ward to explain the substantial decadal-scale variability in the surface temperature record, especially the period of relatively flat temperatures from the early 1940s through the late 1970s. Probably the most widely cited hypothesis, which is supported by some statistical analyses and model simulations, is that increasing levels of sulfate aerosols from fossil fuel combustion introduced a cooling effect that offset much of the positive forcing from GHGs during the “flat” part of the record (e.g., Hegerl et al., 2007). This hypothesis seems to be supported by the more pronounced “flattening” in the Northern Hemisphere, relative to the more steady increase in the Southern Hemisphere (where aerosol levels are generally much lower). However, other recent analyses (e.g., Swanson et al., 2009) suggest that natural variations in ocean circulation might also give rise to some of the decadal-scale variations in the global temperature record.
The observed warming is also unevenly distributed around the planet (Figure 6.13). In general, the largest increases in temperature worldwide have occurred over land areas and over the Arctic, which is consistent with the horizontal pattern of warming expected from a positive climate forcing. In the continental United States, on average temperatures rose by 1.5°F (0.81°C) between the first decade of the 20th century and the first decade of the 21st century, or about the same as the global temperature change over this period. There is also a rich tableau of ongoing regional, seasonal, diurnal, and local temperature changes associated with these large-scale, long-term, annual-mean surface warming trends:
Recent analyses of temperature trends over the Midwest and northern Great Plains have revealed that winter temperatures in that region have increased by 7°F (4°C) over the past 30 years (USGCRP, 2009a).
Late spring and early summer daytime maximum temperatures in the southeastern United States, on the other hand, declined slightly from the 1950s to the mid-1990s (Portmann et al., 2009).
An analysis of daily temperature records reveals that during the last decade nearly twice as many extreme record high temperatures have been recorded globally than extreme record low temperatures (Meehl et al., 2009c).
Hot days and nights have become warmer and more common, while cold days and nights have become warmer and fewer in number (IPCC, 2007a).
Many of these changes are consistent with the spatial and temporal patterns of temperature change expected to result from increasing GHG concentrations.