Many lines of evidence support the conclusion that most of the observed warming over at least the last several decades is due to human activities:
• Both the basic physics of the greenhouse effect and more detailed calculations using sophisticated models of atmospheric radiative transfer indicate that increases in atmospheric GHGs should lead to warming of the Earth's surface and lower atmosphere (NRC, 2005d).
Earth's surface temperature has unequivocally risen over the past 100 years, to levels not seen in at least several hundred years and possibly much longer (NRC 2006b), at the same time that human activities have resulted in sharp increases in CO2 and other GHGs (as discussed above). Detailed observations of temperatures, GHG increases, and other climate forcing factors from an array of instruments, including Earth-orbiting satellites, reveal an unambiguous correspondence between human-induced GHG increases and planetary warming over at least the past three decades, in addition to substantial year-to-year natural climate variability (Hegerl et al., 2007). The vertical pattern of atmospheric temperature change over the past few decades, with warming in the lower atmosphere and cooling in the stratosphere (see Figure 6.15), is consistent with the pattern expected due to GHG increases and inconsistent with the pattern expected if other climate forcing agents (e.g., changes in solar activity) were responsible (Roble and Dickinson, 1989). Estimates of changes in temperature and forcing factors over the first seven decades of the 20th century are slightly more uncertain and also reveal significant decadal-scale variability (see Figure 6.12), but nonetheless indicate a consistent relationship between long-term temperature trends and estimated forcing by human activities.
The horizontal pattern of observed surface temperature change over the past century, with stronger warming over land areas and at higher latitudes (Figure 6.13), is consistent with the pattern of change expected from a persistent positive climate forcing (see, e.g., Schneider and Held, 2001). Detailed numerical model simulations of the climate system (see the following section for a discussion of climate models) are able to reproduce the observed spatial and temporal pattern of warming when anthropogenic GHG emissions and aerosols are included in the simulation, but not when only natural climate forcing factors are included (Randall et al., 2007).
Both climate model simulations and reconstructions of temperature variations over the past several centuries indicate that the current warming trend cannot be attributed to natural variability in the climate system (Jansen et al., 2007; NRC, 2006b).
As discussed earlier in this chapter, estimates of climate forcing and temperature changes on a range of time scales, from the several years following volcanic eruptions to the 100,000+ year Ice Age cycles, yield estimates of climate sensitivity that are consistent with the observed magnitudes of observed climate change and estimated climate forcing.
• Finally, there is not any compelling evidence for other possible explanations of the observed warming, such as changes in solar activity (Lean and Woods, in press), changes in cosmic ray flux (Benestad, 2005), natural climate variability (Hegerl et al., 2007), or release of heat stored in the deep ocean or other climate system components (Barnett et al., 2005a).
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