This article originally was published on Yale Environment 360.
It is not often you meet a scientist breathless with excitement about new findings. But it happened to me in September at the National Institute for Space Research in the Brazilian research city of Sao Jose dos Campos. Atmospheric chemist Luciana Gatti was rushing to tell her colleagues the result of her latest analysis of carbon dioxide emissions from the Amazon rainforest, which she had completed that morning.
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.
Each year it gets worse ... We have to stop deforestation while we work out what to do.
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 previously had 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."
Gatti asked me to keep silent for the time being while she prepared her data for publication. When I contacted her in April, her paper was still being finalized. But now I can tell the story. It vividly illustrates a growing dismay among climate scientists, who are seeing ecosystems around the world going the way of the Amazon.
The scientists are warning that past climate models used by the U.N.’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, already could 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, due — coronavirus permitting — in April 2021.
Our planet’s land and oceans take up about half of all the CO2 we put into the atmosphere. The gas dissolves in seawater and is absorbed by growing plants. Without these "carbon sinks," warming to date would have been twice as great. We already would have exceeded the 2-degree-Celsius target. But the question now is whether the take-up will remain as it is, or diminish.
That depends on how ecosystems respond to the extra gas in the air. This response takes two competing forms. First, the extra CO2 speeds up plant growth. This fertilization effect means that forests absorb more CO2 as they grow, slowing the build-up in the air. Good news.
But the bad news is that the higher temperatures, also brought about by the added CO2, are pulling in the other direction, reducing nature’s ability to soak up CO2. This happens because warmer ocean waters dissolve less CO2, while soils release more of the gas and some forests suffer heat stress and die or catch fire.
Both these feedbacks are in play. But the debilitating effects of the warming, especially when combined with deforestation, are becoming increasingly dominant, ecologists say. That is what Gatti has seen in the Amazon. And the trend is often happening faster than expected.
Gatti’s findings, while relating to the southeast of the Amazon, the region’s most heavily deforested area, suggest that the rainforest as a whole could be close to flipping from a sink to a source of CO2. The ability of intact areas of the rainforest to absorb CO2 already have halved since the 1990s, says Carlos Nobre of the University of Sao Paulo, Brazil’s most noted climate scientist. Passing the tipping point for the whole forest would release more than 50 billion tons of carbon, he recently said, which is the equivalent of five years of global fossil-fuel and industrial emissions.
Non-tropical forests remain largely in carbon "sink" mode. But other tropical rainforests appear to be following the Amazon in moving toward becoming carbon sources. Wannes Hubau, now at the Royal Museum of Central Africa in Belgium, reported recently that "overall, the uptake of carbon into Earth’s intact tropical forests peaked in the 1990s" and has been declining since. The jungles of tropical Africa began showing increased carbon losses around 2010, he found.
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.”
Some researchers think these alarming findings are unlikely to be realistic in future predictions and should be dismissed.
The risks of such rapid runaway carbon releases to the atmosphere have been worrying ecologists for a while. That worry is being reinforced by the projections of a new generation of climate models designed to factor in how ecosystems respond to climate change.
Until now, most climate models largely have confined themselves to assessing how our CO2 emissions warm the air, and how that warming interacts with physical feedbacks such as reduced ice cover, elevated atmospheric water vapor and changes to clouds. This remains a work in progress. I wrote here on Yale Environment 360 in February how new field research suggests that the ability of clouds to keep us cool could be drastically reduced as the world warms, pushing global heating into overdrive.
When ecological feedbacks have been included in the models, it mostly has been in a very simplistic way. But new models being developed for the next IPCC assessment of climate science are changing that. For the first time, they capture the full range of possibilities for how nature’s ability to soak up CO2 may change as the climate changes, says Richard Betts of Britain’s Met Office Hadley Centre, one of the world’s top climate modeling groups. His initial assessment of the early outcomes of these new models is sounding alarm bells.