As rivers snake and slice through landscapes, the ever-moving water undergoes seasonal changes that have impacts on ecosystems, water systems, agriculture, and economies.
In middle to high latitudes, rivers can form ice shells, capping the underlying flow. This river ice is a multifaceted powerhouse: It influences nutrient cycling in rivers, can function as winter roads for travel, and can help block greenhouse gases from escaping from stream surfaces.
The importance of river ice has long been clear, but researchers have now peered into how global river ice cover might be changing in a warmer climate. Using 400,000 satellite images from 1984 to 2018, the researchers found that on average, ice cover declined by almost a week over those decades.
They also projected ice cover losses for the turn of the next century and found that as temperatures increase, global river ice cover will continue to decline by about 16 days. Less river ice will impact the hydrology and ecology of rivers and hurt those who rely on ice for transportation.
Decades of Changing River Ice
In winter, ice does more than blanket the riverine system. “River ice plays an important role in the hydraulics, geomorphology, and ecology of rivers,” said Ian Knack, a cold regions hydraulic engineer at Clarkson University in Potsdam, N.Y., who was not involved in the new research.
Considering the importance of river ice, researchers have now determined how the global extent of river ice might be changing. In their paper in Nature, researchers looked at the history of river ice cover around the world.
The team used Landsat imagery from 1984 to 2018 to calculate the percentage of river ice cover. The decades-long record provided the team with “global coverage and multitemporal observations,” said Xiao Yang, a paleoclimatologist at the University of North Carolina at Chapel Hill and lead author of the study.
They used 400,000 images in their analyses, said Yang. “By analyzing 34 years of data, we estimated that about 56% of the rivers globally are affected by seasonal ice,” he said. In addition, the team estimates “that there is 2.5 percentage points decline of river ice globally during this time.”
“The paper presents an interesting means of evaluating global changes to river ice extent and timing,” said Knack. “The use of Landsat for evaluating river ice conditions has become quite popular in the past few decades, but its use at the global scale to establish the extents of ice cover is novel.”
The researchers also developed a model for the future of river ice based on temperature and the seasons. “We used climate simulation data to predict what will happen to river ice,” said Yang. “What we found was that in a business-as-usual scenario, in terms of climate simulation, we’re expecting to see 16.7 days decline of river ice.”
He added that this is an average value for all rivers globally. If you factor out the rivers that never experience ice, the number of lost river ice days rises considerably. “The actual ice duration decline for rivers that have ice will probably be over a month,” said Yang.
Knack said he was “surprised how strong the relationship is between surface air temperature and river ice extent.” He added that although temperature and ice formation are “strongly linked physically, river ice is also influenced by precipitation and solar radiation.”
“Rain or snow events and clear spring days have led to significant ice breakup and jamming events in the past few decades, and I would have expected that those factors would have reduced the relationship with air temperature alone,” Knack explained.
Although Knack found the approach to be an interesting start, he said the researchers focused on relatively large rivers. “While these rivers are very significant when it comes to river ice processes, small rivers are also strongly affected by river ice processes.”
He also noted that Landsat sometimes has trouble telling the difference between smooth, clear ice and open water. “Although these areas were excluded from the study to reduce errors, it may have also excluded important rivers in the overall conclusions about the rate of change of river ice extents.”
Future of River Ice
Yang said that although there is a plethora of research on sea ice and ice sheet monitoring with regard to climate change, he hopes that their research might spur more attention to freshwater ice changes.
“Lakes and rivers are actually the hot spots for a lot of geochemical processes happening on the landscape,” said Yang. “And a lot of the things that we care about, for example, of greenhouse gas emission from these systems, are an important contribution to global carbon budget.”
Yang noted wintertime is an integral part of the annual cycle of the river but is often understudied. “It’s really important that we understand how the hydrology, the ecosystems, the climate associated with these systems operate,” he said.
—Sarah Derouin (@Sarah_Derouin), Science Writer