Effects of orbital decay on satellite-derived lower-tropospheric temperature trends

The 17-year lower-tropospheric temperature record derived from the satellite Microwave Sounding Unit (MSU)^1-3 shows a global cooling trend, from 1979 to 1995, of -0.05 K per decade at an altitude of about 3.5 km (refs 4,5). Air temperatures measured at the Earth's surface, in contrast, have risen by approximately +0.13 K per decade over the same period^4,6. The two temperature records are derived from measurements of different physical parameters, and thus are not directly comparable. In fact, the lower stratosphere is cooling substantially (by about -0.5 K per decade)⁵, so the warming trend seen at the surface is expected to diminish with altitude and change into a cooling trend at some point in the trophosphere. Even so, it has been suggested that the cooling trend seen in the satellite data is excessive^4,7,8. The difficulty in reconciling the information from these different sources has sparked a debate in the climate community about possible instrumental problems and the existence of global warming^4,7,9. Here we identify an artificial cooling trend in the satellite-derived temperature series caused by previously neglected orbital-decay effects. We find a new, corrected estimate of +0.07 K per decade for the MSU-based temperature trend, which is in closer agreement with surface temperatures. We also find that the reported⁷ cooling of the lower trophosphere, relative to the middle troposphere, is another artefact caused by uncorrected orbital- decay effects.