Raper, 2001; Forest et al., 2002; Gregory et al., 2002; Anderson et al., 2003a).

SOLAR VARIABILITY

At present, the observational database is too short to detect long-term solar irradiance trends, nor is it possible to predict future solar activity with any skill. Empirical evidence suggests that the approximately 11-year activity cycle of the Sun might be expected to produce future changes in total solar irradiance of order 1 W m−2 or less. This is the change observed in the past three solar cycles, two of which are the second and third largest since 1610 (Lean, 2001).

Unknown is whether solar activity will increase or decrease in the future and how long-term secular changes, if they exist, might evolve. That current levels of solar activity are at overall high levels, according to both sunspot numbers and cosmogenic isotopes, suggests that future solar irradiance values may not be significantly higher than seen in the contemporary database. A projection of future solar activity based on spectral synthesis of the cosmogenic isotope record confirms that solar activity is presently peaking and in 2100 will reach levels comparable to those in 1990 (Clilverd et al., 2003). However, projections of combined 11-, 88-, and 208-year solar cycles suggest that an overall increase in solar activity from 2000 to 2030 will produce a top-of-the-atmosphere (TOA) climate forcing of +0.45 W m−2, followed by decreasing activity until 2090 with climate forcing −1 W m−2 (Jirikowic and Damon, 1994). In contrast, a numerical model of solar irradiance variability that combines cycles related to the fundamental 11-year cycle by powers of 2 predicts a 0.05 percent decrease in irradiance during the next two decades (Perry and Hsu, 2000), for which associated climate forcing is −0.1 W m−2. A lack of physical understanding of how the dynamo-driven solar activity produces the competing effects of sunspot blocking and facular brightening cautions against future predictions, even of 11-year cycle amplitudes.

LAND-USE AND LAND-COVER CHANGE

With continued population growth in the twenty-first century, the conversion of land into agricultural, urban, and other human uses will continue and could even accelerate. The dynamics of anthropogenic land-use change is quite complex and involves socioeconomic forcings and feedbacks (Lambin et al., 2003; Napton et al., 2003; Sohl et al., 2004), as well as climate forcings and feedbacks. For example, Figure 5-3 demonstrates how highway construction and other major infrastructure projects might contribute to change in Brazilian vegetation. The availability of satellite moni-



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