The combination of heat and drought is particularly dangerous for people and nature: forest fires, crop losses and water shortages threaten ecosystems and food security. Due to climate change, such extreme weather conditions are becoming more and more common. A study now shows that heat-drought events caused by heat waves have increased eightfold since the beginning of the millennium. The results also point to both global trends and regional differences and can help better understand the dynamics of such events.
The atmosphere and the surface of our earth influence each other. This so-called land-atmosphere coupling can cause extreme weather events such as heat waves and droughts to reinforce each other and become worse in combination than individually. When it’s hot, more moisture evaporates from the soil, which dries out the soil and can cause droughts. Conversely, droughts can contribute to even greater heat because when soil moisture is low, evaporative cooling is absent and surface temperatures continue to rise.
But regardless of whether the trigger was a drought or a heat wave, the resulting so-called compound dry and hot events (CDHEs) increase the risk of wildfires and harm natural ecosystems as well as agriculture and human health. However, in order to better counteract these dangers, it can be helpful to know the trigger. However, previous surveys have predominantly relied on monthly rather than daily data. The question of which came first, the drought or the heat, was therefore difficult to answer.
Heat waves as a trigger
In order to gain more detailed insights into the dynamics of such hot-dry events, a team led by Yong-Jun Kim from Hanyang University in South Korea has now used daily global weather data for the period from 1980 to 2023 to evaluate where, when and in what order heat waves and droughts occurred in combination. The researchers found that in most cases a drought preceded the heat wave. Cases where the event began with the heat wave were still rare in the 1980s and 1990s.
But since the early 2000s, this proportion has increased rapidly. “The recent increase in CDHEs is primarily due to a rapid increase in events triggered by heat waves,” the team reports. Hot-dry events triggered by heat waves have increased eightfold since the beginning of the millennium and their spatial extent has more than doubled.
Strengthening interactions
From the researchers’ perspective, this development is problematic. According to the results, heat-drought events triggered by heat waves are often more dangerous than heat-drought events triggered by droughts. “CDHEs triggered by heat waves can have disproportionately severe impacts and pose a significant risk to socio-ecological systems,” the researchers write.
According to the study, the northern part of South America as well as the southern USA, Eastern Europe, Central Africa and South Asia are particularly affected. These regions were already vulnerable to heat-drought events, which are now becoming more frequent and more intense due to climate change. But even regions like the northwest of America, where such events rarely occurred in the past, are now increasingly suffering from combinations of heat and drought. The effects are stronger than would be expected due to rising temperatures alone. According to Kim and his colleagues, this is because land-atmosphere interactions are becoming stronger, leading to an exponential increase in heat-drought events.
“Our results underscore the urgent need to rethink current strategies for managing combined hazard risks,” the researchers write. “As they continue to intensify, heat wave-induced CDHEs could become the dominant driver of overall CDHE risks, triggering chain reactions such as declining crop yields, increased wildfire frequency, and severe public health crises.”
Source: Yong-Jun Kim (Hanyang University, Anan, South Korea) et al., Science Advances, doi: 10.1126/sciadv.aea3038