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 century1, with 1995 being the warmest year on record2. But the rate of global annual mean surface warming of 0.13 °C per decade during the period 1979–95 differs substantially from the global lower-tropospheric cooling trend of – 0.05 °C per decade3 Inferred from the record (MSU-2R) of radiance measurements by the satellite Microwave Sounder Unit (MSU)4,5. Accordingly, the satellite record has been widely cited by sceptics as evidence against global warming6–10. However, a substantial fraction of the measured radiance originates not from the atmosphere but from the Earth's surface11, and gives rise to high noise levels. This noise can lead to errors when merging temperature time series obtained from different satellites. Here we present comparisons among different MSU retrievals, sea surface temperatures (SSTs), and equivalent MSU temperatures derived from an atmospheric general circulation model forced with observed SSTs. The comparisons, focused on the tropics where atmospheric temperatures are closely tied to SSTs, strongly suggest that two spurious downward jumps occur in the MSU-2R record coinciding with changes in satellites, and that the real trend in MSU temperatures is likely to be positive, albeit small.