Integrated Chinese Level 1 Part 1 Workbook Keys.839
Margaret Sieverding
Why is it necessary and even vital to maintain the global temperature increase below 1.5C versus higher levels? Adaptation will be less difficult. Our world will suffer less negative impacts on intensity and frequency of extreme events, on resources, ecosystems, biodiversity, food security, cities, tourism, and carbon removal.
Extra warming on top of the approximately 1C we have seen so far would amplify the risks and associated impacts, with implications for the world and its inhabitants. This would be the case even if the global warming is held at 1.5C, just half a degree above where we are now, and would be further amplified at 2C of global warming. Reaching 2C instead of 1.5C of global warming would lead to substantial warming of extreme hot days in all land regions. It would also lead to an increase in heavy rainfall events in some regions, particularly in the high latitudes of the Northern Hemisphere, potentially raising the risk of flooding. In addition, some regions, such as the Mediterranean, are projected to become drier at 2C versus 1.5C of global warming. The impacts of any additional warming would also include stronger melting of ice sheets and glaciers, as well as increased sea level rise, which would continue long after the stabilization of atmospheric CO2 concentrations.
Change in climate means and extremes have knock-on effects for the societies and ecosystems living on the planet. Climate change is projected to be a poverty multiplier, which means that its impacts are expected to make the poor poorer and the total number of people living in poverty greater. The 0.5C rise in global temperatures that we have experienced in the past 50 years has contributed to shifts in the distribution of plant and animal species, decreases in crop yields and more frequent wildfires. Similar changes can be expected with further rises in global temperature.
Thermal expansion of the oceans means sea level will continue to rise even if the increase in global temperature is limited to 1.5C, but this rise would be lower than in a 2C warmer world. Ocean acidification, the process by which excess CO2 dissolves into oceans and makes them more acidic, is expected to be less damaging in a world where CO2 emissions are reduced and warming is stabilized at 1.5C compared to 2C. The persistence of coral reefs is greater in a 1.5C world than that of a 2C world, too.
This chapter builds on findings of AR5 and assesses new scientific evidence of changes in the climate system and the associated impacts on natural and human systems, with a specific focus on the magnitude and pattern of risks linked for global warming of 1.5C above temperatures in the pre-industrial period. Chapter 3 explores observed impacts and projected risks to a range of natural and human systems, with a focus on how risk levels change from 1.5C to 2C of global warming. The chapter also revisits major categories of risk (Reasons for Concern, RFC) based on the assessment of new knowledge that has become available since AR5.
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) 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
Climate models project robust2 differences in regional climate between present-day and global warming up to 1.5C3, and between 1.5C and 2C4 (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 3C for 1.5C of global warming, i.e., a factor of two) and at high latitudes in the cold season (with increases of up to 4.5C at 1.5C 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 1.5C global warming (high confidence). Limiting global warming to 1.5C instead of 2C 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 1.5C would limit risks of increases in heavy precipitation events on a global scale and in several regions compared to conditions at 2C global warming (medium confidence). The regions with the largest increases in heavy precipitation events for 1.5C to 2C 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 2C compared to 1.5C of global warming (medium confidence). Heavy precipitation, when aggregated at a global scale, is projected to be higher at 2C than at 1.5C of global warming (medium confidence) 3.3.3, 3.3.6
Limiting global warming to 1.5C 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 2C than at 1.5C 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 1.5C than at 2C of global warming (high confidence). This difference is due to the smaller rates and magnitudes of climate change associated with a 1.5C 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
Global warming of 2C would lead to an expansion of areas with significant increases in runoff, as well as those affected by flood hazard, compared to conditions at 1.5C (medium confidence). Global warming of 1.5C 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 Ocean5 during summer is substantially higher at 2C compared to 1.5C 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 2C, with the frequency decreasing to one sea-ice-free Arctic summer every 100 years under 1.5C (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
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 1.5C 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 1.5C, as compared to today, with adaptation required now and up to 1.5C. However, risks would be larger at 2C 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 1.5C 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 1.5C and return to 1.5C later in the century, as compared to pathways that stabilize at 1.5C 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
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