Sink to source

Why the carbon cycle is set for a U-turn

It seemed too good to be true. Throughout the 1980s and 1990s, evidence grew that wherever forests survived around the world, they were growing faster. And as they did so, they were soaking up ever more carbon dioxide from the air. Despite deforestation in the tropics, the world's forests overall were a strong carbon sink. Most researchers assumed that the extra growth happened because rising concentrations of carbon dioxide in the atmosphere made it easier for trees to absorb the gas from the air. Provided that the other ingredients for photosynthesis, such as water and nutrients, were available, the sky was the limit for growing plants. The "C02 fertilization effect" entered the climate scientists' lexicon.

In 1998, at the height of this enthusiasm, a group of carbon modelers at Princeton University scored what looked like a political as well as a scientific bull's-eye. Song-Miao Fan and colleagues claimed in a paper in Science to have discovered "a large terrestrial carbon sink in North America." The U.S. and Canada, they said, had become a hot spot for carbon absorption, as trees grew on abandoned farmland and previously logged forests, and carbon dioxide in the air boosted growth. They calculated the sink at a stunning 2.2 billion tons a year—more than enough to offset the two countries' total annual emissions from power plants, cars, and the rest. Thanks to their trees, the biggest polluters on the planet were "carbon neutral."

To many, it seemed an outrageous claim. And on examination, it turned out to involve some fairly heroic assumptions about where carbon dioxide in the atmosphere was coming from, where it was going, and how it moved around. Carbon-cycle specialists poured cold water on the notion. The figure of 2.2 billion tons was not far off the total amount of carbon that North America's trees absorbed in a year in order to grow. If it was accurate, it meant that no North American trees were dying; they weren't even breathing out—because both processes release carbon dioxide back into the air. But the findings came less than a year after the Clinton administration had signed up for tough carbon-dioxide-emissions targets at Kyoto, without any clear idea of how it was going to achieve them. They seemed like manna from heaven.

And yes, it was too good to be true. The authors agreed that their data were sparse and their analytical techniques largely untried. Nobody, it turned out, could repeat the results. A plethora of researchers demonstrated that U.S. forests could never have stashed away more than a fifth of the nation's emissions. After a while, nobody stood up to defend the original results, and they disappeared from view as fast as they had arrived.

The final nail entered the coffin when it emerged later that 1998, when the report was published, was just about the worst year on record for nature's ability to soak up carbon dioxide from the air. Forests and peat bogs had burned from the Amazon to Borneo. If there had been a big sink, it was disappearing even as it was uncovered. And it wasn't just in the tropics that carbon had been seeping out of the biosphere. There were major forest fires from Florida to Sardinia, and from Peru to Siberia—where Russian foresters revealed that a conflagration on a par with Borneo's had been taking place virtually unnoticed. The world's largest stretch of forest, which for 200 years had been soaking up a fraction of Europe's industrial emissions as they poured east on the prevailing winds, was giving up what it had previously absorbed. As 1998 closed, the idea of a huge carbon sink in the U.S. or anywhere else seemed absurd.

The next episode in the story of the amazing disappearing carbon sink came in the summer of 2003, when Europe suffered a massive heat wave. Temperatures averaged 1 o°F above normal during July. In France, the mercury soared above io4°F. With the high temperatures accompanied by less than half the usual rainfall, Europe's beech trees and cornfields, grasslands and pine forests, were expiring.

Philippe Ciais, a Paris-based environmental scientist, followed events. He was a key player in CarboEurope, a project begun a couple of years earlier to measure Europe's carbon sink. It was launched in the aftermath of the purported discovery of the large North American carbon sink. European politicians, like their U.S. counterparts, were keen to discover if nature was helping them meet their own Kyoto Protocol targets. Ciais's initial assessment was that, thanks to warmer temperatures, higher carbon dioxide levels in the air, and a longer growing season, Europe's ecosystems were absorbing up to 12 percent of its man-made emissions.

But in 2003, the carbon sink blew a fuse. During July and August that year, when Europe's ecosystems would normally have been in full bloom and soaking up carbon dioxide at their fastest, around 550 million tons of carbon escaped from western European forests and fields. This was roughly equivalent to twice Europe's emissions from burning fossil fuels during those two months. All the carbon absorbed in recent years was being dumped back into the atmosphere in double-quick time. The rapid exhaling of the continent's ecosystems was "unprecedented in the last century," said Ciais. But he judged that it was likely to be repeated "as future droughts turn temperate ecosystems from carbon sinks into carbon sources."

Europe seemed to have fast-forwarded into a nightmare future strapped to a runaway greenhouse effect. And it soon emerged that Europe's carbon crisis was part of a more general story of summer stress across the Northern Hemisphere. Ning Zeng, of the University of Maryland, found an area of drought stretching from the Mediterranean to Afghanistan. It had lasted from 1998 to 2002, and had eliminated a natural carbon sink across the region that had averaged 770 million tons a year over the previous two decades.

Alon Angert, of the University of California at Berkeley, explained the big picture. Through the 1980s and into the early 1990s, the "C02 fertilization effect" had been working rather well, with increased photosynthesis in the Northern Hemisphere soaking up ever more carbon dioxide. But sometime around 1993 that had tailed off, probably because of droughts and higher temperatures. And since the mid-1990s, the carbon sink had been in sharp decline. From the Mediterranean to central Asia, and even in the high latitudes of Siberia and northern Europe, the added uptake of carbon by plants in the early spring was canceled out by the heat and water stress of hotter, drier summers. The findings, Angert said, dashed widespread expectations of a continuing "greening trend" in which warm summers would speed plant growth and moderate climate change. Instead, "excess heating is driving the dieback of forests, accelerating soil carbon loss and transforming the land from a sink to a source of carbon to the atmosphere."

And further north, beyond the tree line, where some of the fastest warming rates in the world are currently being experienced, fear is growing about the carbon stored in the thick layers of permanently frozen soil known as permafrost. The carbon comprises thousands of years' accumulation of dead lichen, moss, and other vegetation that never had a chance to rot before it froze. David Lawrence, of the NCAR, reported in 2005 that he expected the top 3 yards of permafrost across most of the Arctic to melt during the twenty-first century. This will leave a trail of buckled highways, toppled buildings, broken pipelines, and bemused reindeer; it will also unfreeze tens and perhaps hundreds of billions of tons of carbon. As the thawed vegetation finally rots, most of its carbon will return to the atmosphere as carbon dioxide. In those bogs and lakes where there is very little oxygen, most of the carbon will be converted into methane—which, as we will see in the next chapter, is an even more potent greenhouse gas.

We should not write off the carbon sink entirely. It won't die altogether. Especially in higher latitudes, warmer and wetter conditions will sometimes mean that trees grow faster and farther north than before—at least where plagues of insects don't get them first. Right now, the best guess is that, on average, forests are still absorbing more carbon dioxide than they release. Up to a fifth of the carbon dioxide emitted by burning fossil fuels may still be being absorbed by soils and forests. But the sink is diminishing, not rising as many anticipated. And many believe that the sink is doomed as we face more and more years like 1998 and 2003.

One of those who fear the worst is Peter Cox, a top young British climate modeler who left the Hadley Centre to spend more time investigating the carbon cycle at the Centre for Ecology and Hydrology, at Winfrith, in Dorset. He believes he is on the trail of the disappearing carbon sink, and is prepared to put a date on when it will disappear. "Basically, we are seeing two competing things going on," he says. "Plants absorb carbon dioxide as they grow through photosynthesis; but they give back the carbon dioxide as they die and their wood, leaves, and roots decompose. The speed of both processes is increasing."

First, the extra carbon dioxide in the atmosphere encourages photosynthesis to speed up. So plants grow faster and absorb more carbon dioxide. But that extra carbon dioxide is also warming the climate. And the warming encourages the processes that break down plant material and release carbon dioxide back into the air. Because it takes a couple of decades for the extra carbon dioxide to bring about warmer temperatures, we have seen the fertilization effect first. Now the process of decay is starting to catch up.

The processes do not involve plants alone. Soils have their own processes of inhaling and exhaling carbon. And they, too, will switch from being a net sink to a net source—eventually releasing what carbon they have absorbed in recent decades. Ultimately, "you can't have the one without the other," Cox says. "If you breathe in, eventually you have to breathe out." And soon, most of the rainforests and soils of the world will be breathing out, pouring their stored carbon back into the air. If the climate gets drier and more fires occur, then the release of the carbon dioxide will happen even more quickly. But it will happen anyway.

The entire land biosphere—the forests and soils and pastures and bogs—has been slowing the pace of global warming for some decades. Soon the biosphere will start to speed it up. The day the biosphere turns from sink to source will be another tipping point in Earth's system. Once under way, the process, like collapsing ice sheets, will be unstoppable. Potentially, hundreds of billions of tons of carbon in the biosphere could be destabilized, says Pep Canadell, a carbon-cycle researcher for the Australian government research agency CSIRO.

Nobody is quite sure when the tipping point might occur. "It is possible," says Cox, "that the 2003 surge of carbon dioxide into the atmosphere is the first evidence." But while some parts of the biosphere may now be irrevocably stuck as carbon sources, the entire system is likely to take a few decades to switch. But of course, much will probably depend on how fast we allow temperatures to rise.

Cox suggests that 2040 is probably when the biosphere will start taking its revenge on us for relying on its accommodating nature. He calculates that by the end of the century, the biosphere could be adding as much as 8 billion tons of carbon to the atmosphere each year. That is roughly the amount coming each year from burning fossil fuels today, and probably enough to add an extra 2 or 3°F to global temperatures—degrees that are not yet included in the IPCC forecasts.

Only one country, so far as I am aware, has completed anything like a national study of the current impact of these changes on its carbon budget.

Perhaps understandably, such studies have a lower priority since nature was shown to be unlikely to offer a helping hand in meeting Kyoto targets. But Guy Kirk, of the National Soil Resources Institute, part of Cranfield University, has done the job for Britain. He surveyed 6,000 test plots across forest and bog, heath and farmland, scrub and back gardens, to see how much carbon dioxide is leaving the biosphere and how much is entering it. His conclusion is that the British biosphere is releasing about 1 percent of its carbon store into the atmosphere every year. Enough, in other words, to turn the whole country into desert in one century.

Kirk rules out altered methods of farming or land use as the predominant cause. The increase is so universal that it can only be owing to climate change. He puts the national release at around 14 million tons a year. That, he points out, is roughly the amount of carbon dioxide the British government has kept from the atmosphere each year in its efforts to comply with the Kyoto Protocol. As the German researcher Ernst-Detlef Schulze, of CarboEurope, puts it—rather gloatingly, I think—this "completely offsets the technological achievements of reducing carbon dioxide emissions, putting the UK's success in reducing greenhouse gas emissions in a different light." True enough. But Britain is not alone.

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