Drought is the single most damaging type of natural disaster on Earth: responsible for over 650,000 deaths worldwide since 1970, causing billions of dollars in economic losses, and inflicting further devastation on natural ecosystems. As the Earth’s climate warms, climate scientists now predict that droughts will only become more frequent and intense in the coming decades, making it more and more crucial to understand the atmospheric phenomena which trigger them in the first place.

Altogether, there are three main reasons why a drought might occur: reductions in the amount of moisture available for rainfall; reductions in atmospheric instabilities which prevent moisture-laden air from rising up and condensing; or a combination of both. So far, most studies have focused on weather and climate patterns which reduce atmospheric instabilities.

Understanding the role of moisture content is more challenging, since the amount of water vapour in the air at any given time does not necessarily indicate how much precipitation it can produce. Instead, a lack of water reaching one region likely points to a deficit in moisture transported from other regions.

Since this effect involves interactions between multiple atmospheric phenomena on global scales, it is far more challenging for climate scientists to predict and study. Ultimately, without a deeper understanding of how droughts are triggered by deficits in moisture transport, millions of people could be put at greater risk from their potentially devastating effects as Earth’s climate continues to change.

For the first time, Gimeno-Sotelo’s team have clearly highlighted the role played by reduced moisture transport from the ocean in causing droughts. Their results led them to predict that if the world’s carbon emissions continue to rise at their current rate, the effects of moisture deficits from the ocean will be felt strongly across Southern Europe and the Mediterranean.

To uncover the link between drought and atmospheric moisture transport, Gimeno-Sotelo’s team analysed a dataset combining satellite and ground-based observations of global precipitation, over a period ranging from 1980 to 2018.

Another important factor for the team to consider was how moisture tends to travel through the atmosphere along specific pathways. These can range from small-scale corridors to atmospheric rivers: narrow highways of concentrated water, which transport vast amounts of vapour from water-rich regions.

Through their exhaustive approach, Gimeno-Sotelo and his colleagues revealed a key link between drought conditions in regions connected by the same moisture pathways. For the first time, they showed how a drought in one region may be intrinsically linked to a deficit in moisture in another region, upstream on the same atmospheric pathway.

This discovery revealed key insights into the probability of drought emerging in certain regions. Across most land areas, the team showed that this chance increases notably– often above 10% – when there is a deficit in moisture arriving from oceans and continents. In areas linked to major moisture pathways such as atmospheric rivers, this probability rises to over 15%, and can even exceed 20% in some cases.

Altogether, these results suggest that climate scientists can improve their drought predictions by tracking deficits in moisture from key sources. Building on their initial findings, Gimeno-Sotelo’s team zoomed in on the world map to focus specifically on the ‘Euromediterranean’ region: which spans much of Southern Europe, as well as coastal regions of North Africa.

According to the team’s previous research, the emergence of drought should be strongly linked to a reduction in moisture transport from the two major sources of the region, namely the North Atlantic Ocean and Mediterranean Sea. In this second study, the researchers explored how these relationships could change as the Earth’s climate continues to heat up. To do this, they considered a ‘business-as-usual’ scenario, where greenhouse gas emissions continue along their current trajectory.

Although most researchers now agree that this scenario is unlikely under current government policies and economic trends, it still provides a useful benchmark for climate scientists to test their predictions, and helps them to understand the full extent of the potential impacts of manmade climate change.

Under this scenario, Gimeno-Sotelo’s team predicted that by the mid-21^st century, the effects of moisture transport would have a moderately higher impact on extreme winter rainfall in the Euromediterranean region. For droughts, however, they expected this relationship to strengthen more quickly: with the probability of droughts driven by moisture deficits from the North Atlantic and Mediterranean sources increasing by 2 to 3 times compared with current levels.

Altogether, the team’s results further strengthen existing evidence that the Euromediterranean region will experience a worsening in the frequency and intensity of droughts in the coming decades.

Yet although these effects are now most likely unavoidable by now, the team’s insights into the role of moisture transport deficits in triggering droughts could make it easier for climate scientists to predict exactly when and where droughts are most likely to occur in this region, and across the world.

In turn, their findings could help planners and policymakers to better manage their water resources – potentially helping millions of people to prepare for coming droughts. On top of this, the results present fresh calls for global economies to reduce their greenhouse gas emissions as quickly as possible, ensuring the worst possible outcomes remain far from the most likely scenario.