Abstract
It is predicted by both theory and models that high-altitude clouds will
occur higher in the atmosphere as a result of climate warming. This
produces a positive longwave feedback and has a substantial impact on
the Earth’s response to warming. This effect is well established by
theory, but is poorly constrained by observations, and there is large
spread in the feedback strength between climate models. We use the NASA
Multi-angle Imaging SpectroRadiometer (MISR) to examine changes in
Cloud-Top-Height (CTH). MISR uses a stereo-imaging technique to
determine CTH. This approach is geometric in nature and insensitive to
instrument calibration and therefore is well suited for trend analysis
and studies of variability on long time scales. In this article we show
that the current MISR record does have an increase in CTH for
high-altitude cloud over Southern Hemisphere (SH) oceans but not over
Tropical or the Northern Hemisphere (NH) oceans. We use climate model
simulations to estimate when MISR might be expected to detect trends in
CTH, that include the NH. The analysis suggests that according to the
models used in this study MISR should detect changes over the SH ocean
earlier than the NH, and if the model predictions are correct should be
capable of detecting a trend over the Tropics and NH very soon (3 to 10
years). This result highlights the potential value of a follow-on
mission to MISR, which no longer maintains a fixed equator crossing time
and is unlikely to be making observations for another 10 years.