As our planet continues to warm, the amount of rain that will fall in tropical regions will increase, research suggests.
The study found that global climate models may underestimate the amount of rain that will fall in these regions, because they underestimate decreases in high clouds over the tropics seen in recent NASA observations.
High-altitude tropical clouds trap heat in the atmosphere, but global warming appears to create fewer of these clouds, which would lead to the tropical atmosphere cooling – leading to increased tropical rainfall.
But how can fewer clouds lead to more rainfall?
Rainfall isn’t just related to the clouds that are available to make rain – but also to the Earth’s ‘energy budget’ – incoming energy from the sun compared to outgoing heat energy.
High-altitude clouds trap heat in the atmosphere, but if there are fewer of these clouds in the future, the tropical atmosphere will cool, and cooling water vapor condenses it, turning it into a liquid – rain.
The research, led by Dr Hui Su of NASA’s Jet Propulsion Laboratory in Pasadena, California, also found that the increased tropical rainfall would warm the air up again to balance the cooling from the high clouds shrinking.
Rainfall warming up the air also sounds counter-intuitive, as people are used to rain cooling the air around them.
However, several miles up in the atmosphere, a different process occurs.
When water evaporates into water vapor on Earth’s surface and rises into the atmosphere, it carries with it the heat energy that made it evaporate.
But when the vapor reaches the cold upper atmosphere, it condenses into liquid droplets or ice, releasing its heat and warming the atmosphere.
The study, published in the journal Nature Communications, says the decrease in high tropical cloud cover is one result of a planet-wide shift in large scale-scale air flows occurring as Earth’s temperature warms.
These air flows are called ‘atmospheric general circulation,’ and they include a wide zone of rising air centered on the equator.
Observations carried out over the last 30 to 40 years have shown that this zone is narrowing as the climate warns, causing the decrease in high clouds.
Dr Su, colleagues at the Jet Propulsion Laboratory, and researcher at four universities compared climate data from the past few decades with 23 climate model simulations of the same time period.
Simulating past conditions like this can help check how well the models are able to reproduce observations.
The researchers used data from NASA’s spaceborne Clouds and the Earth’s Radiant Energy System (CERES) – which measures outgoing thermal radiation – and other satellite instruments, as well as ground level observations.
Dr Su’s team found that most of the climate models they tested underestimate the rate at which rainfall would increase for each degree of surface warming that has occurred in recent decades.
The models that came closest to matching NASA’s current cloud observations of clouds showed a greater increase in rainfall for the future than other models.
‘This study provides a pathway for improving predictions of future precipitation change,’ said Dr Su.