Nature's carbon 'sink' smaller than expected

When it comes to global warming, nature's help is limited.

While the continents and oceans have absorbed much of the carbon dioxide that humanity has pumped into the atmosphere so far, they won't be able to keep up with the expected rise in greenhouse-gas emissions over the next several decades. Indeed, some recent studies suggest that current scientific estimates about natural absorption are too optimistic: Earth's climate by century's end could be on average up to 1.5 degrees C (2.7 degrees F.) hotter than current "business as usual" projections suggest.

What this implies is that policy and technological measures to cope with climate change will become even more important. This week, scientists and government negotiators are wrestling over those measures in a key international meeting in Bangkok, Thailand. They will lay out their recommendations in a summary statement slated for release Friday.

"We've been getting a free ride from forests and oceans," says Robert Jackson, a Duke University ecologist who heads the southeastern division of the US Department of Energy's National Institute for Climate Change Research. But "I'm not confident – especially as our fossil-fuel emissions continue to grow – that we can rely on natural systems to bail us out of this."

To be sure, few if any in the climate-policy community advocate a hands-off, let-nature-do-it-all approach. But the use of natural "sinks" – oceans, plants, and soil that can hold carbon – is said to appear in the report researchers and politicians are haggling over this week.

The document is a brief policymaker's summary tied to the third of three large volumes published every six years by the United Nations-backed Intergovernmental Panel on Climate Change (IPCC). The first two reports, issued in February and April, focused on the state of climate science and on the effects researchers already are seeing as a result of global warming.

The current volume, by contrast, aims to suggest a target for stabilizing carbon-dioxide concentrations so that global average temperatures in 2100 would only be about 2 degrees C higher than they were before the Industrial Revolution. The volume also describes medium- and long-term approaches that could achieve that goal and lays out estimates of the economic costs and benefits of emissions mitigation. Thus, it is by far the most policy relevant and politically sensitive of the IPCC reports.

"This is where the rubber meets the road," says Peter Altman, who heads the coal campaign at the National Environmental Trust, an environmental group in Washington. One measure of the summary's contentiousness: Some 100 countries participating went into this week's meeting ready to offer 1,500 amendments to a 24-page draft document.

The latest caution about leaning too heavily on natural processes comes from an international team of researchers who have found that the ocean may be far less efficient at storing CO2 at the bottom for long periods of time than previously believed. As a result, models may be overestimating how much long-term carbon storage the oceans will provide.

The group looked at the ocean's "biological pump" in a layer of ocean dubbed the "twilight zone." In the upper layer of ocean – the most studied region – plankton take up CO2 the ocean absorbs from the atmosphere. But much of the carbon gets converted back to CO2 in that layer and is quickly recycled back into the atmosphere. Only about 10 to 20 percent of the carbon in this layer sinks into the twilight zone, which begins some 500 feet below the surface. Using new measuring techniques, the team led by biogeochemist Ken Buesseler from the Woods Hole Oceanographic Institution found that only 20 to 50 percent of that carbon makes it to the ocean's bottom for long-term storage.

The team cautions that it sampled only two sites: one off Hawaii and one in the Northwest Pacific. But if the results were averaged over the entire ocean, annual deep storage could be some 3 billion tons less than current estimates suggest, the team concludes in a paper that appears in the current edition of the journal Science,

The picture is somewhat brighter on land. Changing land-use practices to reduce deforestation, especially in the tropics, and encouraging farmers to convert some farmland to forest cover "represent a bridge to the future," says Brian Murray, director of economic analysis for Duke University's Nicholas Institute for Environmental Policy Solutions. If done right, such efforts could buy perhaps 20 to 30 years' time as policies take hold to reduce humans' carbon footprint, he says.

Still, if emissions continue to rise at their current rates, several studies involving forests and grasslands suggest that these natural systems could become less-efficient "sinks" over time. In essence, these systems become saturated with CO2. "It's not a permanent solution," he says.

On a global level, the carbon cycle will have grown far less efficient in accommodating the extra CO2, concludes a study published last year in the Journal of Climate. Assuming "business as usual" emissions growth, most of the 11 computer models used put the additional warming at 0.1 to 1.5 degrees C.