River ice has declined globally over last three decades, new study says

Image: Wikimedia Commons

In what is believed to be the most comprehensive study of its kind, a team of researchers that includes a Texas A&M University geographer has found the amount of ice in the Earth’s rivers has rapidly declined over the past three decades, and the trend is likely to continue.

George Allen, assistant professor of geography at Texas A&M, is part of the study led by researchers at the University of North Carolina that is published in the current issue of Nature magazine.

More than a third of the Earth’s land mass is drained by rivers that seasonably freeze over, forming river ice. Examining more than 400,000 satellite images over the past 34 years, the team found that the overall amount of river ice has been declining, and will almost certainly continue to do so as global temperatures continue to rise.

The researchers found that for every degree of increase in global temperature, there will be about six days less of ice per year on rivers that freeze.

“We found that 56 percent of Earth’s rivers are seasonally affected by river ice, which amounts to a much greater proportion of rivers than previously thought,” Allen said. “We found that almost all the river ice is located in the Northern Hemisphere with only rivers in southernmost portions of New Zealand, Patagonia and Australia. It is important to note that we only observed rivers with widths over 300 feet, so it is likely that there a lot of smaller rivers and streams that also freeze but we did not detect them.”

Allen said that rivers are natural hotspots for greenhouse gas emissions to the atmosphere, and these emissions typically slow down or completely stop when rivers freeze. The team’s findings that the occurrence of river ice is declining suggests that the global river network is contributing more and more to the atmospheric greenhouse gas emissions.

“The anticipated acceleration in the decline of river ice is not evenly distributed around the world,” Allen said. “In the United States, river ice is expected to decline most rapidly in the Rocky Mountains and in the Northeast.

“If you’ve lived in Texas for a long time, you probably haven’t thought that much about the importance of river ice,” Allen said. “However, in many parts of the world, river ice plays important roles in transportation and in ecosystems. For example, during the winter, many remote Arctic civilizations rely on frozen rivers as ice roads. In the spring, during river ice breakup, ice jams can cause unpredictable and hugely disruptive floods, but these floods actually benefit ecosystems by distributing nutrients across floodplains.”

Allen said the study is made possible by two recent technological developments. One is the first global map of river location and width from optical satellite images, and the second is cloud- based data provided by Google Earth Engine that makes processing hundreds of thousands of satellite images relatively easy.

The study was funded by the NASA/CNES Surface Water and Ocean Topography (SWOT) satellite mission, which is planned for launch in September 2021 and is expected to provide unprecedented observations of Earth’s rivers and lakes.

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