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Pages 11-28

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From page 11...
... The net RF is the sum of positive and negative terms, and each term is defined as the change relative to 1750. These warming influences may be expressed as CO2-equivalent concentrations, corresponding to the concentration of CO2 that would cause the same amount of radiative forcing as a given mixture of CO2 and other forcing components.
From page 12...
... Some climate changes could be beneficial for some people or regions, while being damaging to others. The primary challenge for this study is to quantify, insofar as possible, the outcomes of different stabilization targets using analyses and informa tion drawn from the scientific literature.
From page 13...
... focusing in part on the relationship between accumulated carbon emissions and global mean temperature change. The motivation for this approach is both practical and conceptual.
From page 14...
... Climate change in the very long term: Future stabilization targets cor respond to altered states of Earth's climate that would be nearly irreversible for many thousands of years, even long after anthropogenic greenhouse gas emissions ceased. The capacity to adapt to slow changes is generally greater than for near-term rapid climate change, but different stabilization levels can lock the Earth and many future generations of humans into large impacts that can occur very slowly over time, such as the melting of the polar ice sheets; similarly, some stabilization levels could prevent such changes.
From page 15...
... (see Figure S.1) affect precipitation patterns, extreme hot seasons, streamflow, sea ice retreat, reduced crop yields, coral bleaching, and sea level rise.
From page 16...
... This behavior is unique to carbon dioxide among major radiative forcing agents. Choices regarding continued emissions or mitigation of other warming agents such 5Approaches to "geoengineer" future climate, e.g., to actively remove carbon from the atmosphere or reflect sunlight to space using particulate matter or mirrors are topics of ac tive research.
From page 17...
... Syn1 and S2 top and_Fig3-6 FIGURE S.2 (top) Best estimates and likely range of cumulative carbon emissions that would result in global warming of 1°C, 2°C, 3°C, 4°C, or 5°C (see Figure S.1)
From page 18...
... . The "low" and "high" onset values in the right panel reflect differences between available climate models in the global mean temperature at which the Greenland ice sheet will disappear after thousands of years since the accumulation cannot sustain the ice loss by melt in the ablation area and rapid ice flow-related loss along the margins.
From page 19...
... . Changes in Antarctica are less clear, in part because both the West and East Antarctic ice sheets must be considered: one study suggests that cumulative carbon emission of about 2,000-5,000 GtC could also contribute up to 5 meters of additional sea level rise from West Antarctic ice sheet loss.
From page 20...
... Note that the equilibrium warming indicated in the figure incorporates only feedbacks from water vapor, clouds, sea ice, or snow changes; the slower acting feedbacks incorporated in Earth System Sensitivity may increase the warming (by about 50% over the values shown according to one study by Lunt et al., 2010) {2.1, 3.2, 3.3} century are expected to determine the commitments to these eventual future changes, the sea level rise expected to occur in the 21st century is consid erably smaller, in the range of 0.5 to 1.0 m.
From page 21...
... are required to approximately stabilize carbon dioxide concentrations for a century or so and even greater reductions in emissions would be required in the longer term; this applies for any chosen stabilization target. Observed climate responses in coming decades will be smaller than the longer-term temperature response to any given stabilization level.
From page 22...
... {3.2, 3.3, 6.1} 3. Climate Changes, Future Impacts, and Choices among Stabilization Targets Increases in global mean temperature caused by higher anthropogenic greenhouse gas concentrations would be expected to lead to a diverse range 8The transient climate response is defined as the warming at the time of doubling of CO 2 concentration (compared to a pre-industrial value of 278 ppm this is about 550 ppm)
From page 23...
... climate changes and impacts can be estimated for specific levels of global mean temperature change experienced, illustrating how stabilization at different levels of greenhouse gas forcing would be expected to alter our world (see Figure S.5)
From page 24...
... As in previous figures, for discussion of transient and equilibrium warming see Chapter 3, where it is noted that the probability distribution of climate sensitivity is uncertain; larger or smaller warmings than the estimated likely value for a given carbon dioxide equivalent concentration cannot be ruled out. Ranges are shown for climate impacts over the globe or over large regions; specific regions, crops, river basins, etc.
From page 25...
... • Sea level rise in the range of 0.5 to 1.0 m in 2100 (reached in a scenario corresponding to about 3±1°C of global warming) and an associated increase in the number of people at risk from coastal flooding by 5-200 million11 as well as global wetland and dryland losses of more than 250,000 square kilometers.
From page 26...
... In the Arctic: shortening of land, lengthening of marine transportation season duration & extent INFRASTRUCTURE Temperature and Shoreline development and Risk of impacts from extreme temperature, precipitation, and storms precipitation extremes protection Frequency, intensity & Extreme cold temperature, Increased risk of heat-related illness and death duration of heat waves precipitation HEALTH Average and extreme Human spread, cultivation temperature & Shifts in timing and geographic range of allergens and vector-borne diseases practices precipitation Precipitation, temperature, Water management and WATER Earlier peak streamflow, longer summer dry periods across much of the U.S. snowpack and melt timing demand FIGURE S.6 Our understanding of the impacts of climate change is still evolving, and quantitative information is currently too limited to provide numerical estimates of the scale, scope, and timing of some impacts.
From page 27...
... 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 2°C, but there is an estimated 17% probability that the warming could exceed 2°C for more than 1,500 years.
From page 28...
... . This report does not identify any specific projections of abrupt climate changes that the committee considers to be robustly estab lished, e.g., based on clear physical understanding of processes and multiple models.


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