Dhere will be a striking change in Europe’s summer climate in the next few decades: average temperatures will increase, while at the same time the climate will also be more variable during these months. Researchers at ETH Zurich (1) had already reached this conclusion two years ago when they made a detailed study of the extreme summer of 2003. This revealed that extreme heat-waves will no longer be an unusual occurrence towards the end of the present century.

However, the exact reason why the European summer climate will become more variable remains in dispute. For example a study published in the scientific journal "Science" indicated that shifts in the global circulation pattern could be responsible for the variability. On the other hand, other studies concluded that processes at the ground surface play a prominent role. However, this hypothesis has been disputed up to now, because the Global Land-Atmosphere Coupling Experiment (GLACE) (2) indicated that such processes are of rather secondary importance to the present-day climate of Europe.

Confirming the thesis

However, Sonia Seneviratne and other researchers from the Institute for Atmosphere and Climate (IAC) of ETH Zurich have now been able to confirm this theory. As the scientists reported last week in the scientific journal "Nature" (3), the summer climate in the future will be shaped decisively by the interactions between the ground surface and the atmosphere: the increase in variability cannot be explained without this factor.

Seneviratne’s explanation of the relationship is that "The greater the increase in summer temperatures, the more water evaporates out of the soil. In turn this is exactly what counteracts the warming, because the evaporation absorbs a proportion of the incident solar radiation." Thus soil moisture has a direct influence on the air temperature. Now if the ground becomes dried out during periods of low rainfall, the air can become strongly heated because the cushioning effect of the evaporation is absent. On the other hand, in wet years the evaporation prevents an excessive rise in temperatures.

Contrary to the results of the GLACE Study, the scientists also showed that in the Mediterranean region the interaction between the atmosphere and the ground is already an important climate factor today. This mechanism will become noticeable to a far greater extent under the climatic conditions of the future. A striking increase in variability is to be expected particularly in Central and Eastern Europe.

Far-reaching consequences

The results of the study have far-reaching consequences. If the variability of the climate in summer increases, this poses great challenges especially for agriculture. “The response of vegetation to soil humidity is non-linear,” explains Jürg Fuhrer from the Agroscope Reckenholz-Tänikon Research Station (ART). "If the climate becomes more variable and extreme events become more frequent, there will also be more years in which water logging incidents or especially drought reduce harvest yields."

However, as Fuhrer and his colleagues have shown, farmers can adapt to the new conditions to a certain extent. For example a study of the cultivation of corn maize in Eastern Switzerland has shown that average yields can be maintained for the time being by bringing forward the growing season and selecting warmth-loving varieties. The climate warming in the next 20 to 30 years could have an entirely beneficial effect on other crops as well, e.g. potatoes or grassland. Fuhrer puts it in a nutshell: "Slightly drier and slightly warmer is no bad thing, but it will topple over sooner or later." Moreover, he says that simulations of maize cultivation in the future have shown that adaptability has its limits. “Whatever is done, yields will fluctuate considerably more than in the past." That might pose economic problems for farmers.

A far-reaching shift: the two maps show how important the exchange between the ground and the atmosphere is for the variability of summer temperatures in Europe (expressed as percentages). Under present-day conditions (top), soil humidity plays a large part especially in Southern Europe (Southern France, Italy, the Balkans). Towards the end of the century (bottom) it will be a dominant factor principally in Central and Eastern Europe. large

However, the new study by the ETH climate researchers also raises new questions in the opposite direction. Fuhrer explains that "If soil moisture has such a powerful effect on climate, then there is a need to examine in more detail the extent to which the agricultural industry can modify the climate.” He says there are already initial indications that for example the large-scale irrigation in the Mid-West of the USA and in Asia could already be having an effect on the climate.

Changed landscape

If the climate becomes more unsettled, this also has consequences for forests. Harald Bugmann, Professor at the Institute for Terrestrial Ecosystems of ETH Zurich, explains that depending on the forest type, the size of the effect of an increase in variability on the species composition is similar to that of an increase in the average temperature.

Using the LandClim landscape model, Bugmann and his colleagues studied the effects of the predicted climate change on the mountain forests in Switzerland. (4) "Our study shows that in the Gantertal valley in the canton of Wallis, for example, a completely different landscape may emerge in the next few decades." There will be a striking rise in the tree line, and the spruce forest that predominates at the present time will extend to higher altitudes.

On the other hand mixed stands mainly of deciduous trees and Scots pine will grow at the altitude level from 1100 to 1900 metres – i.e. where many housing estates are situated in Wallis and elsewhere. The effect on the density of the forest is remarkable: today one finds just under 250 tons of biomass per hectare at this altitude level; towards the end of the century it will probably be barely 70 tons. Bugmann’s interpretation of the figures is that “We will encounter an open, 'savannah-like' vegetation in those locations." The main reason for the thinning-out will be the forest fires that will occur with considerably greater frequency because of the more severe climate variability and in particular because of the smaller amounts of precipitation rainfall in summer. Bugmann has calculated that "The landscape will be burnt off once every few decades.” It is clear to him that thinned-out forests of this kind can no longer fulfil their current protective function.