Climate scientists have made dramatic strides over the last two-and-a-half decades in their ability to attribute specific, observable climatic changes to the release of heat-trapping gases by humans. It wasn’t until the mid-1990s that the United Nations’ Intergovernmental Panel on Climate Change definitively linked global warming to a “discernible human influence on the global climate,” and yet in the last few years, scientists have become able to pinpoint the contribution of the greenhouse effect to individual heat waves and fire disasters.
Today, a new study in the journal Science carves out yet another advance in the field of climate change attribution: the ability to blame humanity for observed changes in the world’s rivers.
Scientists from a dozen countries collected data from 7,250 river monitors around the world going back to 1971 and ending in 2010. They then ran complex Earth-system models able to simulate the world’s climate during that time period, drawing on the same methods now commonly used to diagnose weather events as climate-influenced. The models the group tested that didn’t include simulations of humanity’s pollution failed to reproduce the river data. Models that included greenhouse gases, by contrast, “are essential for explaining the observed patterns” of low, average, and high river flows, they wrote.
In other words, heat-related changes to the planet’s water cycle, including rainfall, snowfall, and snowmelt, are causing rivers to swell in some places and shrink in others.
“Rivers are an important indicator for freshwater availability for society and ecosystems,” said the study’s lead author, Lukas Gudmundsson, an environmental systems scientist at Swiss research university ETH Zurich. While the new research assembled a global picture, “zooming in on smaller regions would be both scientifically interesting and practically relevant.”
There are two reasons scientists haven’t done this kind of analysis before—slowness in assembling global data from so many far-flung sites, and also the previous inability of models to factor in how people affect rivers by industrial-scale water and land management. More advanced models couldn’t detect the latter, and the scientists conclude that broader human-caused climate change is influencing river systems.
“This finding is distinct, and important,” wrote Julia Hall and Rui A P Perdigão, researchers at the Meteoceanics Institute for Complex System Science in Vienna who weren’t involved in the study, in an essay that accompanies its publication. They caution, however, that while the team found that river flow patterns are consistent with manmade global warming, “technically that evidence is still circumstantial” until richer analysis drawing on advances in physics and statistics more deeply characterise how the elements of the system are producing what’s been observed.
Another group writing last month in the same journal found that after two centuries of industrial development, nearly a quarter of freshwater fish species are at risk of extinction.