Observed decreasing trend in the upper-tropospheric cloud top temperature

Obtaining the response of cloud top temperature (CTT) to global warming correctly is crucial for understanding the current and future energy budget of the climate system. For a given cloud fraction, colder CTT implies more longwave radiation being capped within the Earth-atmosphere system, consequently heating it. Current climate models predict an almost fixed CTT for upper-tropospheric clouds as the climate is expected to warm up during the 21st century, as explained by the fixed anvil temperature hypothesis. However, our analysis, based on the last 19 years of satellite observations (12.2002-11.2021), reveals a significant decreasing trend in upper-tropospheric CTT with almost no change in the corresponding cloud fraction. This cooling rate is several times larger than the observed surface warming rate. This finding suggests a missing heating component by upper-tropospheric clouds in current climate predictions.