New research suggests that glacier-led, freshwater rivers in northern Canada can sequester carbon from the atmosphere at a rate faster than the Amazon rainforest. The discovery was made by a team of biologists from the University of Alberta, who measured dissolved carbon dioxide in meltwater samples from Lake Hazen (Nunavut), as well as the Rocky Mountains and Greenland.
The findings overturn conventional wisdom on rivers as sources – rather than sinks – of atmospheric carbon. This is attributed to the biological activity of organisms that populate them, such as photosynthesis, respiration and decomposition. Glacial rivers, on the other hand, are mostly abiotic as a result of their freezing temperature and high turbidity, leading to far less carbon output. Their ‘milky’ appearance comes from the vast amounts of mineral-rich sediment created by glacial advance and retreat. As sediment mixes with meltwater and atmospheric gas, it undergoes a series of reactions called ‘chemical weathering’. The weathering of minerals affects water chemistry by producing alkalinity, which in turn prompts CO2 absorption as far away as 42 kilometres from the river source.
The scientists concluded that glacial rivers will absorb up to 40 times more carbon dioxide than the Amazon rainforest during high melt seasons. To be clear, this is on a per-metre-squared basis as the rainforest spans 6.7 million square kilometres in total, eclipsing that of a glacial watershed. Future research will investigate how common this weathering phenomenon is across different high-latitude, high altitude regions around the world.
At first glance, the study offers a surprisingly optimistic message amid the ‘doom and gloom’ of present-day climate change narratives: that there are unseen ways in which the planet regulates greenhouse emissions. But to reframe glacial melt (and rising sea levels) as humanity’s ‘geochemical saviour’ would be a crude misreading.
For one, the sequestration capacities of meltwater are finite and persist only insofar as we still have glaciers. It also cannot keep up with other climate-induced carbon emitters such as thawing permafrost. Previously named the ‘sleeping giant of the global carbon cycle’, arctic permafrost is expected to discharge up to 280 gigatonnes of CO2 and 3 gigatonnes of methane by the end of the century. “If we don’t do anything to curb global warming”, says lead researcher Dr. Kyra St. Pierre, “we are potentially losing something we derive a lot of benefits from, before we actually fully understand it”.
In order to establish more robust carbon budgets in line the Paris Agreement and SDGs, a deeper understanding of how melting glaciers impact freshwater systems and the carbon cycle at large is needed.