Natural signals in the MSU lower tropospheric temperature record

Volcanic and ENSO forcings explain nearly two-thirds (63.5%) of the variance in monthly global temperature anomalies in the MSU lower tropospheric temperature observations from 1979 through 1999. While the raw trend in the drift-adjusted 1979–99 global data is a statistically significant warming of 0.055°C/decade, the combination of volcanism and ENSO account for 25.4% of this trend, or 0.014°C/decade. The remaining trend, 0.041°C/decade, is left unexplained, likely comprising a combination of factors including the anthropogenic alterations to the collection of the earth's greenhouse gases, solar variability, and other forcings internal to the earth/atmosphere system. A recent widely cited attempt to explain the discrepancy between the rate of the observed increase in lower tropospheric MSU temperatures and the much larger value predicted by general circulation models weakens considerably when the known natural forcing mechanisms are more completely specified.