Multi-year droughts increasing in frequency, severity worldwide as climate warms

Persistent multiyear droughts (MYDs) have been worrying increasing the last 40 years and in a changing climate, will continue to pose growing threat to nature and humans, a new study has found.

These long-term droughts are increasing in both frequency and severity worldwide as climate warms and affect agriculture, energy production and ecosystems, the research study led by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) warns.

For 14 years, northern Chile has been suffering from extreme drought. The southwest of the USA recently experienced eight bone-dry years, and southern Australia three. This accumulation is no coincidence: multi-year droughts have become more frequent, longer, and more extreme over the last 40 years. Their extent has increased by 50,000 km2 per year during this period, which is significantly more than the area of Switzerland. “Multi-year droughts cause enormous economic damage, for example in agriculture and power generation”, said Dirk Karger from WSL, who led the study.

The effects on ecosystems are also becoming increasingly severe. Grasslands, in particular, are reacting sensitively to droughts, which is evident from the loss of green color in satellite images. However, grasses can recover quickly. Tropical forests and those in cold zones are better able to buffer dry phases, but in the event of extreme water shortages, the trees can die, explains Karger. The damage is then long-lasting, said an official statement.

Typically, droughts only become noticeable when they damage agriculture or forests. However, there is little observational data for some regions, such as the tropical rainforest or the Andes. So the research team from WSL and the Institute of Science and Technology Austria (ISTA) used meteorological data to determine droughts by calculating anomalies in precipitation and in evaporation from soil and plants (evapotranspiration) over the last 40 years. From this, they created a drought index and also observed changes in vegetation during those drought events using global satellite data.

It was found that their method not only correctly modelled known droughts, such as those in the western United States (2008-2014), Australia (2017-2019) and Mongolia (2000-2011), but also those in less accessible areas such as the Congo rainforest (2010-2018), which are less well documented. The end result is a ranking of the worst multi-year droughts of the last 40 years, along with the underlying precipitation and evaporation patterns and their effects on vegetation.

Unsurprisingly, the reason for this lies in rising temperatures caused by climate change – on the one hand, they increase the variability of precipitation, leading to more extreme dry periods and more heavy precipitation. On the other hand, it increases evaporation from the soil and vegetation, a factor that has been underestimated by previous models. “The severity of perennial droughts will become more and more severe with climate change”, said co-author Philipp Brun of the WSL.

The study, based on 40-year global quantitative inventory, underlines for policy makers the environmental impact of human-induced climate change. The study has also detected previously ‘overlooked’ events.

The work is based on the CHELSA climate data prepared by Dirk Karger, which goes back to 1979 and describes the climate in high resolution on all continents. “Our drought inventory is the most comprehensive in the world for the last 40 years, at a resolution of five kilometres”, said WSL postdoc and lead author of the study Liangzhi Chen. It is publicly available and should help countries to be better prepared for future multi-year droughts.