SPURIOUS TRENDS IN SATELLITE MSU TEMPERATURES FROM MERGING DIFFERENT SATELLITE RECORDS

Analysis of global surface air temperature records has indicated that recent years have been among the warmest since the late nineteenth cen tury(1), with 1995 being the warmest year on record(2), But the rate o f global annual mean surface warming of 0.13 degrees C per decade duri ng the period 1979-95 differs substantially from the global lower-trop ospheric cooling trend of -0.05 degrees C per decade(3) inferred from the record (MSU-2R) of radiance measurements by the satellite Microwav e Sounder Unit (MSU)(4,5). Accordingly, the satellite record has been widely cited by sceptics as evidence against global warming(6-10). How ever, a substantial fraction of the measured radiance originates not f rom the atmosphere but from the Earth's surface(11), and gives rise to high noise levels, This noise can lead to errors when merging tempera ture time series obtained from different satellites. Here we present c omparisons among different MSU retrievals, sea surface temperatures (S STs), and equivalent MSU temperatures derived from an atmospheric gene ral circulation model forced with observed SSTs. The comparisons, focu sed on the tropics where atmospheric temperatures are closely tied to SSTs, strongly suggest that two spurious downward jumps occur in the M SU-2R record coinciding with changes in satellites, and that the real trend in MSU temperatures is likely to be positive, albeit small.