Polar orbiting satellites have monitored the microwave emissions of atmospheric oxygen, a quantity proportional to temperature, since late 1978 utilizing the Microwave Sounding Unit (MSU) instrument. Temperatures from nine different spacecraft have been adjusted and merged into a single 20-year time series. New adjustments for orbit decay, diurnal drift and instrument-body temperature feedback have now been separately applied prior to merging and the resulting time series shows the trend in the lower troposphere is zero for 1979-97 while that of the lower stratosphere has declined at a significant rate of -0.6 C/decade. The warm ENSO of 1998 caused the warmest monthly anomalies of any observed to date, April and May being +0.7 C above the base period mean of 1982-91.
Various validation studies will be presented, including independent comparisons with large datasets of radiosondes along with independent regions of the NCEP and ECMWF Reanalyses all of which verify the MSU's calculated trends. In addition, recent studies which report possible errors in the datasets will be addressed.