Theory and modeling results predict that high-altitude clouds will occur at higher levels as the world warms. This rise is associated with longwave radiative feedback. Observations have not confirmed the prediction yet. This study uses the NASA Multi-angle Imaging and SpectroRadiometer (MISR) to examine changes in Cloud-top-height (CTH). The record is investigated for significant trends over large regions. Two climate models that produced MISR simulator output for a climate projection are used to investigate when trends might be detectable in the observations.
Current observations from MISR show a statistically significant trend in CTH over the Southern Ocean, but not in other regions. By carefully adjusting the variability in the models to account for their biases, an estimate of the Time-of-Emergence of trends is produced. The models differ by only a couple of years and indicate that trends in the tropics and subtropics, as well as the zonal mean in midlatitudes of both hemispheres, will be detectable within the next few years, likely by 2024.
Trends in high-altitude cloud-top height are expected to be robustly detected within the next few years. This work, along with previous results from other satellite records, indicates that a stable time series of 20-30 years is needed to confirm current trends are distinguishable from natural variability. This study uses satellite simulator output from CESM2 and IPSL-CM6A-LR to make an estimate of when MISR will be able to detect these trends. Both models indicate trends that should emerge in the next couple of years. The TERRA satellite that houses MISR, however, is running out of fuel and the orbit is drifting which might mean that MISR will not be able to confirm the trends. The replacement for MISR is planned as part of ACCP, but there may be a data gap depending on how long TERRA lasts.