Felipe III reigned in Spain the pious and, climatologically, a Little Ice Age that froze even the Tagus, was master of Europe. Since then, the beginning of the 17th century, the air has not been as dry as it is now and this time it is not because of the cold, but because of the increase in temperatures. It is the main conclusion drawn by around fifty scientists who have read the anomaly in tree rings throughout Europe. In the last 30 years, the aridity of European skies has been greater than in the past 400. The atmospheric drought thus joins that caused by the lack of rain and the dryness of the soil. The cocktail could be behind both the fires in Central Europe in recent summers and the poor harvests in the south of the continent.
A wide network of 67 scientists has participated in research led by dendroclimatologists from the Swiss Federal Research Institute for Forests, Snow and Landscape, WSL. Dendroclimatology studies the climate in trees. Every spring, they grow and this manifests itself in an annular widening of the trunk. The thickness of each ring depends on how well the tree has done that year, the availability of water, sun, nutrients… The cellulose of this wood is made up of carbon, hydrogen and oxygen atoms. With the variations of the latter, the WSL researcher Kerstin Treydte and their laboratory colleagues have been able to know the humidity that was in the air 10, 100 and even 400 years ago.
Treydte's research, just published today in the scientific journal Nature Geoscience, is based on chronologies obtained from the study of trees from 45 places in Europe. They are mostly oaks and pines, such as the Salereños from Cazorla (Jaén) or the red ones from Windsor (United Kingdom). But they also analyzed Scandinavian firs, beech trees from a forest near Zurich (Switzerland) or larch trees from Slovenian forests. Its cellulose shows different ratios of two oxygen isotopes (variations of the same element according to its atomic weight). The differences are due especially to precipitation and the water that the roots take from the soil, but also to processes specific to the physiology of the plants. Combining this information, the scientists determined the vapor pressure deficit (VPD), which shows the difference between the amount of water present in the air and what it could have when it is saturated, at which time it precipitates into dew shape. It is an indicator of atmospheric drought that offers more information than the classic indices of meteorological drought, precipitation or temperature separately.
Between 1600 and 1640, VPD rose in all European regions, a period replaced by half a century of relatively greater humidity, alternating with phases of decreasing dryness. At the end of the 19th century, another phase of high VPD occurred across the continent. Already in the last century, there were two especially dry periods, one in the years of World War II and another in the decades of the 70s and 80s. But at no time in these 400 years was there a vapor pressure deficit. as pronounced and as generalized as that shown by the cellulose of the rings of the last 30 years. There is no single value, as the indices are very site-specific, depending on factors such as local temperature, latitude, altitude, species, forest type or regional climate. But it is the synchrony between all the regions and the almost parallel trends followed by the chronologies of the 45 sites, which gives forcefulness to the results of this work.
These new data complicate the drought scenario that almost the entire continent has been experiencing for a few years. The mother of all droughts is meteorological drought, caused by the lack of precipitation. With it, the most immediate consequence is agricultural drought and, later, comes the hydrological one, the lack of water in natural reservoirs or created by humans. The latter was only punishing the southern countries, in particular Spain. But water deficits were also occurring in the air in large areas of the rest of western, central and eastern Europe. Now, thanks to the VPD information, the severity and depth of atmospheric drought across the continent has been quantified. From the far north to the Mediterranean, only in the southern fringe of the Scandinavian countries, the increase in this pressure deficit is not so historic.
“The vapor pressure deficit is particularly important for agriculture because the higher it is, the greater the water demand of the crops.
Kerstin Treydte, researcher at the Swiss Federal Research Institute for Forests, Snow and Landscape
“VPD is particularly important for agriculture because the higher it is, the greater the water demand of crops. More irrigation is needed and yields tend to decrease,” says Treydte. This is what has been happening in many European regions, such as the Spanish ones, at least since 2015. That year, together with 2003 and 2018, have been the ones in which the greatest anomalies in the VPD have been recorded in the last 400 years, except for the from 1709 (see below).
With this deficit of available moisture in the air, a complex process with potentially catastrophic consequences is triggered: under normal conditions, the fallen rain is collected by the roots of the plants. Normally, during the day, its leaves respire, capturing oxygen and releasing CO₂. In parallel, photosynthesis takes place, in which there is a reverse exchange: CO₂ is fixed and oxygen is released. At night, there is no sunlight, so photosynthesis stops, but respiration continues. This entire complex system is supported by stomata, cells of the plant epidermis that open or close depending on the concentration of gases that the plant needs. But these are not normal times and if the environment becomes dry, the stomata close to prevent excessive perspiration. This short-circuits photosynthesis and all gas exchange. The plants wither and, if the environmental dryness continues, their lives may be in danger. In the countryside, this can be combated by watering, if there is water. But in nature, there is no lifeguard.
Furthermore, in forests, atmospheric drought steals water from plants and their greater dryness makes them easy prey and facilitators of fires. Also, high VPD reduces the trees' ability to capture CO₂. On the other hand, both in natural environments and in cultivated land and following the laws of physics, the atmosphere tries to quench its thirst for water by searching for it in the soil, causing the surface to dry out even more. This would be related to the proliferation of so-called flash droughts that are breaking out across the planet.
“In regions of northern Europe we now see that a water deficit is also occurring in the atmosphere”
Raúl Sánchez, dendroclimatologist at the Pablo de Olavide University
For the dendroclimatologist at the Pablo de Olavide University, Raúl Sánchez, the advantage that the VPD offers is that “changes in the flows of energy, water and carbon are better detected by this index than by the classic variables of drought indices, precipitation or temperature alone.” For Sánchez, who has not participated in the work, the most relevant thing is that he detects processes not seen by these classic indices. “In regions of northern Europe, where apparently the only effect that had been observed and reconstructed as unprecedented was the increase in temperature, which in many cases had been observed to benefit growth, we now see that, based on the VPD, there is also producing a water deficit in the atmosphere.”
To explain what is happening, we must go back to the year 1709. The trees of the Cazorla mountain range, those of the Pyrenees, the Alps, those of eastern Europe, but also those of the west or those of the Central European plains, recorded the highest VPD of the historical series. By stopping to study it and using climate reconstructions, they observed that such an extreme appeared connected to a deeply negative phase of the North Atlantic Oscillation (NAO), a complex interaction in the ocean and the atmosphere between the Azores (and its anticyclone) and Iceland (low pressure area). When this dance couple is in a positive phase, winds and fronts from the Atlantic loaded with humidity continue to reach most of Europe. But the summer of 1709, the NAO was historically negative, leaving its mark on the trees.
For the authors of this research, the parallel evolution of the VPD in almost all of Europe and its synchrony with the most acute phases of the NAO “indicate a link between the VPD and large-scale climate dynamics.” The highest VPD values in 400 years (they could go back further, as some trees in the sample are more than 1,000 years old, but did not cover the entire continent), the largest expansion of the Azores anticyclone in a millennium and the weakening of the main ocean current point to a climate system in transition and everything points to the climate crisis.
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