Is the climate warming?
Yes. Global surface temperatures have increased about 0.6°C (plus or minus 0.2°C) since the late-19th century, and about 0.4°F (0.2 to 0.3°C) over the past 25 years (the period with the most credible data). The warming has not been globally uniform. Some areas (including parts of the southeastern U.S.) have, in fact, cooled over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Warming, assisted by the record El Niño of 1997-1998, has continued right up to the present, with 2001 being the second warmest year on record after 1998. Linear trends can vary greatly depending on the period over which they are computed. Temperature trends in the lower troposphere (between about 2,500 and 26,000 ft.) from 1979 to the present, the period for which Satellite Microwave Sounding Unit data exist, are small and may be unrepresentative of longer term trends and trends closer to the surface.
Global surface temperatures have increased about 0.74°C (plus or minus 0.18°C) since the late-19th century, and the linear trend for the past 50 years of 0.13°C (plus or minus 0.03°C) per decade is nearly twice that for the past 100 years. The warming has not been globally uniform. Some areas (including parts of the southeastern U.S. and parts of the North Atlantic) have, in fact, cooled slightly over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Lastly, seven of the eight warmest years on record have occurred since 2001 and the 10 warmest years have all occurred since 1995.
Summary form only given. The balance of evidence suggests that the climate is not warming. A US National Research Council report has highlighted, but not explained, the disparity between different data sets: while a variety of surface thermometers report a substantial warming trend, microwave sensing units (MSU) on weather satellites, and also radiosondes carried in weather balloons, show little if any warming of the atmosphere in the last twenty years. As is well understood, however, climate models predict the opposite, namely a stronger warming trend for the atmosphere than for the surface. In further investigating this puzzling discrepancy involving different observing methods as well as theoretical models, the authors hypothesize that the recent surface trends may not be credible. They note that while there is general agreement that the global climate warmed before 1940, and then cooled slightly until about 1975, the well-controlled surface temperature data for the United States an
