How a thirsty atmosphere is making global droughts much worse

Drought is closely tied to how warm air interacts with moisture. Warmer air can pull more water from surfaces, which is why your hair dries faster in dry air than in a steamy bathroom. It’s also why the tropics experience heavy rain while deserts rapidly lose water from their soils.

But now, this basic fact about warm air is causing bigger problems. A new study shows that the atmosphere’s growing thirst for water is making droughts more severe – even in areas where rainfall hasn’t changed.

The research, conducted by a team of scientists at the University of Oxford and UC Santa Barbara, paints a clear picture. Over the past 40 years, droughts have become 40% more severe around the world, largely because the air is demanding more water than it used to.

The hidden force behind drought

Traditionally, droughts are blamed on low rainfall. But there’s another force at work – the atmosphere’s evaporative demand, often called AED. It acts like a giant sponge, pulling moisture from soils, rivers, and plants faster than it can be replaced.

“Drought is based on the difference between water supply (from precipitation) and atmospheric water demand. Including the latter reveals substantial increases in drought as the atmosphere warms,” said co-author Chris Funk, director of the Climate Hazards Center at UC Santa Barbara.

As the air warms, it can hold more moisture. Even if the relative humidity stays the same, warmer air can draw more water. This tug-of-war between rainfall and evaporative demand has raised an important question: Which is increasing faster?

To answer this, the scientists joined forces to examine how AED is affecting droughts around the world.

Measuring the atmosphere’s thirst

Although experts have known about AED for years, there hadn’t been a careful, global measurement of its impact using real-world data. Without this, predicting and preparing for droughts remained difficult.

This new study changed that. The team used more than a century of high-resolution climate data. They applied sophisticated models that account for multiple climate factors – not just temperature.

“We face a big challenge,” explained lead author Solomon Gebrechorkos, a hydro-climatologist at the University of Oxford.

“There’s no direct way to measure how ‘thirsty’ the atmosphere is over time,” he said. “So, we used high-resolution climate data and applied the most advanced models for atmospheric evaporative demand – models that account for multiple climate variables, not just temperature.”

Drought increases faster than rainfall

The researchers compared water supply, based on precipitation, with AED using several top-tier global datasets. They standardized the data to make comparisons easier across wet and dry regions.

“This allowed us to compare wet and dry regions using a common framework,” Funk said.

The analysis showed that the atmosphere’s evaporative demand has increased faster than precipitation. This suggests a worrying trend toward drier conditions.

“I find these results very concerning, but perhaps not terribly surprising,” Funk said. “Most of us are familiar with how air temperatures are increasing rapidly, but most people may not realize the connections between this warming and the desiccating influence of the atmosphere.”

He added that just a small rise in temperature can dramatically boost the air’s ability to draw moisture from crops, rangelands, and forests.

Bracing for drier days ahead

This study supports earlier findings that droughts will likely grow more intense as the atmosphere warms. This isn’t just a problem for farmers. Droughts threaten food and water supplies and can increase political instability and conflict.

The link between increased AED and wildfires is even more direct. Drier plants mean bigger fires.

Looking ahead, the study emphasizes the importance of early warning systems and better drought management strategies. Predicting when AED will rise can help farmers and communities take action in advance.

For instance, farmers might turn to micro-irrigation or use soil treatments that help retain water.

“To counter increasing drought trends, we need to anticipate and manage the extreme events that lead to concerning increases in drought risk,” Funk said.

The team also wants to dig deeper into how evaporation and the atmosphere’s evaporative demand interact with water supplies, beyond just rainfall.

There’s a growing need to understand how farmers, cities, and ecosystems can adapt to an atmosphere that’s constantly demanding more moisture.

The full study was published in the journal Nature.

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