Why ‘Carbon-Cycle Feedbacks’ Could Drive Temperatures Even Higher

https://e360.yale.edu/features/why-carbon-cycle-feedbacks-could-drive-temperatures-even-higher?fbclid=IwAR1EYQERxidwAcwrEKAWzAFYowtG51Rxbnh-oEKqVRfV9bU0_b23nGjxWDw

By Fred Pearce • April 28, 2020

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For a decade, her team had been sampling the air from sensors on aircraft flying over the world’s largest rainforest. Their collating of recent results showed that, perhaps for the first time in thousands of years, a large part of the Amazon had switched from absorbing CO2 from the air, damping down global warming, to being a “source” of the greenhouse gas and thus speeding up warming.

“We have hit a tipping point,” Gatti almost shouted, caught between elation at her discovery and anguish at the consequences.

As she spoke, fires were burning across the Amazon, making headlines around the globe. But her findings were not the short-term result of the fires. They were based on measurements from before the upsurge in fires, and showed a long-term trend. She had previously observed the same thing briefly during drought years. But now it no longer mattered if it was a wet or a dry year, or how many fires there were, the sink had become a source. “Each year it gets worse,” she said. “We have to stop deforestation while we work out what to do.”

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The scientists are warning that past climate models used by the UN’s Intergovernmental Panel on Climate Change (IPCC) have not fully reflected the scale of the warming that lies ahead as carbon sinks die. These revelations are coming from three areas of research:

Studies such as Gatti’s in the Amazon, showing forests turning from sinks to sources of CO2;
A new generation of climate models that incorporate these findings into future projections of climate change, and whose early outputs are just emerging;
Recent revelations that ecosystems are releasing rising volumes of methane, the second most important greenhouse gas and of vital importance for temperatures in the next couple of decades.

The extra emissions, known as carbon-cycle feedbacks, could already be making the prospect of keeping warming below 2 degrees Celsius — the target agreed to in the Paris climate accord in 2015 — all but impossible. The new modeling is likely to result in more pessimistic projections in the next scientific assessment from the IPCC, which is due — coronavirus-permitting — in April 2021.

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Another big concern is the impact of thawing permafrost. This frozen ground, which covers large areas of the far north, holds hundreds of billions of tons of carbon that could be released as the land thaws. How much and how fast is an unresolved question. But the signs are not good. One recent study in northern Canada found thawing had reached depths “already exceeding those projected to occur by 2090.”

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Mark Lunt of Edinburgh University has also found a dramatic increase in emissions from the Sudd, a vast wetland downstream of Lake Victoria on the Nile in South Sudan. The presumption is that warmer temperatures are making microbes more active.

None of this methane increase is built into even the new climate models with carbon-cycle feedbacks. These models mostly assume that methane levels in the air will remain stable. But the concern is growing that, even if technology can reduce industrial emissions, a warmer world will drive a continuing surge in methane levels — and more warming as a consequence.

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Climate models conventionally assess the warming impacts of greenhouse gases over a century. This effectively tunes them to emphasize the importance of C02, and relegates methane to an also-ran. But if they were tuned to the shorter timeframe, methane would appear almost three times more important.
ALSO ON YALE E360

What is causing the recent rise in methane emissions? Read more.

It seems odd that this shorter timeframe is rarely adopted, given that the world risks exceeding its two-degree warming limit by 2050. As Nisbet puts it, if natural ecosystems keep pumping out more methane as the world warms, “it may become very difficult to meet the Paris goals.”

Nature, it seems, is biting back. Having so far absorbed our pollution indiscretions, it now seems to be making them worse. We only have ourselves to blame.