Projections of 21stCentury Climate

The most comprehensive suite of climate modeling experiments performed to date were completed in 2005 as part of the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3; Meehl et al., 2007b) in support of the IPCC's Fourth Assessment Report. CMIP3 included 23 different state-of-the-art models from groups around the world, all of which were run with a specific set of emissions scenarios (based on the SRES report described above) to facilitate comparison and synthesis of results. As described in detail by the IPCC (Meehl et al., 2007a), the CMIP3 climate models project increases in mean surface temperatures over the 21st century ranging from 2.0°F to 11.5°F (1.1°C to 6.4°C), relative to the 1980-1999 average, by the end of the century.

Figure 6.19 shows projected global temperature changes associated with three representative scenarios of high, medium-high, and low future GHG emissions. The separation between the three curves illustrates the uncertainty associated with the choice of scenario, while the uncertainties associated with differences among different models in simulating the climate system can be inferred from the shading surrounding each curve. The "commitment warming" associated with emissions through the year 2000 and, for two of the future forcing scenarios, through 2100 are also shown. These "commitment" runs, which are performed by instantaneously stabilizing atmospheric GHG concentrations, show that the climate system will continue to warm for several centuries after GHG emissions are stabilized—illustrating the inherent time lag between GHG emissions and the long-term climate response.

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 spreadl 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).

1900 2000 2100 2200 2300

Year

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 spreadl 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).

1 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.

B1:201t-2030 B1: 2046-2065 B1 : 2080-2099

B1:201t-2030 B1: 2046-2065 B1 : 2080-2099

A1B: 2011 -2030 A1B: 2046-2065 A1B: 2080-2099
FIGURE 6.20 Worldwide projected changes in temperatures, relative to 1961-1990 averages, under three different emissions scenarios (rows) for three different time periods (columns). Projected warming is much stronger over land areas and high latitudes. SOURCE: Meehl et al. (2007a).

In addition to average temperature, a host of other climate variables are projected to experience significant changes over the 21st century, just as they have during the past century. For example, the frequency and intensity of heat waves is projected to continue to increase, both in the United States (Figure 6.21) and around the world. This projection is considered robust because a shift in the average value of a temperature distribution (or in another climate variable) typically entails an increase in the frequency of extreme and unprecedented events (see, e.g., Solomon et al., 2007). Similarly, there is considerable confidence that the frequency of cold extremes will decrease and that the number of frost days will decline in the middle and high latitudes, following current trends (Meehl et al., 2007a; USGCRP, 2009a). Projections of future climate also indicate that snow cover and sea ice extent will continue to decrease (Meehl et al., 2007a; USGCRP, 2009a; Zhang, 2010), while sea level will continue to rise (see Chapter 7).

Projections of precipitation change are generally more uncertain than projections of temperature and temperature-related changes. However, most models project increased precipitation in northern regions of the United States, while it is considered very likely that the southwestern United States will experience a net decrease in precipitation (USGCRP, 2009a). Another robust projection, which results from the fact

FIGURE 6.21 Projected changes in number of very hot days in the United States for lower- and higher-emissions scenario. The number of very hot days will increase substantially across virtually the entire country, in some places doubling or even trebling the number of days above 90°F. SOURCE: USGCRP (2009a).

Recent Past (1951-1971 Average)

Recent Past (1951-1971 Average)

Projected End-of-Cenfury under Lower Emissions Scenario*1 (2030-2099 Average!
Piojected Ertd-of-Ceriiury under Higher Emissions Scenario'1 (2080-2039 Average)

Number of Days per Year

0 15 3C 45 «0 75 90 105 120 1Î5 ISO 165 180 »1«

CMIP3-B

Number of Days per Year

0 15 3C 45 «0 75 90 105 120 1Î5 ISO 165 180 »1«

that warmer air can hold more moisture, is that the fraction of rainfall falling in the form of heavy precipitation events will increase in many regions (Meehl et al., 2007a). These and other projected changes in precipitation, and the impact of these changes on freshwater resources, are explored in Chapter 8. Later chapters also explore how changes in temperature, precipitation, and other aspects of the physical climate system are likely to affect ecosystems (see Chapter 9), agriculture (Chapter 10), human health (Chapter 11), the urban environment (Chapter 12), transportation (Chapter 13) and energy systems (Chapter 14), and national security (Chapter 16).

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