Key driver increased by climate change is worsening droughts

When we think of droughts, the first thing that comes to mind is lack of rain. But there is another important driver at play – evaporation.

Atmospheric Evaporative Demand (AED) to be exact. As AED increases, the atmosphere becomes thirstier, increasing evaporation, which dries out the landscape, particularly effecting regions with low soil moisture such as Australia.

New research has isolated the effect of AED and found it is playing an increasingly important role in driving severe droughts globally.

According to the paper published in the journal Nature, AED has increased global drought severity by an average of 40% since 1981.

“In the past 5–10 years, this trend has accelerated as a consequence of the strong increase in AED, which is directly related to global warming and an increased vapour pressure deficit, as the water supply to the atmosphere is not enough to compensate for the large temperature increase,” the authors write.

Stabilising the rise in global temperature requires anthropogenic greenhouse gas emissions be reduced to net zero, in which the residual emissions released into the atmosphere by humans are balanced by the amount that is removed by us.

AED is projected to continue to increase with worsening warming, and the authors of the research warn that “socioeconomic and environmental measures are needed to mitigate drought impacts.

“Not only are typically dry regions becoming drier but also wet areas are experiencing drying trends,” they write.

“During the past 5 years (2018–2022), the areas in drought have expanded by 74% on average compared with 1981–2017, with AED contributing to 58% of this increase.

“The year 2022 was record-breaking, with 30% of the global land area affected by moderate and extreme droughts, 42% of which was attributed to increased AED.”

To explore the effect of AED on the magnitude, frequency, duration and real extent of droughts, the researchers developed a high-resolution dataset of global droughts from 1901-2022. 

To do this, they used the most accurate global precipitation datasets and computed AED using a comprehensive model.

They then used the Standardised Precipitation Evapotranspiration Index (SPEI), which captures the supply–demand dynamics of drought through the difference between precipitation and AED, to assess drought trends at the global scale.

SPEI revealed that the influence of increasing AED on the drying trend has been particularly pronounced in large parts of Europe (excluding Norway and Sweden), Asia, Australia, the western United States, southern parts of South America, and parts of East and South Africa.

“Specifically, AED accounts for 44% of the drought trend in Africa and 51% in Australia, playing a significant role in intensifying drought severity in these regions,” the authors write.

AED contributed about 30% to worsening drought trends in North and South America, Europe, and Asia, but has had minimal or no effect in North America (Canada, Midwest and Southeast United States), northern South America (Amazon River Basin) and Central Africa.

“Regionally, drought-affected areas increased by 119% in Australia, 163% in southern South America, and 141% in the western United States in 2018–2022 compared with 1981–2017,” the authors write.

“Similarly, in the past 5 years, drought areas increased by 75%, 80% and 56% in East Africa, Northern Asia and Europe, respectively.

They say that further studies are needed – considering variables such as soil moisture, vegetation stress and hydrological flows – to better understanding the broader impacts of these observed changes on ecosystems and human activities.

“Droughts are also detrimental for vegetation, reducing the carbon uptake of ecosystems, causing widespread plant mortality and leading to significant disruptions in ecosystem functioning and biodiversity loss.

“They also negatively affect the productivity of annual and perennial crops, exacerbating food insecurity and economic instability.

“With climate change, there is an expectation that droughts will be more frequent and intense, with increased impacts on agricultural, environmental and hydrological systems.”