Compound Heatwaves Tripled in 40 Years, Threatening Rivers and Water Quality

A new international study reveals a concerning trend: the occurrence of "compound heatwaves," where both air and river temperatures rise abnormally high simultaneously, has nearly tripled over the past four decades across 796 river basins in the United States and Central Europe. This phenomenon poses a significant threat to aquatic ecosystems and water resources.

“

The frequency and duration of river heatwaves have increased faster than those of air heatwaves.

The research, published in Nature Geoscience, analyzed river data and employed deep learning models from 1981 to 2019. It found that compound heatwaves increased from an average of 0.76 per year in the 1980s to approximately 2.3 per year recently, marking a substantial escalation. Over the past 40 years, river heatwaves have notably outpaced air heatwaves in terms of frequency and duration increases. While air heatwaves saw modest growth, river heatwaves experienced a 113.6% increase in frequency and a 147.8% increase in duration.

Compounding the issue, rivers are cooling down much slower than the air. In 99.6% of compound heatwave cases, the river heatwave persisted longer than the air heatwave, with an average difference of 2.67 days, and in extreme cases, exceeding 12 days. Researchers attribute this to rivers being shallower and having less water volume compared to lakes or oceans, making them more susceptible to rapid temperature fluctuations influenced by ambient air temperature and solar radiation.

“

Higher nighttime low temperatures are the most important variable in explaining the occurrence of compound heatwaves.

Crucially, the study identified nighttime low temperatures as a more significant predictor of compound heatwaves than daytime highs. When nighttime temperatures remain elevated, rivers are unable to cool down sufficiently, leading to a cumulative buildup of heat. This persistent warmth has detrimental effects on water quality, decreasing dissolved oxygen levels essential for aquatic life. Projections under high-emission scenarios suggest that by the end of the century, air and river heatwaves could occur almost concurrently, highlighting the urgent need for climate action.

“

In high emission scenarios, by the end of the century, 98.5% of air and river heatwave days are projected to overlap.