A closer look at United States and global surface temperature change

We compare the United States and global surface air temperature changes of the past century using the current Goddard Institute for Space Studies (GIS S) analysis and the U.S. Historical Climatology Network (USHCN) record [Kar l et al., 1990]. Changes in the GISS analysis subsequent to the documentati on by Hansen et al. [1999] are as follows: (1) incorporation of corrections for time-of-observation bias and station history adjustments in the United States based on Easterling et al. [ 1996a], (2) reclassification of rural, small-town, and urban stations in the United States, southern Canada, and northern Mexico based on satellite measurements of night light intensity [I mhoff et al., 1997], and (3) a more flexible urban adjustment than that emp loyed by Hansen et al. [1999], including reliance: on only unlit stations i n the United States and rural stations in the rest of the world for determi ning long-term trends. We find evidence of local human effects ("urban warm ing") even in suburban and small-town surface air temperature records, but the effect is modest in magnitude and conceivably could be an artifact of i nhomogeneities in the station records. We suggest further studies, includin g more complete satellite night light analyses. which may clarify the poten tial urban effect. There are inherent uncertainties in the long-term temper ature change at least of the order of 0.1 degreesC for both the U.S. mean a nd the global mean. Nevertheless, it is clear that the post- 1930s cooling was much larger in the United States than in the global mean. The U.S. mean temperature has now reached a level comparable to that of the 1930s, while the global temperature is now far above the levels earlier in the century. The successive periods of global warming (1900-1940), cooling (1940-1965), and warming (1965-2000) in the 20th century show distinctive patterns of t emperature change suggestive of roles for both climate forcings and dynamic al variability. The U.S. was warm in 2000 but cooler than the warmest years in the 1930s and 1990s. Global temperature was moderately high in 2000 des pite a lingering La Nina in the Pacific Ocean.