Soil Heat Waves: An Emerging Threat to Our Ecosystem

We often picture sweltering city streets and parched landscapes when we think of heat waves. Unless we're barefoot, most of us experience heat waves through air temperature, and that's essentially how scientists track them, too. However, recent research reveals that this focus on air might have led us to overlook the problematic heat waves occurring under our feet in the soil.

"The world under our feet is heating up, and the consequences could be dire."

Almudena García-García, an Earth system scientist at the Helmholtz Centre for Environmental Research, has been studying how heat waves ripple through the ground, an area much less explored than air temperatures. Her recent findings published in the journal Nature Climate Change underline a troubling phenomenon: soil heat waves are becoming more extreme, and they could have significant implications for our food systems, water processing, and carbon sequestration and even contribute to higher air temperatures in a climatic feedback loop.

Soil vs Air: A Disturbing Trend

García-García's team collected data from 1996 to 2021 on Central Europe's air and soil temperatures. They found that heat extremes at two-thirds of the nearly 120 measuring stations grew more pronounced in the soil than in the air. Each decade, these extremes grew 0.7 degrees Celsius higher in the ground compared to the atmosphere. The number of days the earth experienced heat extremes increased twice as fast.

"This paper opens a lot of questions because now we see that there are differences between the evolution of soil and air heat extremes," says García-García. Understanding these differences could help predict agricultural failures, biodiversity changes, and other climate-change impacts on ecosystem activities.

The Complex Nature of Soil

Soil is a complex entity. No two patches of soil are alike in the world. Its composition can vary from high clay or sand content, different levels of carbon from plants, varying degrees of sunlight absorption, and diverse microbial communities. All these variables combine to determine how soil will heat when the sun rises over the local horizon.

Farmers have always been concerned about soil temperature. The right soil temperature determines if seeds will germinate. Today, with improved modeling and more affordable instruments for measuring temperature and moisture, predicting how soil might heat is more accessible. However, as global warming intensifies, scientists are keen to understand how soil temperatures might change and what that means for our ecosystem.

The Perilous Feedback Loop

As soil heats, it releases accumulated rainwater as vapor and cools the landscape. But once it's dry, it absorbs heat and begins releasing it into the air, much like concrete does in a city. This feedback loop powers the "heat domes" settling over the United States, drawing moisture from the soil and raising temperatures. These heat domes can continue for weeks and are worse in areas with little water in the ground.

"Our climate models predict that this contribution of low moisture of dry soils to heat extremes is only going to increase or strengthen in the future," says ETH Zurich atmospheric scientist Dominik Schumacher. As the climate changes, so will the soils themselves, organically, hydrologically, and microbially, which could disrupt the equilibrium of the soil and its ability to provide services like water storage and food production.

Impact on Carbon Cycle and Agriculture

Changes in soil temperature could disrupt the carbon cycle, where plants absorb CO2 as they grow and add it to the soil for microbes to process and release back into the atmosphere. If higher soil temperatures cause plants to suffer, they won't grow as much and won't capture as much carbon, leading to more carbon loss from the soil than it takes in.

Hot, dry conditions combined with soil that lacks organic matter can also lead to erosion and dust emissions, impacting agricultural growth and food availability. Soil microbial communities will change, too, affecting the functions of species that process carbon or phosphorus, turn nitrogen into fertilizer for plants, and help the roots take up water and nutrients.

Soil heat waves could become a repetitive stress on these underground communities, potentially reducing their capacity to deal with future events. As we continue to grapple with the impacts of climate change, the need to pay closer attention to the world beneath our feet becomes ever more pressing.