks of increases in heavy precipitation events on a global scale and in several regions compared to conditions at 2°C global warming (''medium confidence'').''' The regions with the largest increases in heavy precipitation events for 1.5°C to 2°C global warming include: several high-latitude regions (e.g. Alaska/western Canada, eastern Canada/ Greenland/Iceland, northern Europe and northern Asia); mountainous regions (e.g.,Tibetan Plateau); eastern Asia (including China and Japan); and eastern North America (''medium confidence''). Tropical cyclones are projected to decrease in frequency but with an increase in the number of very intense cyclones (''limited evidence, low confidence''). Heavy precipitation associated with tropical cyclones is projected to be higher at 2°C compared to 1.5°C of global warming (''medium confidence''). Heavy precipitation, when aggregated at a global scale, is projected to be higher at 2°C than at 1.5°C of global warming (''medium confidence'')
{3.3.3, 3.3.6}

'''Limiting global warming to 1.5°C is expected to substantially reduce the probability of extreme drought, precipitation deficits, and risks associated with water availability (i.e., water stress) in some regions (''medium confidence'').''' In particular, risks associated with increases in drought frequency and magnitude are projected to be substantially larger at 2°C than at 1.5°C in the Mediterranean region (including southern Europe, northern Africa and the Near East) and southern Africa (''medium confidence''). {3.3.3, 3.3.4, Box 3.1, Box 3.2}

'''Risks to natural and human systems are expected to be lower at 1.5°C than at 2°C of global warming (''high confidence'').''' This difference is due to the smaller rates and magnitudes of climate change associated with a 1.5°C temperature increase, including lower frequencies and intensities of temperature-related extremes. Lower rates of change enhance the ability of natural and human systems to adapt, with substantial benefits for a wide range of terrestrial, freshwater, wetland, coastal and ocean ecosystems (including coral reefs) (''high confidence''), as well as food production systems, human health, and tourism (''medium confidence''), together with energy systems and transportation (''low confidence''). {3.3.1, 3.4}

'''Exposure to multiple and compound climate-related risks is projected to increase between 1.5°C and 2°C of global warming''' with greater proportions of people both exposed and susceptible to poverty in Africa and Asia (''high confidence''). For global warming from 1.5°C to 2°C, risks across energy, food, and water sectors could overlap spatially and temporally, creating new – and exacerbating current – hazards, exposures, and vulnerabilities that could affect increasing numbers of people and regions (''medium confidence''). Small island states and economically disadvantaged populations are particularly at risk (''high confidence''). {3.3.1, 3.4.5.3, 3.4.5.6, 3.4.11, 3.5.4.9, Box 3.5}

'''Global warming of 2°C would lead to an expansion of areas with significant increases in runoff, as well as those affected by flood hazard, compared to conditions at 1.5°C (''medium confidence'').''' Global warming of 1.5°C would also lead to an expansion of the global land area with significant increases in runoff (''medium confidence'') and an increase in flood hazard in some regions (''medium confidence'') compared to present-day conditions. {3.3.5}

'''The probability of a sea-ice-free Arctic Ocean <sup>[[#fn:4|4]]</sup> during summer is substantially higher at 2°C compared to 1.5°C of global warming (''medium confidence'').''' Model simulations suggest that at least one sea-ice-free Arctic summer is expected every 10 years for global warming of 2°C, with the frequency decreasing to one sea-ice-free Arctic summer every 100 years under 1.5°C (''medium confidence''). An intermediate temperature overshoot will have no long-term consequences for Arctic sea ice coverage, and hysteresis is not expected (''high confidence''). {3.3.8, 3.4.4.7}

'''Global mean sea level rise (GMSLR) is projected to be around 0.1 m (0.04 – 0.16 m) less by the end of the 21st century in a 1.5°C warmer world compared to a 2°C warmer world (''medium confidence'').''' Projected GMSLR for 1.5°C of global warming has an indicative range of 0.26 – 0.77m, relative to 1986–2005, (''medium confidence''). A smaller sea level rise could mean that up to 10.4 million fewer people (based on the 2010 global population and assuming no adaptation) would be exposed to the impacts of sea level rise globally in 2100 at 1.5°C compared to at 2°C. A slower rate of sea level rise enables greater opportunities for adaptation (''medium confidence''). There is ''high confidence'' that sea level rise will continue beyond 2100. Instabilities exist for both the Greenland and Antarctic ice sheets, which could result in multi-meter rises in sea level on time scales of century to millennia. There is ''medium confidence'' that these instabilities could be triggered at around 1.5°C to 2°C of global warming. {3.3.9, 3.4.5, 3.6.3}

'''The ocean has absorbed about 30% of the anthropogenic carbon dioxide, resulting in ocean acidification and changes to carbonate chemistry that are unprecedented for at least the last 65 million years (''high confidence'').''' Risks have been identified for the survival, calcification, growth, development and abundance of a broad range of marine taxonomic groups, ranging from algae to fish, with substantial evidence of predictable trait-based sensitivities (''high confidence''). There are multiple lines of evidence that ocean warming and acidification corresponding to 1.5°C of global warming would impact a wide range of marine organisms and ecosystems, as well as sectors such as aquaculture and fisheries (''high confidence''). {3.3.10, 3.4.4}

'''Larger risks are expected for many regions and systems for global warming at 1.5°C, as compared to today,''' with adaptation required now and up to 1.5°C. However, risks would be larger at 2°C of warming and an even greater effort would be needed for adaptation to a temperature increase of that magnitude (''high confidence''). {3.4, Box 3.4, Box 3.5, Cross-Chapter Box 6 in this chapter}

'''Future risks at 1.5°C of global warming will depend on the mitigation pathway and on the possible occurrence of a transient overshoot (''high confidence'').''' The impacts on natural and human systems would be greater if mitigation pathways temporarily overshoot 1.5°C and return to 1.5°C later in the century, as compared to pathways that stabilize at 1.5°C without an overshoot (''high confidence''). The size and duration of an overshoot would also affect future impacts (e.g., irreversible loss of some ecosystems) (''high confidence''). Changes in land use resulting from mitigation choices could have impacts on food production and ecosystem diversity. {3.6.1, 3.6.2, Cross-Chapter Boxes 7 and 8 in this chapter}