Dry Regions Challenge Climate Change Moisture Predictions: Unraveling the Desert Paradox

Contrary to the predictions of climate models, latest studies reveal an unexpected outcome: atmospheric moisture has not substantially increased over arid and semi-arid regions as would be presumed with global warming. This observation, courtesy of SciTechDaily.com, stands in contrast to common climate science theories.

A recent study discovered that the predicted increase in atmospheric moisture due to global warming in dry regions is not taking place. This presents a fresh challenge for climate science and underlines the potential for amplified risks of wildfires and extreme weather.

Based on the principles of thermodynamics, a warmer atmosphere should hold more water vapor, but the new research identifies the absence of expected growth in atmospheric moisture over the more arid parts of the world as the climate warms.

This outcome is strikingly odd, since climate models have previously implied that the atmosphere would develop a higher moisture level, even in dry regions. Arid and semi-arid regions could therefore be more susceptible to future wildfires and severe heat than initially forecasted if the atmosphere is drier than expected.

Such an inconsistency leaves scientists from U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR), the leading authors of the study, perplexed.

Isla Simpson, an NSF NCAR scientist and the lead author, shares her concern, “This issue is global and quite unprecedented considering what our climate models have indicated so far. The impacts could be potentially severe”.

The authors suggest more research to comprehend why moisture levels have not increased. One reason could be that moisture does not circulate from the Earth's surface into the atmosphere as initially projected, or it could circulate in ways not yet identified. Another possible cause could be a totally different, previously unconsidered mechanism at play.

Adding to this puzzle, the new study indicates that while humid regions have seen an increase in water vapor, the rise is not as marked during the driest months of the year as initially expected.

The research, funded by the National Science Foundation, NOAA, and the U.S. Department of Energy, features collaborations from scientists from various institutions and appears in this week's Proceedings of the National Academy of Sciences.

Climate science predicts that as the atmosphere warms, it can contain more moisture - a fact known as the Clausius-Clapeyron relationship. This is the basis for models predicting an increase in water vapor as the climate warms.

However, while working on a 2020 NOAA report about climate change in the southwestern United States, Simpson noticed that atmospheric aridity was more significant than climate models had predicted.

In light of this revelation, Simpson's team investigated global atmospheric levels to verify if water vapor was following the trajectory of the model’s predictions. The team made extensive use of multiple data sources spanning a 40-year period from 1980 to 2020. Surprisingly, they found the moisture content over arid regions was not experiencing an expected increase of about 7% for each 1° Celsius (1.8° Fahrenheit) of warming as the Clausius-Clapeyron relationship would suggest. They even observed a decline in water vapor over the southwestern United States, an area experiencing a long-term reduction in precipitation.

"The results are counter to all climate model predictions, which anticipated a rise near the theoretical rate, even in arid regions," state the authors in their paper. As water vapor has a significant impact on wildfires, temperature extremes, and the overall functioning of ecosystems, they stress the need to resolve this issue to provide credible climate projections for arid and semi-arid regions.

The study disclosed that the increased vapor pressure deficit, the difference between how much moisture the atmosphere can hold and the actual moisture content, can boost wildfires and ecosystem stress. This could lead to even higher risk levels than initially predicted for arid and semi-arid regions like the Southwest, a region already affected by extreme water shortages and wildfire seasons, states Simpson.

Although they observed a rise in atmospheric water vapor in humid climates, it was in line with initial climate model projections only during the wetter seasons. Nevertheless, during the driest months, the increase was not as severe as it was in arid regions.

As for the question of why the water vapor in the atmosphere is not increasing over dry regions as expected, the authors broadly suggest two possibilities: the amount of moisture that is being moved from the land surface to the air may be lower than in models, or the way that the atmosphere is transporting moisture into dry regions may differ from the models. 

Issues with atmospheric transport are less likely, they conclude, because that wouldn’t necessarily explain the common behavior among all arid and semi-arid regions worldwide, which receive moisture from differing locations. 

That leaves the land surface as the most likely culprit. The authors speculate several possible causes: the land may have less water available to the atmosphere in reality than in models, it may be drying out more than anticipated as the climate warms, or plants may be holding on to moisture more effectively and releasing less into the atmosphere.

The authors also considered the possibility that there is an error in the observations. But they concluded this was unlikely since the discrepancy is closely tied to the dryness of regions all over the world, and it is consistently found even when dividing up the record into shorter time segments to avoid errors due to instrumentation changes. 

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

“It is a really tricky problem to solve, because we don’t have global observations of all the processes that matter to tell us about how water is being transferred from the land surface to the atmosphere,” she said.  “But we absolutely need to figure out what’s going wrong because the situation is not what we expected and could have very serious implications for the future.”