Tree ring data shows worsening ‘vapour pressure deficit’ across Europe in 21st century

Research shows the atmosphere across Europe has become markedly drier in recent decades compared to pre-industrial times exacerbating drought and wildfire risks.

Tree rings

The investigation of tree-ring data reaching back to the 1600’s by the Swiss Federal Institute for Forest, Snow & Landscape Research has showed that since the beginning of the 21st century, the air over large parts of Europe has become drier compared to any previous period. A trend which is predicted to continue and cause for concern, according to Kerstin Treydte, lead author of the study in ‘Nature Geoscience’ and a WSL researcher.

The Vapour Pressure Deficit (VPD) is used to measure air dryness and describes the difference between the actual and the maximum possible water content of the air – basically the air’s ‘thirst for water’. A high VPD draws more water from soils and plants, reduces vegetation growth and if extended, can cause tree death. The result is poor crops, desiccated vegetation and a greater risk of wildfires with irrigation the only option to keep crops alive.

Along with 67 international researchers, Treydte reconstructed changes in VPD across Europe over 400 years, compiling oxygen isotope data in tree rings from various European regions into a large network. Isotopes are variants of an atom with different weights that are assimilated during water uptake by the roots and modified during transpiration in the leaves.

The scientists have shown that the problem of dry air in the 21st century is exceptionally high compared to pre-industrial times due to the increase in greenhouse gases from fossil fuels. In Northern Europe, the water thirst of the air relative to pre-industrial times has increased the least compared to other European regions because the air is cooler and can take up less water. In the Central European lowlands and in the Alps and Pyrenees the VPD increase is particularly strong, with highest values ​​in the drought years 2003, 2015 and 2018.

According to the scientists, further increases in VPD poses a long-term threat to many vital ecosystem functions. “VPD is particularly important for agriculture because the higher it is, the greater the water demand of crops. More irrigation is necessary and crop yields decrease. In forests, wood supply and carbon sequestration are at risk, leading to uncertainties regarding climate regulation and carbon storage of these ecosystems in the future”, explains Treydte. This is of particular concern in the densely populated regions of Europe and adds to the urgent need of reducing emissions and adapting to climate change.