Identifying the world’s most at-risk river basins

Globally, rivers support numerous ecosystems that in turn provide important health and ecosystem services, such as water purification, that bolster local economies. However, Earth’s warming climate puts many of these ecosystems at risk. A new study found major river basins across the globe, including the Amazon and Australia’s Darling River, may be hot spots for ecological shifts under climate change.

By combining five global climate models, nine global hydrological models, and an environmental flow approach, Thompson et al. showed that the risk of a river’s ecological change dramatically varies across the world. The authors looked at 321 river basins covering about 50% of Earth’s land surface and evaluated how they might change under 1°C, 1.5°C, 2°C, and 3°C global temperature rise scenarios.

They found that river basins in South America, southern Africa, Australia, southern Europe, and the central and eastern parts of the United States are at high risk of ecological disturbances—including changes in high flows (wet periods) and, especially, low flows (dry periods)—because of global warming. On the other hand, northern boreal regions are at relatively lower risk. The spatial differences in risk are partially because rivers in northern latitudes are much less affected in periods of prolonged low-flow, dry weather than in periods of high flow. The authors do note that although northern boreal regions appear to be at relatively low risk of ecological change, permafrost melting was not always accounted for, suggesting that this low risk might not be as low as their results show.

The risk of changes in both high- and low-flow conditions increases significantly with each higher amount of predicted global warming in the climate models. Simon Gosling, coauthor and climate risk scientist at the University of Nottingham, said that the risks for some rivers are particularly high if the 1.5°C and 2°C goals of the U.N. Paris climate agreement are not met, which highlights the need for ambitious goals to reduce global greenhouse gas emissions.

The authors compared the historical characteristics of a river’s flow with its flow following warming global temperatures. They found that ecological risk is much higher for low-flow conditions than for high flows. Changes in the extent and quality of refugia, those areas that serve as a habitat for plants and animals during periods of low flow, could have particularly significant impacts on riverine ecosystems. Changes in flow can alter other conditions within rivers, like temperature and oxygen concentration, which in turn can affect ecosystems and their services.

Although future studies are needed to better understand these potential changes, particularly in individual river basins, these results highlight the need for ambitious global greenhouse gas emissions reductions to avoid ecological degradation of some of the world’s largest rivers, according to the authors. This work, plus future studies related to climate change–induced alterations to river flow, can provide important insight for identifying hot spots and targeting ecosystem conservation efforts.

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