From -
http://www4.nationalacademies.org/news.nsf/isbn/0309068916?OpenDocument
"The differences between the surface and upper-air trends in no way invalidates the conclusion that the Earth's temperature is rising," said John M. Wallace, chair of the panel that wrote the report and professor of atmospheric sciences at the University of Washington in Seattle. "But the rapid increase in the Earth's surface temperature over the past 20 years is not necessarily representative of how the atmosphere is responding to long-term, human-induced changes, such as increasing amounts of carbon dioxide and other 'greenhouse' gases. The nations of the world should develop an improved climate monitoring system to resolve uncertainties in the data and provide policy-makers with the best available information."
While a combination of human activities and natural causes has contributed to rising surface temperatures, other human and natural forces may actually have cooled the upper atmosphere. For example, natural events such as the eruption of Mt. Pinatubo in 1991 tended to decrease atmospheric temperature for several years. And burning coal and oil for energy produces tiny aerosol particles in the atmosphere that can have a cooling effect. Upper-air temperatures also can be reduced by depletion of ozone in the stratosphere caused by chlorofluorocarbons and other chemicals being emitted into the atmosphere. When these variables are accounted for in atmospheric models, satellite and balloon data more closely align with surface-temperature observations."