that choices about stabilization targets depend upon judgments regarding the degree of acceptable risk.

The likely range of cumulative carbon emissions corresponding to a given warming level is estimated to lie between –30% to +40% of the best estimate. This range is due mainly to uncertainties in the carbon cycle response to emissions and the climate response to increased radiative forcing. For a cumulative anthropogenic emission of 1,000 GtC, our best estimate of the warming remains below 2ºC, but there is an estimated 17% probability that the warming could exceed 2ºC for more than 1,500 years. When cumulative emissions are increased to 1,500 GtC, the best estimate of the anthropogenic warming remains above 2ºC for more than 3,500 years, and the very likely upper end warming is still above 2.5ºC for more than 10,000 years. Higher values cannot be excluded, implying additional risk that cannot presently be quantified. On the other hand, at the lower end of carbon-climate likely uncertainty range, there may be about a 17% chance that warming could remain below 2ºC even if as much as 1,700 GtC are emitted. Figures S.3 and S.5 provide some scientific reasons why global warming of a few degrees could be considered dangerous to some aspects of nature and society, but the corresponding uncertainty ranges should be emphasized here. For example, while the best estimate of a stabilization target corresponding to a long-term warming of 2ºC is 430 ppm, the likely uncertainty range for this value spans from 380 ppm (below current observed levels) to 540 ppm (almost a doubling of carbon dioxide relative to pre-industrial times). {3.4, 6.1}


Many important aspects of climate change and its impacts are expected to be approximately linear and gradual, slowly becoming larger and more significant relative to climate variability as global warming increases.

This report highlights the importance of 21st century choices regarding stabilization targets and how they can be expected to affect many aspects of Earth’s future. Progressively warmer temperatures are expected to slowly lead to larger and more significant changes for impacts including wildfire extent, decreases in yields of some (but not all) crops, streamflow changes, decreased Arctic sea ice extent, increases in heavy rainfall occurrence, and other factors presented. However, it should be noted that many climate changes and impacts remain poorly understood at present. For example, the record of past climates suggests that major changes such as dieback of the Amazon forests or substantial changes in El Niño behavior can occur. This report identifies some areas where recent science suggests reduced effects compared to earlier studies (including e.g., projected future changes in



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