Climate change not producing expected increase in atmospheric humidity in dry regions: study

The laws of thermodynamics dictate that a warmer atmosphere can hold more water vapor, but new research has shown that atmospheric humidity has not increased as expected in arid and semi-arid regions of the world in as the climate warmed.

The results are particularly puzzling because climate models predict the atmosphere will become wetter, even in dry regions. If the atmosphere is drier than expected, arid and semi-arid regions could be even more vulnerable to future wildfires and extreme heat than expected.

The authors of the new study, led by the National Center for Atmospheric Research (NSF NCAR) of the US National Science Foundation, are not sure what causes this discrepancy.

“The impacts could be potentially severe,” said Isla Simpson, NSF NCAR scientist and lead author of the study. “This is a global problem, and it’s something completely unexpected given the results of our climate model.”

Simpson and his co-authors say more research is needed to determine why water vapor isn’t increasing. The reasons could be that moisture is not moving from the Earth’s surface into the atmosphere as expected or that it is circulating in the atmosphere in unexpected ways. It is also possible that a completely different mechanism is responsible.

Adding to the mystery, the new study showed that although water vapor increases in humid regions of the world, it does not increase as much as expected during the driest months of the year.

The study appears in the Proceedings of the National Academy of Sciences.

A surprising discovery

A basic rule of climate science is that the atmosphere can hold more moisture as it warms. This is called the Clausius-Clapeyron relationship, and it’s why climate models consistently predict that atmospheric water vapor will increase as the planet warms.

But while Simpson was working on a report for NOAA in 2020 on climate change in the southwest United States, she realized that the atmosphere there had become much drier than anyone would expect. expected based on climate model simulations.

Intrigued, Simpson and his co-authors studied the atmosphere on a global scale to determine whether water vapor was increasing in line with climate projections. The research team looked to several sources of observations from 1980 to 2020. These included networks of weather stations as well as datasets that estimate humidity based on observations from sources such as weather balloons and satellites.

To their surprise, scientists found that water vapor over arid and semi-arid regions generally remained constant instead of increasing by almost 7% for every 1° Celsius (1.8° Fahrenheit) warming, as might be expected from the Clausius study. -Clapeyron relationship. Water vapor actually decreased in the southwestern United States, which experienced a long-term reduction in precipitation.

“This is contrary to all climate model simulations in which it increases at a rate close to theoretical expectations, even in dry regions,” the authors write in the new paper. “Given the strong links between water vapor and wildfires, ecosystem functioning, and temperature extremes, this problem must be addressed in order to provide credible climate projections for the world’s arid and semi-arid regions. “

The study noted that the situation leads to an increase in vapor pressure deficit, which is the difference between the amount of moisture the atmosphere can hold and the amount actually present in the air. When the deficit increases, it can become a critical driver of wildfires and stress on ecosystems.

“We could face even higher risks than predicted for arid and semi-arid regions like the Southwest, which have already been hit by unprecedented water shortages and extreme wildfire seasons “Simpson said.

She and her colleagues discovered a more complex situation in humid regions, where atmospheric water vapor increased as climate models projected during the wetter seasons. This increase leveled off somewhat during the drier months, but did not stabilize as much as in arid and semi-arid regions.

Looking for the culprit

As for the question of why water vapor in the atmosphere does not increase as expected in dry regions, the authors broadly suggest two possibilities: The amount of moisture moved from the Earth’s surface into the air may be lower than the models, or the way the atmosphere transports moisture to dry regions may differ from the models.

Problems with atmospheric transport are less likely, they conclude, because it would not necessarily explain the common behavior of all the world’s arid and semi-arid regions, which receive moisture from different locations.

This leaves the earth’s surface as the most likely culprit. The authors speculate on several possible causes: the earth may have less water available in the atmosphere in reality than in models, it may dry out more than expected as the climate warms, or plants may retain water. moisture more efficiently and release less into the atmosphere. the atmosphere.

The authors also considered the possibility of error in the observations. But they concluded that this was unlikely since the discrepancy is closely linked to the drought of regions around the world, and it is consistently found even when dividing the record into shorter time segments to avoid errors due to changes in temperature. ‘instrumentation.

Simpson stressed that more research is needed to determine the cause.

“This is a really tricky problem to solve because we don’t have global observations of all the important processes to tell us about how water is transferred from the Earth’s surface to the atmosphere,” he said. -she declared. “But we definitely need to understand what is wrong, because the situation is not what we expected and could have very serious consequences for the future.”