May 17, 2015, by Tim Radford
Where does all the carbon go? Only half stays in the atmosphere
Image: Ra Boe via Wikimedia Commons
Climate change has intensified more slowly than scientists had expected because the continents and oceans are absorbing more atmospheric carbon dioxide. LONDON, 17 May, 2015 − Half of all the carbon emissions from burning fossil fuels remain in the atmosphere. The good news is that only half remain in the atmosphere, while the rest have been taken up by the living world and then absorbed into the land, and the ocean. That is, as carbon dioxide levels in the atmosphere have risen, so also has the planet’s capacity to soak up atmospheric carbon. The implication is that what engineers call “positive feedback” – in which global warming triggers the release of yet more greenhouse gases into the atmosphere to accelerate yet further warming – doesn’t seem to be at work yet. The implication, too, is that the world’s governments still have time to launch determined programmes to sharply reduce fossil fuel use, and switch to wind, solar and other renewable energy sources before climate change disrupts the planet’s food security and exacts what could be a devastating toll on the biosphere. But most climate scientists know all this anyway: the real significance of a new study in the journal Biogeosciences is that US and British scientists have narrowed some of the uncertainties in what climate scientists like to call the carbon budget: how much gets into the atmosphere, where it goes, and how long it stays. That is because although the big picture – that carbon dioxide levels in the atmosphere are beginning to rise steeply – has been confirmed repeatedly by systematic measurements since 1956, the potential margin of error has been considerable.
“This increased uptake by land and ocean is not only surprising; it’s good news”
“There is no question that land and oceans have, for at least the last five and half decades, been taking up about half of the carbon emitted each year. The outstanding question is, Why?” said Richard Houghton of the Woods Hole Research Center in Massachusetts, one of the authors. “Most of the processes responsible for that uptake would be expected to slow down as the Earth warms, but we haven’t seen it yet. Since the emissions today are three times higher than they were in the 1960s, this increased uptake by land and ocean is not only surprising; it’s good news. “Without it, the concentration of CO2 in the atmosphere would be twice what it is, and climate change would be much farther along. But there’s no guarantee that it will continue.” The carbon budget is an integral part of the climate puzzle: all simulations of how climate will change with increasing emissions from fossil fuels depend on an understanding of how much carbon dioxide concentrates in the atmosphere and what happens to it after that. In the last few months researchers have reported a dramatic uptake of atmospheric carbon by new forests and the growth of woodland on the world’s savannahs and pinpointed the fjords – those steep, still stretches of sea in mountainous coastlines in the high latitudes – as prime “sinks” for atmospheric carbon.
At the same time others have once again confirmed fears that thawing permafrost could release vast quantities of carbon stored for millennia is semi-decayed and now frozen vegetation. But these have been studies of small pieces of the big puzzle. What the Biogeosciences authors did was to refine two global uncertainties. One is how much fossil fuel is burned each year and the other is how much is stacking up in the atmosphere. Both sound simple, but the first question is complicated by differences in the ways nations maintain their own energy inventories, and the way they report the details, and the second depends on how the use of land has changed, how the oceans are responding to higher levels of acidification and how carbon dioxide levels vary according to region, and to season. With greater certainty in the answers to the second question – which began with one single set of measurements at the top of a mountain in Hawaii now replicated worldwide – researchers found they could make more sense of the first question, and narrow the uncertainties to a point where they could write that they were “93% confident that terrestrial C uptake has increased and 97% confident that ocean C uptake has increased in the last five decades. “Thus it is clear that arguably one of the most vital ecosystem services currently provided by the biosphere is the continued removal of approximately half of atmospheric CO2 emissions from the atmosphere.” − Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.