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Sass et al. (2002) measured emissions at a single site in the 25 U.S. over nine years and observed a year-to-year flux variability of approximately ±50% of the annual mean 26 over the entire period.

In the last 50 years, several regions of the world have seen cropland 4 areas stabilize, and even decrease (Figure 2.17). In the U.S.A., as cultivation shifted from the east to the 5 Midwest, croplands were abandoned along the eastern seaboard around the turn of the century, the eastern 6 forests have undergone a regeneration over the last century.

Hence although the overall sign of the trends is robust, the 22 details of humidity trends are sensitive to network choice, suggesting local influences, and time period. For -1 23 1961-1990, annual mean U.S. dew point temperature trends were 0.22 K decade but data after 1990 were 24 not included because of the likelihood of discontinuities caused by the introduction of automated 25 measurement systems.

The positive PNA is associated with an enhanced Aleutian 3 Low, a strengthened and extended Asian jet, and a tendency for the Pacific storm track to extend farther east 4 and equatorward, resulting in enhanced precipitation in California and relatively dry, warm conditions over 5 the northwest U.S. and southwestern Canada. During the negative PNA, the Pacific storm track curves north, 6 favouring wintertime blocking events over the Alaskan region, and an increased frequency of cold air 7 outbreaks over the western United States (Compo and Sardeshmukh, 2004).

Meehl and Tebaldi (2004) 17 showed that the pattern of future changes of heat waves, with greatest increases of intensity over western 18 Europe and the Mediterranean, the southeast and western U.S., was related to base state circulation changes 19 due to the increase in GHGs. Schär et al. (2004), Stott et al. (2004) and Beniston (2004) used the European 20 2003 heat wave as an example of the types of heat waves that are likely to become more common in a future 21 warmer climate.

The longest-lasting and most comprehensive such effort has 50 been the Atmospheric Radiation Measurement (ARM) Program in the U.S.A., which has established 51 elaborately instrumented observational sites to monitor the full complexity of cloud systems on a long-term 52 basis (Ackerman and Stokes, 2003).

However, the PSA pattern is 13 present at all times of year, lying from Australasia over the southern Pacific and Atlantic (Kiladis and Mo, 14 1998; Mo and Higgins, 1998; Kidson, 1999; Mo, 2000). 15 16 The positive PNA, or a variant of that (Straus and Shukla, 2002), is associated with an enhanced Aleutian 17 Low, a strengthened and extended Asian jet, and a tendency for the Pacific storm track to extend farther east 18 and equatorward, resulting in enhanced precipitation in California and relatively dry, warm conditions over 19 the northwest U.S. and southwestern Canada. During the negative PNA, the Pacific storm track curves north, 20 favouring wintertime blocking events over the Alaskan region, and an increased frequency of cold air 21 outbreaks over the western United States (Compo and Sardeshmukh, 2004).

Hsu et al. (2003) 24 used SeaWiFs, TOMS and CERES data to show that biomass burning aerosol emitted from S.E. Asia is 25 frequently lifted above cloud leading to a reduction in outgoing solar radiation over cloudy areas by up to -2 26 100 W m and points out that this effect could be due to a combination of direct and indirect effects. 27 Similarly, Haywood et al. (2003a) showed that remote sensing of cloud liquid water and effective radius 28 underlying biomass burning aerosol off the coast of Africa are subject to potentially large systematic biases. 29 This may have important consequences for studies that use correlations of aerosol optical depth and cloud 30 effective radius in estimating the indirect radiative effect of aerosols. 31 32 That the biomass burning direct RF can exert a significant positive direct RF when above cloud is 33 documented by the non-AEROCOM and AEROCOM models in Table 2.4.4.

Since the TAR, coupled ocean-atmosphere 35 models have been used to simulate the LGM (Figure 6.2; Table 6.1), although with varying prescriptions of 36 the changed forcings (Kitoh et al., 2001; Hewitt et al., 2003; Kim et al., 2003; S.I. Shin et al., 2003; Peltier 37 and Solheim, 2004). These simulations generally produce a global cooling that falls within the range of the 38 PMIP-1 results that were discussed in the TAR (IPCC, 2001), except for one simulation that exhibits very 39 cold conditions, 10°C cooling (Kim et al., 2003).

Meehl and Tebaldi (2004) showed that the pattern of 22 future changes of heat waves, with greatest increases of intensity over western Europe and the 23 Mediterranean, the southeast and western U.S., was related in part to base state circulation changes due to the 24 increase in GHGs. An additional factor for extreme heat is drier soils in a future warmer climate (Brabson et 25 al., 2005; Clark et al., 2006).