Changes Observed in Cloud-Top Heights by MISR From 2002 to 2021

We analyzed cloud heights measured by the Multiangle Imaging SpectroRadiometer on the Terra satellite from 1 April 2002–31 March 2021. Throughout these 19 years, the equatorial crossing time of Terra's orbit varied by less than 1 min from its mean value. This variation created a homogeneous time series of deseasonalized and deregionalized height anomalies unaffected by sun-glint position changes. We analyzed the changes in effective cloud height (i.e., the integral of cloud occurrence weighted by cloud-top height) and the influence of all altitudes, from the surface to 20 km, on the effective height. We show that cloud fractions tended to decrease at low altitudes, especially in the tropics, and to increase at high altitudes for most latitudes. The globally effective height has very likely risen, at an average rate of about 1 m/yr. The rise was most significant at high latitudes, reaching 5 ± 1 m/yr between 45°N and 65°N. Tropical high clouds have risen, but this was offset by a reduction in tropical low clouds, resulting in an insignificant change in tropical effective height. The increase in effective height typically reduces the outgoing longwave radiation, thereby augmenting the cloud greenhouse effect. The observed height increase may be an adjustment to the radiative forcing of the stratosphere over 19 years that presumably causes changes in upper tropospheric stability, the tropopause, the Brewer-Dobson circulation, etc. If so, the observed height increase should be included in modeling the effective radiative forcing.