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As with observed climate change to date, regional manifestations of future climate projections vary substantially, with stronger warming over higher latitudes and land areas (Figure 6.20). The similarity between the three panels on the left-hand side of Figure 6.20 also illustrates how temperature increases over the next few decades reflect past emissions as well as somewhat similar GHG emissions over the next few decades for the three selected SRES scenarios (none of which include explicit policy interventions). By midcentury and especially at the end of the century, however, the medium- and high-emissions scenarios clearly lead to much warmer temperatures than the lower-emissions scenario. U.S. temperatures are projected to warm substantially over the 21st century under all emissions scenarios (USGCRP, 2009a).

FIGURE 6.19 Model simulations of changes in global average temperature from 1900 to 2300. The black line and gray shading shows the average and spread1 of 23 model simulations of 20th-century climate using estimates of actual climate forcing. The colored lines and shading show average and spreads for projected global average temperatures for the 21st century under four different scenarios of future forcing: a “high-emissions” scenario (red), a “medium-high” scenario (green), a “low-emissions” scenario (blue), and a “commitment” scenario (orange), which assumes that GHG concentrations remain constant at year 2000 values. The green and blue curves also show commitment experiments for the 22nd and 23rd centuries (i.e., with the forcing at year 2100 held constant thereafter). Changes are relative to the 1960-1979 average. See text for additional discussion. SOURCE: Meehl et al. (2007a).

FIGURE 6.19 Model simulations of changes in global average temperature from 1900 to 2300. The black line and gray shading shows the average and spread1 of 23 model simulations of 20th-century climate using estimates of actual climate forcing. The colored lines and shading show average and spreads for projected global average temperatures for the 21st century under four different scenarios of future forcing: a “high-emissions” scenario (red), a “medium-high” scenario (green), a “low-emissions” scenario (blue), and a “commitment” scenario (orange), which assumes that GHG concentrations remain constant at year 2000 values. The green and blue curves also show commitment experiments for the 22nd and 23rd centuries (i.e., with the forcing at year 2100 held constant thereafter). Changes are relative to the 1960-1979 average. See text for additional discussion. SOURCE: Meehl et al. (2007a).

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 The spreads in this figure indicate the 90 percent statistical confidence range of the model experiments (i.e., the annual average temperature traces from 90 percent of the included model experiments fall within the shaded bands). This spread is indicative of the uncertainty that the underlying models and forcing scenarios are able to resolve, but not the unresolved uncertainties discussed in the next section.



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