The National Academies Press

Currently Skimming:

Pages 59-82

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.

From page 59... ... . Here we briefly summarize how major forcing agents contribute to current and future CO2-equivalent target levels and explore implications for global mean temperature increases. Read the entire page →
From page 60... ... Best estimates and very likely uncertainty ranges for aerosols and gas contributions to CO2-equivalent concentrations for 2005, based on the radiative forcing given in Forster et al. Read the entire page →
From page 61... ... . Thus carbon dioxide is the main forcing agent in all of the stabilization targets discussed here, but the contributions of other gases and aerosols to the total CO2-equivalent remain significant, motivating their consideration in analysis of stabilization issues. Read the entire page →
From page 62... ... . In this example test case, carbon dioxide emissions increase at cur rent growth rates of about 2% per year to a maximum of about 12 GtC per FIGURE 2.2 Illustrative calculations showing CO2 concentrations and related warming in two EMICS (the Bern model and the University of Victoria model, see Methods) Read the entire page →
From page 63... ... Box 2.1 discusses how emissions of non-CO2 greenhouse gases could affect attainment of stabilization targets. Figures 2.2 and 2.3 illustrate a fundamental change in understanding stabilization of climate change that has been prompted by the scientific literature of the past two years or so (see Jones et al., 2006; Matthews and Caldeira, 2008) Read the entire page →
From page 64... ... shows emissions required for stabilization without accounting for the impact of climate on the carbon cycle, while panel (c) included the climate impact on the carbon cycle, showing that emission reductions in excess of 80% (relative to peak values) Read the entire page →
From page 65... ... EMISSIONS, CONCENTRATIONS, AND RELATED FACTORS 65 BOX 2.1 STABILIZATION AND NON-CO2 GREENHOUSE GASES Because carbon emissions reductions of more than 80% are required to stabilize carbon dioxide concentrations, small continuing emissions of carbon dioxide, or emissions of CO2-equivalent through other gases, could have surprisingly important implications for stabilizing climate change. For example, emissions of the hydrofluorocarbons (HFCs) Read the entire page →
From page 66... ... S greenhouse gases is the transportation sector, again due to the combustion of fossil fuels. Read the entire page →
From page 67... ... . Figure 2.6 highlights the importance of the carbon budget, that is, the area under the allowable emissions curve associated with a particular radiative forcing target. Read the entire page →
From page 68... ... Here the decline is shifted out in time. carbon budget and a greater array of low-carbon, economically competitive alternatives, which are assumed to become available in the future. Read the entire page →
From page 69... ... 2.3 SHORT-LIVED RADIATIVE FORCING AGENTS: PEAK TRIMMING VERSUS BUYING TIME The role of CO2 emissions in Earth's climate future is unique among the major radiative forcing agents, because the impact of the CO2 emitted into the atmosphere will continue to alter Earth's energy budget for millennia to come. As noted above, Earth's energy budget is also subject to the influence of a number of short-lived radiative forcing agents whose radiative effect would decay to zero on a time scale of weeks to decades if their sources where shut off. Read the entire page →
From page 70... ... 70 FIGURE 2.7 This figure illustrates components of radiative forcing (in CO2-equivalent concentration units) Read the entire page →
From page 72... ... The climate effects of short-lived radiative forcing agents are thus more reversible than those of CO2, and therefore actions reducing emissions of short-lived agents have different implications for Earth's climate future than actions that affect CO2 emissions. Insofar as it is perceived that control of methane or black carbon may be technically easier or less economically dis ruptive than controlling CO2 emissions, mitigation of the short-lived warm ing influences has sometimes been thought of as a way of "buying time" to put CO2 emission controls into place. Read the entire page →
From page 73... ... It would be unrealistic to contemplate policies that would reduce black carbon emissions while leaving reflecting aerosol emissions intact, given that the diverse sources of emission yield an interlinked stew of absorbing and Read the entire page →
From page 74... ... If the long-term situation instead includes a recalci trant methane emission rate that is stabilized but not brought to zero, then the long-term warming is brought above the CO2-only case for a period as long as the methane emissions continue. 2.4 CARBON CYCLE The evolution of atmospheric carbon dioxide concentrations depends on the balance of human emissions, natural processes that remove excess carbon dioxide from the atmosphere, and the sensitivity of land and ocean carbon reservoirs to climate change and land use (see Box 2.2) Read the entire page →
From page 75... ... The only long-term sink of CO2 is silicate weathering, which is a very slowly increasing function of temperature. It would require over 20°C of warming to balance a steady state fossil fuel emission of only a half Gt of carbon per year, so that even an emission as low as this would lead to a steady accumulation of CO2 in the atmosphere. Read the entire page →
From page 76... ... (b) CO2 emissions from fossil fuel combustion and cement production and from land-use change. Read the entire page →
From page 77... ... Theoretical arguments and numerical models suggest that the efficiency of both the land and ocean carbon sinks may decline in the future under warmer climate conditions, which would act to amplify climate warming (Fung et al., 2005; Friedlingstein et al., 2006) Read the entire page →
From page 78... ... In coupled carbon-climate models, biogeochemical feedbacks to a warmer climate tend to partially offset physical-chemical effects and act to reduce the overall strength of ocean climate-carbon cycle feedbacks. In the South ern Ocean, enhanced outgassing of natural CO2 due to stronger winds and upwelling may more than compensate for increased anthropogenic CO2 uptake, leading to a net reduction in ocean uptake (Le Quéré et al., 2008; Lovenduski et al., 2007, 2008) Read the entire page →
From page 79... ... . Climate warming may cause land ecosystems to lose carbon because respiration is more temperature sensitive than photosynthesis, but there is a wide range of estimates for the climate sensitivity of land carbon stocks. Read the entire page →
From page 80... ... The C4MIP simulations did not address the full suite of interactions between the carbon cycle and other changes in the Earth System; for example, ocean carbon storage can be influenced by ozone driven changes in Southern Ocean winds (Le Quéré et al., 2008; Lovenduski et al., 2008; Lenton et al., 2009) Read the entire page →
From page 81... ... (f ) FIGURE 2.10 Predicted atmospheric CO2 and climate-carbon cycle feedback parameters. Read the entire page →

From page 59...

... . Here we briefly summarize how major forcing agents contribute to current and future CO2-equivalent target levels and explore implications for global mean temperature increases.