A study published on November 4th provides valuable insight into the role of vegetation on the evolution of soil water content in the context of climate change. For mid-latitudes, it challenges the paradigm that plants slow down soil drying. These disturbing results support the idea of a future that is facing significant tensions over access to drinking water.
The climate is changing and it’s not just about temperature. Indeed, the disruption of the Earth’s radiative budget affects all the energy flows in the environment. However, the cycle of water concretizes a latent heat flow. He is therefore forced to readjust as well as the temperature. This implies the precipitation, but also the water content of the soil.
Simple thermodynamic arguments indicate that the hydrological cycle is faster in a warmer climate. This is concretized by more extreme hydric extremes. On the other hand, the contrast between dry and humid regions tends to increase. These evolutions have been detected in observations for a few decades now.
The uncertain role of vegetation
However, to predict changes more precisely and more regionally, it is necessary to go beyond purely thermodynamic considerations. In particular, the influence of atmospheric circulation and vegetation must be taken into account.
For the latter, the paradigm is that in warmer climates, it tends to decrease the rate at which soils dry up. Indeed, with a higher CO2 level, stomata (the “pores” of plants) are less open and evapotranspiration is reduced. Thus, water consumption is reduced and reserves less solicited.
However, a new study published in the journal Nature on November 4 qualifies this vision. The described process would be dominant only in certain tropical areas and at high latitudes. For temperate latitudes (North America, Europe, Asia), it would be rather the opposite. In other words, an aggravating factor of soil drying, even if the rainfall does not change or even increase.
Antagonistic effects according to the regions
The researchers studied the issue using several high-resolution models coupling climate and vegetation. Their analyzes reveal that if the excess of CO2 limits the instantaneous flow of water from the plants to the atmosphere via a contraction of the stomata, it does not stop there. In mid-latitudes, the lengthening of the growing season and the acceleration of photosynthesis more than offset the process mentioned above. Finally, vegetation consumes more and more water and accelerates the drying of soils.
At high latitudes and near the equator where competition for water is less, the net effect is to reduce total evapotranspiration. The researchers are therefore observing soil humidification, all other things being equal. In the tropics, it can be seen that this only associates locally with a notable increase in river flow. It is the opposite at high latitudes with a general rise – largely due to an earlier melting of snow.
The results obtained underline the importance of better understanding and understanding the influence of vegetation in changes associated with the water cycle. Understanding the future evolution of water reserves is a particularly sensitive subject since they regulate socio-economic life, geopolitical stability and the environment. However, the signals converge more and more in the direction of increasing tension at this level. Especially in the subtropics and some temperate areas like Southern Europe.
