The Southwest, already the driest region in the United States, has become even drier on the hottest days, according to new research since the mid-20th century.
Humidity has decreased in the summers for the past seven decades, research shows, and the decline has accelerated since 2000, a period of prolonged drought in the region.
Extreme heat combined with lower humidity increases the risk of forest fires, said Karen McKinnon, a climate scientist at the University of California, Los Angeles and lead author of an article in Nature Climate Change describing the research and findings.
“Days with high temperatures and low humidity help dry out the vegetation,” she said. “And the fire weather itself is worse.”
The region and other parts of the west have seen more severe fires and longer forest fire seasons as the climate warms. Another dangerous fire season is expected this summer, when much of the west has been hit by extreme drought.
The results of the study contradict a basic idea of climate change – that humidity will increase as the world warms from human-induced greenhouse gas emissions because warmer air contains more moisture. While this may apply globally, the study shows that there may be regions where the opposite is the case.
“There’s this basic expectation of higher humidity in an average sense,” said Dr. McKinnon. “It is all the more surprising that the southwest stands out as the region where the humidity drops in summer.”
But it makes sense, given the mechanism for the decline that Dr. McKinnon and her colleagues uncovered in their research.
In most parts of the world, evaporation from the sea is responsible for most of the humidity. “The ocean is an amazing source of moisture,” said Dr. McKinnon.
In the southwest, however, they found that soil moisture was the dominant source. “What we see on these hot days in the southwest is that the source of moisture is not the ocean, but the surface of the land,” she said.
Soil moisture is lower in summer and even lower with increasing temperatures. “You just don’t have the water to evaporate,” said Dr. McKinnon. This makes the air drier.
Dr. McKinnon described the lower humidity in summer as a “consequence” of the drought rather than a cause.
“If we think about the origins of the drought, we have to go back before summer,” she said of winter and spring. Soil moisture is higher during these seasons, but warming makes it drier than it used to be. This prepares the summer for very dry soils.
“It is clear that the fall in soil moisture in summer is due to the fall in soil moisture in winter and spring,” she said.
Park Williams, a climate scientist at the University of California at Los Angeles, had studied the humidity in the southwest years ago and found that it would decrease for a few decades.
“But I had no idea why,” said Dr. Williams, who was not involved in the current research. Dr. McKinnon’s study “solves a lurking question,” he said.
“This paper shows that it’s a pretty straightforward effect from drier soils,” he said.
Lower soil moisture should also lead to a rise in temperature, said Dr. Williams, as little or no moisture can evaporate and the evaporation has a cooling effect.
“We often see the most intense heat waves over dry soils,” he said. “Soil moisture in summer is shockingly low this year. The cubes are charged for extremely intense heat waves. “