Other Indicators of Climate Change

Additional direct indicators of a warming trend over the last several decades can be found in the cryosphere and oceans. As discussed in detail in Chapter 7, the vast majority of the total heating associated with human-caused GHG emissions has actually gone into the world's oceans, which have warmed substantially over the last several decades (Levitus et al., 2009). In the cryosphere, mountain glaciers and icecaps are melting (these changes are also discussed in detail in Chapter 7), rivers and lakes are thawing earlier and freezing later in the year (Rosenzweig et al., 2007), and winter snow cover (Trenberth et al., 2007) and summer sea ice (Figure 6.15) are both decreasing in the Northern Hemisphere. Analyses of recently declassified data from naval submarines (as well as more recent data from satellites) show that the aver age thickness of sea ice in the Arctic Ocean has declined substantially over the past half-century, which is yet another indicator of a long-term warming trend (Kwok and Rothrock, 2009). Warming can also be inferred from a host of ecosystem changes: flowers are blooming earlier, bird migration and nesting dates are shifting, and the ranges of many insect and plant species are expanding poleward and to higher elevations— these and other trends in biological systems are discussed in detail in Chapter 9.

Scientists have collected a wide array of indirect evidence of how temperature and other climate properties varied before instrumental measurements became available. These so-called "proxy" climate data are derived from a diverse range of sources including ice cores, tree rings, corals, lake sediments, records of glacier length, borehole temperature measurements, and even historical documents. A recent assessment of these data and the techniques used to analyze them (NRC, 2006b) concluded that, although proxy data generally become scarcer, less consistent, and more uncertain going back in time, temperatures during the past few decades were warmer than during any other comparable period for at least the last 400 years, and possibly for the last 1,000 years or longer (Figure 6.16). Proxy-based temperature and forcing estimates for the past millennium, and for longer time periods such as the Ice Age cycles described above, illustrate the natural variability of the climate system on a wide range of time scales. These estimates are also used to help constrain estimates of climate sensitivity.

While temperature and temperature-related changes are the most widely cited and typically the best-understood changes in the physical climate system, a host of concomitant and related changes have also been observed. For example, the absorption of CO2 by the oceans is causing widespread ocean acidification, with significant implications for natural ecosystems and fisheries (as discussed in Chapters 9 and 10, respectively). There have also been significant changes in the overall amount, patterns, and timing of precipitation both globally and in the United States, and the characteristics of these precipitation changes are consistent with what would be expected for GHG-induced warming (see Chapter 9). A number of changes in atmospheric circulation patterns have also been observed (e.g., Fu et al., 2006).

Finally, it should be noted that the observed changes in the climate system to date represent only a fraction of the total expected changes associated with the GHGs currently in the atmosphere: Even if the current climate forcing were to persist indefinitely, it is estimated that the Earth would warm another 0.6°C (1.1°F) over the next several decades as the oceans slowly warm in response to the current GHG forcing, with concomitant changes in other parts of the Earth system (this so-called "commitment warming" is discussed in further detail below). In addition, since CO2 and many other GHGs remain in the atmosphere for hundreds or even thousands of years

-Borehole temperatures (Huang et al. 2000) -Multiproxy (Mann and Jones 2003a) Multiproxy (Hegerl et al. 2006) -Instrumental record (Jones et al. 2001)

-Glacier lengths (Oerlemans 2005b) -Multiproxy (Moberg et al. 2005a) -Tree rings (Esper et al. 2002a)

-Borehole temperatures (Huang et al. 2000) -Multiproxy (Mann and Jones 2003a) Multiproxy (Hegerl et al. 2006) -Instrumental record (Jones et al. 2001)

-Glacier lengths (Oerlemans 2005b) -Multiproxy (Moberg et al. 2005a) -Tree rings (Esper et al. 2002a)

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FIGURE 6.16 Estimates of surface temperature variations for the last 1,100 years derived from different combinations of proxy evidence (colored lines). Each curve portrays a somewhat different history of temperature variations and is subject to a somewhat different set of uncertainties that generally increase going backward in time (as indicated by the gray shading), but collectively these data indicate that the past few decades were warmer than any comparable period for at least the last 400 years, and possibly for the last 1,000 years. SOURCE: NRC (2006b).

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FIGURE 6.16 Estimates of surface temperature variations for the last 1,100 years derived from different combinations of proxy evidence (colored lines). Each curve portrays a somewhat different history of temperature variations and is subject to a somewhat different set of uncertainties that generally increase going backward in time (as indicated by the gray shading), but collectively these data indicate that the past few decades were warmer than any comparable period for at least the last 400 years, and possibly for the last 1,000 years. SOURCE: NRC (2006b).

(Solomon et al., 2009), an additional 2.5°F (1.4°C) of global warming is possible over the next several centuries due to ice sheet disintegration, vegetation change, and other long-term feedbacks in the climate system (Hansen et al., 2008). However, these processes are generally less well understood than the feedbacks that give rise to climate change on shorter (e.g., decadal) time scales.

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