Temperatures are rising, but the soil is becoming wetter: why?

Soil moisture can determine how quickly a wildfire spreads, how quickly a hillside turns into a mudslide, and, perhaps most importantly, the productivity of our food systems. As temperatures rise due to human-caused climate change, some researchers worry that soils will dry out. However, between 2011 and 2020, soil moisture increased across 57% of the United States during summer, the hottest time of the year.

Why has the soil become wetter even as the planet warms?

A recent study by Harvard University researchers found that precipitation, rather than temperature, largely explains soil moisture trends. While it’s not surprising that more rain means wetter soils, the research challenges the long-held assumption that rising global temperatures would lead to drier soils.

The work is published in the journal Natural water.

“Atmospheric water has often been used as an indicator of drought, but this paper highlights the distinctions between the hydroclimate of soils and the temperature and hydroclimate of the atmosphere,” said Peter Huybers, professor of science at Earth and planets at the Faculty of Arts and Sciences. and Environmental Sciences and Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences, and lead author of the paper.

The research team found that drying due to higher temperature was largely offset by CO₂ fertilization, which allows plants to use water more efficiently. These two effects are secondary to precipitation and tend to cancel each other out, leaving precipitation as the primary driver of soil moisture.

One of the challenges of studying soil moisture is the scarcity of data and the frequent disconnect between satellite data and ground-level observations. The team compared ground-level observations between 2011 and 2020 – the short period when many soil moisture measurements are available in the United States – with satellite data and found a similar increase in soil moisture. soil moisture.

These results highlight the importance of improving predictions of long-term changes in precipitation in response to climate change, particularly as it relates to food production.

“We don’t have very precise measurements of long-term soil moisture, but the consequences of high temperatures on agricultural yields have a lot to do with water availability,” said Lucas Vargas Zeppetello, a researcher at the Harvard University Center for the Environment and is the first author of the study. “Plants are generally less sensitive to temperature if they have enough water, but in dry conditions they can have big problems.”

Vargas Zeppetello is now an assistant professor at UC Berkeley.

“Our results suggest that reductions in surface soil moisture are far from inevitable given the uncertainty in global precipitation trends,” Huybers said. “With uncertainties in interannual precipitation variability and uncertainties in long-term precipitation forecasts, it is virtually impossible to predict soil moisture in coming decades.”

This uncertainty makes it difficult to predict crop growing conditions, making it all the more important to focus on water management strategies, Vargas Zeppetello said.

The research was co-authored by Aleyda M. Trevino, who received her Ph.D. He received his PhD from Harvard in 2023 and is currently a postdoctoral researcher at San Francisco State University.