Running water sheds light on carbon budget

Bani Khalid, Oman: Wadis like this are potent sources of CO2. Image: By Richard Bartz, via Wikimedia Commons

A surprising stretch of terrestrial rock and soil is actually covered by running water. New measurements could mean new accuracies for climate science.

LONDON, 6 July, 2018 – Researchers have a new map of waterworld, the running water that is a key part of the carbon budget. The rivers and streams that feed life and drain landscapes across the planet cover an area almost half as great again as previous estimates – and wash over an area of rocks and soil that adds up to 773.000 square kilometres, an area slightly bigger than the land surface of Turkey.

This finding has consequences for those climate scientists who are trying to make accurate calculations of the planet’s carbon budget: the trafficking between life, rocks, ocean and atmosphere of carbon dioxide and other greenhouse gases.

That is because rivers and streams are active players in the climate machine. They release roughly 20% of the carbon dioxide levels emitted by humans when they burn fossil fuels or turn limestone rock into cement.

And although such research counts as basic – just another, hopefully more accurate, picture of the planetary surface and its moving parts – it fits into an increasingly complex picture of the world and its relationship with freshwater.

“It’s really important that we clearly understand where the carbon that we are emitting goes, and that requires us to accurately quantify the global carbon cycle”

A second, entirely separate study has found that dry riverbeds – those wadis and gulches that carry intermittent flash floods across arid landscapes – could add up to as much as half of the global river network.

But it also found that the dry and parched plant litter in the riverbeds decomposes rapidly when the rains arrive, to add carbon dioxide emissions that reach as much as 10% of the daily discharge from those permanently watery watercourses.

And a third study has found that carbon dioxide is not the only concern: as the planet warms as a consequence of relentless human use of fossil fuels to raise greenhouse gas levels in the atmosphere, the world’s ponds, lakes and marshes could release significantly more methane. For every 1°C temperature increase, the releases of this potent greenhouse gas could rise by between 6% and 20%.

Such studies are intended to provide a more reliable basis for the calculations of other scientists. Hydrologists from the US report in the journal Science that they used decades of satellite data, ground measurements, flow studies and some complex mathematics to make estimates not just of rivers’ lengths, but their widths, to arrive at their grand total of 773,000 sq km.

Helping mitigation

“As we try to mitigate the effects of climate change, it’s really important that we clearly understand where the carbon that we are emitting goes, and that requires us to accurately quantify the global carbon cycle,” said Tamlin Pavelsky, of the University of North Carolina at Chapel Hill.

“Our new calculation helps scientists better assess how much carbon dioxide is moving from rivers and streams into the atmosphere each year.”

A team from Australia and 22 other nations asked a slightly different question: what about those rivers that don’t flow most of the time? The researchers report in the journal Nature Geoscience that they looked at 212 dry riverbeds across a range of climate zones to sample the pool of dried organic material at risk of decomposition once it gets wet.

“Taking rivers and streams that only flow at certain times into account would improve estimates of the consequences of global climate change on carbon cycling – given that the extent of these rivers and streams will increase, and periods of drying will become more prolonged in many regions,” said Nathan Waltham of James Cook University.

Watching methane bubble

And another international team – this time led by researchers from the Netherlands – report in the journal Nature Communications that they collected data on methane bubbles from fishing ponds nearby, from post-glacial lakes in Sweden and forest ponds in Canada, and then simulated lake behaviour in a series of laboratory mini-lakes in which they could control the temperature, and then watch the methane bubble.

Their hunch was right. Higher temperatures meant more methane, and at predictable levels.

“Never before have such unequivocal strong relationships between temperature and emissions of methane bubbles been shown on such a wide, continent-spanning scale,” said Sarian Kosten, of Radboud University.

“Every tonne of greenhouse gas we emit leads to additional emissions from natural sources such as methane bubbles. Luckily, the opposite is also true: if we emit less greenhouse gas and the temperature drops, we gain a bonus in the form of less methane production.” – Climate News Network

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