By Carol Rasmussen,
NASA’s Jet Propulsion Laboratory
A new study suggests that most global climate models may underestimate the amount of rain that will fall in Earth’s tropical regions as our planet continues to warm. That’s because these models underestimate decreases in high clouds over the tropics seen in recent NASA observations, according to research led by scientist Hui Su of NASA’s Jet Propulsion Laboratory in Pasadena, California.
Wait a minute: how can fewer clouds lead to more rainfall? Globally, rainfall isn’t related just to the clouds that are available to make rain but also to Earth’s “energy budget” — incoming energy from the sun compared to outgoing heat energy. High-altitude tropical clouds trap heat in the atmosphere. If there are fewer of these clouds in the future, the tropical atmosphere will cool. Judging from observed changes in clouds over recent decades, it appears that the atmosphere would create fewer high clouds in response to surface warming. It would also increase tropical rainfall, which would warm the air to balance the cooling from the high cloud shrinkage.
Rainfall warming the air also sounds counterintuitive — people are used to rain cooling the air around them, not warming it. Several miles up in the atmosphere, however, a different process prevails. When water evaporates into water vapor here on Earth’s surface and rises into the atmosphere, it carries with it the heat energy that made it evaporate. In the cold upper atmosphere, when the water vapor condenses into liquid droplets or ice particles, it releases its heat and warms the atmosphere.
The new study is published in the journal Nature Communications.