Kolumne "Zur Sache"

What soil moisture can teach us about heatwaves

Jasper Denissen © J. Denissen

Plants are the connection between the Earth’s surface and the atmosphere. They can severely dampen extreme temperatures – if the soil moisture is right.

An editorial view from Jasper Denissen, Max Planck Institute for Biogeochemistry

I’d like to introduce my research with a thought experiment: Given the same amount of sunlight, would it be hotter in the desert or in a forest? In the desert, for sure. But why? In the forest there are plants, that can reach the moisture in the soil with their roots and transpire it into the atmosphere through their leaves. So, the available energy from the sunlight is used to heat up the air and at the same time transpire moisture into the atmosphere. Whereas in the desert, the same amount of sunlight reaching the Earth’s surface is used only for heating up the air. Hence, the desert is hotter than the forest. Through this mechanism, figuratively, not technically speaking, the forest could behave as an air conditioning system.

But this is not the entire story. Plants will transpire at their maximum rate until the soil gets so dry that transpiration is decreased: this is called critical soil moisture. The dryer the soil gets from the critical soil moisture, the more sunlight will be used for heating up the air and less and less for plant transpiration, which eventually will increase the temperature in the forest. This critical soil moisture is a very important property, for example, for forecasting how hot a heatwave will be: A dryer soil in the forest leads to higher temperatures during heatwaves than wetter soils.

In my research I try to determine this critical soil moisture on a continental spatial scale using satellite data. Satellites can be used to 1) get an idea of how wet the soil is, 2) how green the plants are, which relates to how much plants are transpiring and 3) how much sunlight reaches the Earth’s surface. If at some location the satellite would measure an abnormal high amount of sunlight, co-occurring with an abnormally high plant transpiration, the plant clearly needed sunlight to transpire, which makes the vegetation energy-limited. Next to that, we say plants are water-limited when high soil moisture co-occurs with high transpiration rates. The critical soil moisture then distinguishes between water- and energy-limitation. When we have determined this critical soil moisture, we can make better estimates about the extreme temperatures during future heatwaves, which are increasing already. The determined critical soil moisture can also help to improve climate models, with which we forecast future heatwaves.

This text was created during a workshop by DKK Managing Director Marie-Luise Beck at the Max Planck Institute for Biogeochemistry, in which PhD researchers spent two days working on climate communication.

 

About the author
Jasper Denissen is researching Land-Atmosphere Interactions in a Changing Climate for his PhD at the Department of Biogeochemical Integration of the Max Planck Institute for Biogeochemistry (MPI-BGC) in Jena.

 

20. may 2020

Picture credits: Jasper Denissen

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