Climate Change May Make Carbon Sinks Less Effective, Studies Say

By Alyson Kenward

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This week, U.S. politicians are negotiating the terms of the federal debt. The consequences of raising the debt ceiling (or not) are complicated. The underlying budget dilemma, however, is not: there is a lot more money coming out of federal coffers than there is going in.

In order to balance the global carbon budget, scientists are studying how much human carbon emissions are going into the oceans and forests around the world. Credit: o5com/flickr.

In terms of global greenhouse gas emissions, the problem is the opposite: human activities that burn fossil fuels like coal and oil are pumping carbon dioxide (CO2) into the atmosphere much faster than is removed by natural processes. This poses a problem, of course, because as the concentration of CO2 builds up in the atmosphere, it traps more of the sun’s heat, which is one of the reasons why average global temperatures have been climbing over the last few decades.

It’s helpful to think of the amount of CO2 in the atmosphere as water in a bathtub. According to this analogy, burning fossil fuels is part of the “faucet,” pouring CO2 into the atmosphere. The world’s oceans and land-based ecosystems (like forests, wetlands and grasslands) help “drain” the CO2 back out of the atmosphere, by absorbing it and storing it away. These are called carbon sinks.

“The global sinks are important because together they absorb about 55 percent of the carbon emissions coming from humans each year,” says Corinne Le Quéré, director of the UK’s Tyndall Center for Climate Change Research and chair of the Global Carbon Project. “And the sinks, the land and the ocean, are taking up about equal amounts of that [55 percent].”

Scientists can estimate how much CO2 goes into the atmosphere because most of it — about 80 percent — comes from burning fossil fuels. Turning down or shutting off this carbon faucet would help lower the concentration of greenhouse gases in the atmosphere, but there is another way to help balance the CO2 budget. The size of the CO2 drain, or sink, also matters, but that depends on exactly how much carbon the oceans, trees, and plants are taking up.

So, imagine how much more complex the federal debt issue would be if the government didn’t know how much money was being spent? It’s awfully tough to balance a budget without that information. That's the kind of challenge scientists and policy makers face when seeking to balance the carbon budget, because there's a lot of uncertainty regarding how big the sink is, and how large it can continue to function at its present efficiency. 

In fact, among the researchers studying the global carbon cycle, Le Quéré says there is still intense debate over whether global warming has changed how much carbon the sinks are taking up. Some recent research, however, has revealed surprising new details on which natural carbon sinks are doing the heaviest lifting, and some not-so surprising details about how effective these carbon sinks are likely to be in the future, depending on how the climate changes. 

According to a new international study led by the US Forest Service, global forests are responsible for taking up about 2.4 billion tons of carbon each year, which turns out to be about one third of all global fossil fuel emissions. Previously, researchers had estimated that all the land around the world (including agricultural space, wetlands, and grasslands) were taking in this amount, so they were surprised to find that trees are doing almost the entire job on their own.

New research shows that one third of human carbon emissions each year gets absorbed by global forests. Credit: Ivan Mlinaric/flickr.

“What we found was that forests are playing a much bigger role than we originally thought,” says Yude Pan, from the US Forest Service. Pan and her colleagues published their findings in the journal Science. 

It’s not as though other plants aren’t absorbing CO2 during photosynthesis — of course they are, because all plants rely on CO2 to grow. But compared to the vast expanses of tropical, temperate, and boreal forests around the world, the CO2 uptake from other land barely makes a dent in human emissions.

How much CO2 forests take out of the atmosphere is offset by the amount of carbon that gets released back into the air during deforestation or wildfires. Over the past decade, an average of 1.3 billion tons of carbon has been sent back into the atmosphere each year by these disturbances. But Pan’s study also illustrated that when deforested land was abandoned or allowed to regrow, it regained a lot of its ability to absorb CO2.

Scientists studying the global carbon balance sheet have also known for years that the oceans play a critical role in taking CO2 out of the atmosphere. Generally speaking, the global oceans absorb another 30 percent of human carbon emissions, although how cold the water is, and how well surface water mixes with the deep ocean influences how much each ocean can absorb.

Even though the oceans and forests together remove over half of the manmade greenhouse gases, a sizable portion of the emissions remain in the atmosphere. And researchers aren’t sure whether the sinks can keep on taking half of all the carbon humans are pumping out.

The oceans also take up about 30 percent of human carbon emissions, but a warming climate may limit how much more the oceans can keep absorbing. Credit: NASA.

“Basically, people don’t understand that these natural sinks are mitigating what is happening in our atmosphere,” says Galen McKinley, a University of Wisconsin at Madison professor studying how carbon cycles through the oceans. McKinley has recently investigated how well the North Atlantic Ocean has kept up with rising fossil fuel emissions. In a paper published last week in the journal Nature Geoscience, McKinley and her colleagues showed that warming waters in the North Atlantic may have caused the ocean to take up less carbon in recent years. Previous studies have found that the warming trend in the North Atlantic was caused in part by global warming.

Moreover, Le Quéré says the global forests are particularly sensitive to climate variability. From year to year, the forest sink can be large, absorbing well over 30 percent of human emissions. On the other hand, years with devastating tropical droughts, like 2005, or when global temperatures have been well above average because of El Niño, like in 1998, the forest sink has shrunk to nearly nothing. Stunted tree growth, increased wildfire burning, and insect infestations all contributed to the significant decrease in carbon uptake in those years.

This fluctuating sink is a consequence of natural climate variability and human disturbances, but it may also be a signal of things to come in the future, as a warmer climate becomes the new normal.

“We’re emphasizing the values of the forests in lowering atmospheric carbon levels,” says US Forest Service scientist Richard Birdsey, who co-authored the new Science paper on global forests, “but they may not stay that way forever.”

In general, Le Quéré says research suggests that if the ocean and forest sinks can't keep pace with rising greenhouse gas emissions, then the extra carbon staying in the atmosphere could increase warming by 5 to 30 percent more than if the sinks keep absorbing at their current rate.

“The current consensus, among the people that study the global carbon cycle, is that as the climate changes in the future, the carbon sink should weaken,” explains Le Quéré. “Overall, we expect that the ocean and land will absorb a bit less [carbon].”

While there is still uncertainty in how much the sinks are going to change in the future, it's possible that as more carbon pours into the atmosphere, it is becoming increasingly difficult for the natural sinks to drain it away. With more carbon emissions going in than coming out, balancing the carbon budget looks like tougher than ever.