Most global climate models may underestimate the amount of precipitation that will fall out in tropical regions as the planet warms up further. This is because these models do not take into account the reduction of high-altitude clouds over the tropics, as recorded in recent NASA observations.
You probably think how a decrease in clouds can lead to more precipitation? The fact is that on a global scale, precipitation is associated not only with the presence of rain clouds, but also with the “energy budget” of the Earth – the energy coming from the sun, in comparison with the outgoing heat energy.
High-altitude tropical clouds catch heat in the atmosphere. If in the future there will be less of them, the tropical atmosphere will cool down. Judging by the observed changes in clouds over the past decades, the atmosphere has reduced the formation of clouds in response to surface warming. This entails an increase in the amount of tropical rainfall that warms the air to balance cooling from shrinking high-altitude clouds.
Precipitation that warms the air – it seems, too, sounds illogical, because people are used to the fact that the rain is connected with the coolness. However, in the atmosphere at an altitude of several kilometers, a different process prevails. When the water evaporates here, on the surface of the Earth, and rises into the atmosphere, it carries a thermal energy, because of which it evaporates. In the cold upper atmosphere, when water vapor condenses into liquid droplets or ice particles, it releases its heat and heats the atmosphere.
Reducing the tropical cloud cover is one of the consequences of global airflow shifts that occur when the temperature of the Earth’s surface rises. These large-scale flows are called the general circulation of the atmosphere and include a wide zone of ascending air concentrated on the equator. Observations over the last 30-40 years have shown that this zone narrows as the climate warms, causing a decrease in high-altitude clouds.
Scientists from NASA’s Jet Propulsion Laboratory and four universities compared climate data over the past few decades with 23 climate models of the same period. Climate models use a retrospective to check how well their numerical schemes are able to reproduce observations. To collect data, the team used observations of the outgoing heat radiation obtained with NASAs spaceborne Clouds, Earths Radiant Energy System (CERES) and other satellite instruments, as well as ground equipment.
They found that most climate models underestimate the rate of increase in rainfall for every degree of surface warming that has occurred in recent decades. Models that were closest to cloud observations in today’s climate showed a greater increase in precipitation in the future than other models.
A study called Tightening of Tropical Ascent and High Clouds was published in the journal Nature Communications.