Temperatures are rising, but soil is getting wetter, why?

The amount of water in the soil affects how fast fires and landslides can happen, and how well crops can grow. Some scientists worry that human-made climate change will make soils drier as the world gets warmer. But from 2011 to 2020, the soil was wetter in 57% of the US during the hottest season of the year. The finding is reported in the journal Nature Water.

Why can a warmer world be a wetter world?

Deke Arndt, Chief of NCEI’s Climatic Science and Services Division, explains, "It’s complicated because several atmospheric ingredients and processes end up causing precipitation.” Oversimplifying, he says, "A warmer atmosphere can hold more water vapour, and an atmosphere that holds more water vapour can produce more precipitation."

He told Ground Report that in parts of the world where water vapour currently limits the amount of precipitation, a warmer world would result in greater precipitation. This applies to much of the middle and higher latitudes of Earth.

A new study by researchers from Harvard University shows that rainfall, not temperature, is the main factor that determines soil moisture changes. This makes sense since more rain means more water in the soil. However, this study contradicts a common belief that warmer temperatures will cause soils to dry out.

Peter Huybers, a professor at Harvard University and the paper’s senior author, said that people often use atmospheric water to measure drought, but this paper shows the differences between soil and air hydroclimate and temperature.

"Atmospheric water has often been used as a proxy for drought, but this paper highlights distinctions between the hydroclimate of soils and the temperature and hydroclimate of the atmosphere," said Peter Huybers.

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

Soil has carbon. Farming and heat let out carbon. Rain may do that too

A study from 2017 said that farming has let out a lot of carbon from the soil in the last 12,000 years. About half of that carbon was lost in the last few hundred years. Scientists think that climate change is also bad for the soil.

New research shows that some soils lose carbon faster when it gets warmer. Scientists think that when it gets hotter, some tiny living things in the soil eat more organic matter and let out carbon.

A new study says that more rain may also make this happen faster. The study, which came out this week in Nature, looked at old records from a big river area in India going back almost 20,000 years. The study says that when it rained more in the past, the soil carbon changed faster.

That means more carbon goes into the air instead of staying in the soil. To do their study, the researchers—led by Christopher Hein from a marine science institute in Virginia—took samples of old dirt from deep in the ground. They did tests to find out how old the carbon in the dirt was and if the carbon age changed at different times in the past.

Older carbon, at any time, would mean that the carbon stayed in the soil longer. Younger carbon would mean that it changed faster. The older carbon went into the air, while newer carbon—from dead leaves or animals, for example—took its place.

Rainfall matters most for soil

The researchers discovered that CO2 fertilization, which helps plants save water, mostly offsets the drying from higher temperatures. These two effects are less important than rainfall and tend to balance each other out—making rainfall the main factor for soil moisture.

In their analysis, scientists found that internal precipitation variability emerged as the primary driver of observed soil moisture trends during the past decade. While surface air warming and the response of plants to rising atmospheric CO2 levels also played a role, their effects were minor over decadal timescales and often offset each other.

Researchers suggest that internal precipitation variability will continue to shape decadal soil moisture trends. However, they caution that predicting centennial soil moisture trends remains challenging, as they are largely contingent upon uncertain changes in precipitation patterns.

The study shows that predicting long-term changes in rainfall due to climate change is important for food production. So scientists are not sure how climate change changes the soil carbon, where it changes the most and how much heat or rain makes a difference.

In general, natural things that affect the climate are hard to predict for the future. Scientists also wonder how the world’s trees or seas will react to more heat and how much carbon they will take in or let out.

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