

Why Is the Boreal Forest Breathing CO2 More Deeply?

The flow of CO2 into and out of the vast northern forests has been increasing in recent years, thanks to climate change

© David Biello

The boreal forest seems to like the higher levels of carbon dioxide that result from fossil-fuel burning. All that carbon appears to be enabling growth rates not seen in human history for the northernmost forest, according to a new study.

"Boreal forests are more active than 50 years ago," says geochemist Heather Graven of the Scripps Institution of Oceanography, lead author of the new analysis, published online August 8 in Science. She and her colleagues documented a dramatic uptick in the flow of carbon into and out of the forests, which cover most of Canada as well as large swathes of Russia and Scandinavia. "The ecosystems might be changing how much carbon is allocated to different structures, such as leaves, wood or roots, changing their extent or composition, or changing the timing of photosynthesis and respiration as a result of warmer temperatures," she adds.

In the famous saw-toothed Keeling curve depicting increasing levels of CO2 in the atmosphere, the saw-teeth—rising and falling as the Northern Hemisphere breathes in and out with the seasons—have become longer. That is, the difference between low CO2 levels in summer and high atmospheric concentrations at the end of winter have become more pronounced; the amplitude of the change has increased by roughly 50 percent over the last 50 years. Northern forests appear to be taking in more CO2 in summer and releasing more in winter—as shown by Graven and her colleagues who compared atmospheric measurements taken in the far north during and after the International Geophysical Year between 1958 and 1961 with a series of airplane flight measurements taken between 2009 and 2011. All told, at least 32 percent more carbon—an additional 1.3 billion metric tons—moves back and forth between atmosphere and forest, which is roughly two thirds as much as is thought to be stored in boreal forests annually and more than a tenth of the total carbon emitted to the atmosphere by human activity in a year.

Such CO2 ebb and flow suggests that the boreal forest is growing more, although the increase in amplitude is a surprise. Whether this results from more growth overall or growth in response to wildfire or pests remains unclear, but scientists had anticipated that higher levels of CO2, which acts like a fertilizer up to a point, would promote plant growth. As a result, some research has found that portions of the world's deserts have begun to green.

More surprises
In the past decade average surface temperatures have hit a warming plateau after rising rapidly in the 1990s. The main reason seems to be that the deep ocean stores more heat more quickly than scientists had originally anticipated—with waters down to 2,000 meters in depth warming by roughly 0.1 degree Celsius (the equivalent of some 36-degree C atmospheric warming) in recent decades. "This is fairly new," explained climatologist Kevin Trenberth of the National Center for Atmospheric Research this past May. "It means less short-term warming at the surface but at the expense of a greater earlier long-term warming, and faster sea level rise."

Despite any "pause," as the leveling off of warming has been called, 2012 still ranked as one of the 10 warmest years since the late 1800s—all of which have occurred during the pause in the early years of the 21st century. Arctic sea ice reached record lows and global sea level reached record highs, as noted in the U.S. National Oceanic and Atmospheric Administration's State of the Climate report for 2012. And whatever happens in the short term, the CO2 put into the atmosphere today will help shape the weather for a long time to come. "Our past, present and future emissions will influence the climate system for millennia," as the American Geophysical Union put it in a revised consensus statement on climate change released on August 5. "Higher emissions of greenhouse gases will lead to larger warming, and greater risk to society and ecosystems."

In the case of the great northern forests, it is unclear how much more carbon, if any, the boreal region actually removes from the atmosphere and sequesters in leaves, roots and wood—or in the ecosystem as a whole. "It's like the difference between how deeply you are breathing and whether you are gaining weight," Graven explains. "We are not measuring weight gain." But the depth of that breathing has increased in the most recent years, suggesting there is no slowdown in this uptick in amplitude at present—much as CO2 emissions from human activities continue to grow at roughly 3 percent per year and atmospheric concentrations of the gas touched 400 parts per million for the first time this past spring. No matter how deeply the boreal forests breathe, fossil-fuel burning and other human activities continue to fill the air with ever more greenhouse gases.

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