Effects of multiyear droughts not equal across the globe
Droughts are becoming longer and more frequent, with a range of adverse effects on green spaces: lawns turn dry and yellow, and crops require more irrigation. But is this happening everywhere in the world? The answer turns out to be more complicated than previously thought. “A tree reacts differently to a drought than grass does, and a tree in Norway reacts differently than a tree in the Netherlands.”
Green areas
Denise Ruijsch, a PhD candidate at Utrecht ľϸӰ, studied global multiyear droughts over the past 20 years. She used satellite images to observe how green vegetation appeared from space. “During droughts, as you would expect, many areas become less green. But in the higher latitudes of the Northern Hemisphere, vegetation actually became greener on average during multiyear droughts,” says Ruijsch.
These were mainly areas where there was still sufficient groundwater available. During droughts, it is often warmer and sunnier, which causes plants to flourish in those areas. “But eventually those plants also turn brown if the drought persists.” Dry regions such as Australia, South Africa and areas around the Mediterranean Sea experienced a more direct negative effect from multiyear droughts, and also a greater impact.
Balance
Different types of plants also respond differently to multiyear droughts. “A tree stays green longer and takes longer to respond to drought than grass, but grass also rebounds more easily,” says Ruijsch. Areas that are managed by humans also suffer from the effects of drought. Ruijsch observed that although crops and urban greenery wither less quickly, “those plant species are still affected, and at some point the groundwater will also run out.”
Predictions
With climate change, it is likely that multiyear droughts will occur more frequently. How will the world’s vegetation respond to this? To answer that question, Ruijsch is now looking to the past to ultimately make better predictions for the future. She is doing this with a model that simulates carbon and water cycles and vegetation. “We will feed climate data from the past 125 years into that model, which will allow us to analyse many more droughts than we currently have satellite data for.” By then adding future climate scenarios to the model, Ruijsch will ultimately be able to make accurate projections about how vegetation will respond to multiyear droughts in the future.
Publication
Ruijsch, D., van Mourik, J., Biemans, H., Hauswirth, S. M., & Wanders, N. (2025). Thrive or wither: Exploring the impacts of multiyear droughts on vegetation. Journal of Geophysical Research: Biogeosciences, 130, e2025JG008992.