Mediterranean area dries out faster

Santorini

View of the coast of the Greek island of Santorini (Image: spooh / iStock)

Many regions of the Mediterranean are already suffering from increasing drought. Now a study reveals that the lack of rainfall in these areas could worsen faster than previously thought. Accordingly, the winter rains, which are a main source of the annual water replenishment, do not decrease linearly with the rise in global average annual temperatures, but much faster. This is apparently to blame for shifts in regional atmospheric circulation. However, climate protection measures could have a positive effect almost immediately, as the researchers report.

Typical of the Mediterranean climate is a dry, hot summer and a mild winter, in which the majority of the annual precipitation falls. If this rain fails to appear or falls less than normal, an acute lack of water is the result. Studies already show that many Mediterranean countries have already used up more than 80 percent of their groundwater reserves and suffer from acute water shortages almost every year. The Levant even suffers from the worst drought in 900 years. Forecasts by climate researchers also show that the Mediterranean region could enter a new, drier climate regime with a warming of 1.5 degrees compared to pre-industrial values. If the temperature rises by two degrees, large parts of southern Spain could become a desert. In other areas, evergreen forests would be replaced by sparse dry vegetation.

The time span ranges from years to centuries

However, many of these forecasts use the development of the global mean annual temperature as a measure of warming. However, this may fall short, as Guiseppe Zappa from the University of Reading and his colleagues report. “It is generally believed that changes in water availability are proportional to global warming,” they explain. But there are additional effects, for example through changes in radiation incidence or regional circulation, which have a much faster consequence. The increasing greenhouse gas values ​​in the atmosphere can therefore affect local climate conditions almost immediately – in the span of a few years – or gradually over several decades or centuries. So far, however, little has been done to determine which time scale applies to precipitation and the climate in the Mediterranean and similar climates.

Zappa and his team have now made up for this. For their study, they used several model simulations to reconstruct the changes in annual precipitation in the Mediterranean, as well as in two climatically similar areas in Chile and California. In doing so, they specifically investigated how the precipitation develops in temperate climate change according to the emission scenario RCP 4.5 to 2100 and, above all, how quickly and by which influencing factors this occurs. The scenario RCP 4.5 assumes that the CO “emissions will increase to 650 ppm by 2100 and that the global mean annual temperature will increase by 2.6 degrees compared to the pre-industrial values.

Fast reaction

The evaluations showed that while precipitation in higher latitudes developed almost linearly with the warming of the earth’s surface, this was not the case in the Mediterranean and other regions with a Mediterranean climate pattern. “In the Mediterranean and Chile, dehydration is progressing substantially faster than global warming,” reports Zappa and his colleagues. Instead of developing linearly with the global rise in mean temperatures, precipitation in these climates follows the rise in greenhouse gas emissions within a few years. The researchers identified the reason for this as being rapid changes in sea temperatures and the associated atmospheric circulation. It is already known that under the influence of climate change, the west wind zones of the temperate latitudes are shifting further to the pole, the researchers explain. Among other things, this means that rainy low-pressure areas no longer move across the Mediterranean area as often in winter.

“Our results indicate that global warming is not always sufficient to predict the future development of regional precipitation,” said Zappa and his colleagues. Instead, changes can occur locally and regionally on significantly different time scales. After all, there is also positive news in the case of the Mediterranean region: the negative effects of climate change show up very quickly there – but they can improve just as quickly. “Our results imply that if we stabilize greenhouse gas emissions, these regions will benefit almost immediately,” explains co-author Paulo Ceppi from Imperial College London. As soon as the CO2 values ​​no longer rise or even decrease again, the rapid decrease in precipitation is stopped. In other words: climate protection measures not only have a positive long-term effect, they could also have an impact after just a few years, ”says Ceppi.

Source: Guiseppe Zappa (University of Reading, UK) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1911015117

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