For observational temperature data, sensitivities to the choice of ENSO index and to uncertainties in key volcanic parameters lead to ranges of 'residual' trends at the surface (0.214 to 0.245oC/decade), in the lower troposphere (2LT; 0.068 to 0.186oC/decade), and in the surface-minus-lower troposphere trend differential (0.060 to 0.144oC/decade). Residual surface and lower tropospheric trends are invariably larger than trends in the raw obervational data. The residual surface-minus-2LT trend difference is generally smaller than in the raw data. These results suggest that volcanoes and ENSO significantly depressed both globally-averaged surface and tropospheric temperatures over 1979-1999, and exerted a larger cooling influence on the troposphere than on the surface.
ENSO and volcanic forcing effects can hamper reliable assessment of the true correspondence between modeled and observed trends. In the second part of our study, we use our iterative approach to remove these effects from model data. After removal, the residual modeled and observed global-mean surface temperature trends are in good agreement. For the lower troposphere, the statistical significance of differences between modeled and observed residual trends depends on the experiment considered, the choice of ENSO index, and the volcanic signal decay time. Residual surface-minus-2LT trend differences are significantly smaller than in observations in 11 out of 12 cases considered.
Finally, we consider results from recent model experiments with relatively complete estimates of natural and anthropogenic forcing. As for residual trends, we find close agreement between raw observed and simulated surface temperature trends. In 25 out of 30 cases, simulated lower tropospheric trends are not statistically different from observations. While simulated surface-minus-2LT trend differentials are significantly smaller than observed, they are positive (larger warming aloft than at the surface) in 5 of 30 cases, behavior qualitatively similar to observations.