1973-1996 trends in depth-averaged tropospheric temperature

The analysis of thickness derived from the National Center for Environmental Prediction (NCEP) reanalysis indicates that there has been no statistically significant warming trend in layer-averaged global tropospheric temperatures during the period 1979-1996. While this result is at variance with data based on surface information, NCEP trends and interannual variations are closely related to tropospheric mean-layer temperatures obtained from the satellite-based microwave sounding unit (MSU) lower-trospospheric data set. When a longer period, 1973-1996, is used for the NCEP analysis, a warming trend within a 95% confidence interval is derived. However, it is nearly half the magnitude of the surface-derived trends. Strong correlations of the yearly global surface temperature anomalies and those calculated for several layers in the NCEP data indicate coincident interannual temperature variations throughout the depth of the troposphere. The disagreement of both our analysis and satellite-derived estimates with surface data could be due to a land surface measurement bias, a result of land use changes such as deforestation and agricultural expansion whose effects are not yet accounted for in the global temperature records, and/or a decoupling between surface and lower-tropospheric temperature over the last several years.