The role of Arctic rivers in global climate change | Polarjournal
The Kolyma in eastern Siberia is frozen to a depth of several meters for more than eight months of the year. When the ice melts in June, huge quantities of sediment and organic material are released into the Arctic Ocean. Photo: Norman Kuring/NASA’s Ocean Color Web

Arctic rivers carry more water and contain more carbon due to warming, which will lead to changing ecology in the Arctic lagoons, more freshwater in the Arctic Ocean and possibly a slowdown in the Atlantic overturning circulation – effects of the thawing permafrost that a research team has studied in such detail for the first time.

Arctic rivers play a crucial role among the countless changes associated with climate change in the Arctic. A study published on March 5 in the scientific journal The Cryosphere reports on the changes these rivers are currently undergoing and the far-reaching consequences for the environment and climate, not only in the Arctic but worldwide.

The authors of the study investigated the effects of warming on Arctic rivers based on historical data and a computer model that combined climate models with a permafrost water balance model.

“We found that thawing permafrost and intensifying storms will change how water moves into and through Arctic rivers. These changes will affect coastal regions, the Arctic Ocean and, potentially, the North Atlantic, as well as the climate,” write authors Michael A. Rawlins, Associate Director of the Climate System Research Center and Associate Professor of climatology at the University of Massachusetts Amherst, and Ambarish Karmalkar, Assistant Professor of earth sciences at the University of Rhode Island, in an article in The Conversation.

Rawlins and Karmalkar looked at the period up to the end of the century and used two different scenarios for the modeling: a moderate scenario in which greenhouse gas emissions are reduced and the rise in temperature is slowed, and a pessimistic scenario with high emissions and strong warming. They discovered that the growing thickness of the active layer of permafrost will fundamentally change the hydrology of the region.

In the northern hemisphere, permafrost soils still cover an area of more than 22 million square kilometers, but the researchers assume that the extent of permafrost in the Arctic will decrease by 42 to 63 % by the end of the century. Map: GRID-Arendal/Nunataryuk

“A thicker active layer creates a bigger bucket for storing water,” Rawlins said in a university press release. “Our work shows that as precipitation intensifies, the water will be stored longer in thawed soils and released at a later time via subsurface pathways, instead of running off immediately into rivers and streams, as much of it does now.”

The active layer of permafrost, which regularly thaws in summer, becomes thicker due to the rapid warming and lasts longer before it freezes again. This fundamentally changes the hydrology of the Arctic, the researchers explain: precipitation and meltwater increase with rising temperatures, penetrate the unfrozen ground over a longer period of time per year and increasingly flow into the rivers via subsurface paths. Their calculations showed that the surface runoff of the large Arctic rivers will increase by up to 25 % and the subsurface runoff by as much as 30 %.

The intensification of the water cycle is also partly attributed to increased evaporation over the increasingly ice-free Arctic Ocean. For the southern Arctic, the researchers predict a general drying out of the landscape, as much of the additional precipitation will be returned to the atmosphere through evaporation and plant transpiration due to the strong warming.

The Arctic river basins: The width of the blue lines illustrates the relative discharge of the rivers, with the thickest lines indicating the rivers with the largest volume. The figures on the map indicate the discharge in cubic kilometers per year. Map: NOAA Arctic Climate Impact Assessment Report

But the effects go even further: according to the study, the Arctic rivers – especially the largest ones such as the Ob, the Yenisei, the Lena and the Mackenzie – will take in proportionately more water from their northern sections. This will also increase the input of fresh water, millennia-old carbon and other substances bound in the soil into the species-rich Arctic coastal areas, which will have an impact on the ecology of coastal waters. While the salinity will decrease, more nutrients will be introduced with effects on organisms throughout the food chain.

“River water will also be warmer as the climate heats up and has the potential to melt coastal sea ice earlier in the season,” Rawlins and Karmalkar note in The Conversation.

It is possible that the increased freshwater input into the ocean could also influence the Atlantic Meridional Overturning Circulation (AMOC), which ensures a temperate climate in northern Europe, and contribute to its slowdown.

There are still many unanswered questions: “More field observations are needed from the small- and medium-sized rivers near the Arctic coast to better understand how warming will alter the land-to-ocean transport of freshwater and, in turn, impact Arctic environments and the flora, fauna and Indigenous populations that call the region their home,” says Rawlins.

Julia Hager, PolarJournal

Link to the study Michael A. Rawlins, Ambarish V. Karmalkar. Regime shifts in Arctic terrestrial hydrology manifested from impacts of climate warming. The Cryosphere, 2024; 18 (3): 1033 DOI: 10.5194/tc-18-1033-2024

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