Anvil clouds are more than the beautiful giant umbrella shape for which they are known. Although they play a large role in Earth’s radiation balance, their effect on global warming is uncertain.

New research from atmospheric scientists at Lawrence Berkeley National Laboratory and UC Berkeley calls into question a conventional theory that these clouds in the upper tropical troposphere, one of the cloudiest places on Earth, could exacerbate rising global temperatures by maintaining a relatively constant temperature themselves.

As Earth warms, the distribution of clouds changes. Some of those changes amplify the original warming, while others damp it. Anvil clouds, which are the high clouds generated by deep convection, would tend to amplify warming if their area and temperature do not change as the planet warms. This absence of change would exacerbate global warming because the clouds would continue to send energy to space, in the form of longwave radiation, at the same rate regardless of how much the planet warms, thereby making it harder for the Earth to reach a new energy balance.

In a paper published online last month in the journal Geophysical Research Letters, EESA research scientist David Romps and colleagues evaluated the widely accepted Fixed Anvil Temperature (FAT) hypothesis that posits that anvil clouds do, indeed, maintain a constant temperature as the planet warms. The researchers tested the FAT hypothesis using a cloud-resolving model that simulates the radiative-convective equilibrium of the tropical atmosphere. This model allows scientists to simulate clouds at a spatial resolution of a few tens to hundreds of meters. Moreover, the authors’ simulations were the first to include only the basic physics on which the FAT hypothesis rests.

Rather than confirming that the clouds have a fixed temperature as hypothesized by FAT, the modeling revealed that the anvil clouds’ temperature varied. While the changes in the anvil temperatures were damped relative to changes in the sea-surface temperature (SST), they were far from invariant. In fact, the changes in the anvil temperatures were about 40% as large as the changes in the SST.

On further analysis, however, the researchers identified a different property of the atmosphere that had an invariant temperature.

“The results don’t support the FAT hypothesis,” said lead author Jacob Seeley, “but we do find this very intriguing result: that the interface between the troposphere and the stratosphere occurs at a temperature that is independent of Earth’s surface temperature.”

This interface — called the tropopause — caps the troposphere and abuts the stratosphere above it. It was this “lid” that had an invariant temperature, not the temperature of the anvil clouds below it. Based on this result, the researchers proposed the Fixed Tropopause Temperature (FiTT) hypothesis. The authors’ work suggests the importance of focusing future investigations on the tropopause, which affects many important aspects of the climate system, from the intensity of hurricanes to the amount of greenhouse-warming water vapor in the stratosphere.