Drought risk in the Northern Hemisphere rises with intensified warming

Droughts primarily manifest in summer, since their occurrence largely depends on three factors: on the air temperature – the warmer the air is, the more moisture it can absorb, which theoretically leads to more evaporation; on the (most often absent) precipitation; and on the amount of moisture that evaporates on the Earth’s surface.

“In the latest generation of global climate models, all three parameters are much more accurately represented than in previous models. Accordingly, our goal was to use these CMIP6 models to investigate how these factors, and with them the drought frequency in the Northern Hemisphere, will change. We looked at three warming scenarios, in which the average global temperature climbs by 1.8 degrees, 2.7 degrees and 4.4 degrees Celsius by the end of the century,” explains first author Daniel Balting, a climate expert at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

The warming scenarios used belong to the Shared Socioeconomic Pathways (SSPs), which depict global societal, demographic and economic developments. Climate modellers around the globe use these SSPs as input data for climate simulations. For the new study, simulations for three different developments were analysed: a path characterised by early and effective climate policy and low greenhouse-gas emissions (SSP1-2.6), a future with delayed implementation of climate protection measures and moderate emissions (SSP 2-4.5), and a world shaped by a high degree of industrialisation, lack of climate protection measures and correspondingly high emissions (SSP5-8.5).

The results of the simulations show a clear trend. “The more greenhouse gases are released, the hotter summers will become. Even in the scenario with moderate emissions, the climate models predict a temperature rise of three to five degrees Celsius for the subtropical and middle latitudes of the Northern Hemisphere. It will be especially warm in the Mediterranean, central North America, parts of Greenland and Siberia, and in large areas in Central Asia,” Balting reports.

The researchers see parallel developments in precipitation patterns. The changes are least pronounced in the low-emissions scenario. The corresponding climate simulations show more precipitation in the Sahara and in North Asia, in Alaska and on the eastern coast of North America. In contrast, the models predict a significant decline in precipitation for central North America and large parts of the Mediterranean. “The scale of precipitation extremes will grow with intensified warming and affect larger and larger regions. According to the model results, people, flora and fauna in Middle America, many parts of Europe and Central Asia and in North Africa will have to adjust to having far less rain than they’re used to,” says AWI climate expert and co-author Dr Monica Ionita.

As warming worsens, evaporation will also intensify, prompting the researchers to predict a major increase in drought frequency in the hardest-hit areas: “Regions like the Mediterranean will become drought hotspots by the end of the 21^st century, areas where moderate to extreme droughts can, in the worst case, become an annual phenomenon, depending on the amount of greenhouse gases human beings emit in the decades to come,” Daniel Balting explains.

The researchers’ findings also indicate that the arid zones of the subtropics will spread to the north, posing enormous new challenges for the environment and human beings alike.

For Central Europe, the climate simulations only predict an increase in drought frequency when greenhouse-gas emissions remain at a very high level. “But this finding should be taken with a grain of salt,” Balting explains. Even the latest generation of global climate models still have difficulties realistically depicting the droughts actually observed in Central Europe and tend to underestimate them. “Consequently, we assume that the models also produce unrealistically low values for Central Europe when it comes to future drought frequency.”