What lies beneath
December 3, 2009
From The Economist
AS THE world gathers in Copenhagen over the coming weeks to discuss how much carbon dioxide people should be putting into the atmosphere, the Benguela Stream will be docking in the Windward Isles to bring bananas to Europe for Christmas, and doing her bit to help ascertain where a large part of that CO2 ends up. The worldâ€™s oceans and plants absorb about 60% of the CO2 emitted as a result of human activities, which has helped keep the extent of climate change in check over the past century or so. But exact figures are hard to come by. Estimates of just how much carbon ends up in plants, in soil and in the oceans are frustratingly sketchy.
The oceans suck up CO2 because it is soluble in water. Plants suck it up because they photosynthesise. As CO2 becomes more available, other things being equal, they will photosynthesise more. And a warmer, more polluted, more disrupted world can encourage growth in other ways, too. But none of these things can go on indefinitely. At some point the oceans and plants will have had their fill.
At that moment, the problems being addressed in Copenhagen will get worse. They will get worse still if the â€œland sinkâ€, as researchers call the plants and soils, actually goes into reverse, adding to net emissions rather than reducing them as the respiration of microbes (which increases as temperatures rise) outstrips the photosynthesis of plants. Some think that a slowdown in the sucking up has already begun. The poverty of the data and the complexity of the problem make it hard to be sure.
To further complicate matters, the size of the sinks depends not just on the concentration of CO2 in the atmosphere, the chemistry and circulation of the oceans, and the appetites of plants and soil organisms, but also on the way in which all these things vary, in turn, in response to climate change. The complexity of the problem is illustrated by the degree to which the sinks change from year to year. In some years 80% of emitted carbon is absorbed; in other years none of it is.
Thanks to the Benguela Stream, and a few other ships, an international research team now thinks it knows how to monitor the oceansâ€™ appetite for CO2 much more precisely. The ships, which regularly plied various routes in the North Atlantic during 2005, were kitted out by a number of European research organisations with instruments that measured the CO2 content of water taken into the engine rooms and air collected upwind of the funnels. Over 100,000 such measurements, along with the shipsâ€™ positions, were sent to shore by satellite. The whole procedure, including checking and recalibrating the instruments on a regular basis, and dealing with the data, costs about â‚¬100,000 ($150,000) a year for each ship.
The crews have little to do with the measurements, but are pleased to give them ship-room, according to Andrew Watson of the University of East Anglia, one of the organisers of the project. â€œEngine rooms are pretty dull, and this way they have some more things to look at,â€ he says. The instruments provide a measure of the â€œpartial pressureâ€ of CO2 in the water, from which the rate at which more of the gas is being absorbed from the atmosphere can be calculated.
In this weekâ€™s issue of Science Dr Watson and his colleagues use these dataâ€”combined with data from elsewhere on sea-surface temperatures and the amount of mixing going on under the surfaceâ€”to produce figures for the total size of the North Atlantic carbon sink during 2005. The various methods they employed to do this all gave answers of about 250m tonnes of carbon.
There are two things about that number which are more important than its magnitude. One is that by comparing the results from 2005 with results from trips to the Windward Isles in different years, the team has shown that the size of the sink changes markedly over time, which is not what previous studies of sinks have assumed. The reason for the change is likely to be the climate itself, but that does not mean that the recent reduction is due to global warming. The climate in the North Atlantic shifts naturally over the decades, and it is probably in response to such shifts that the sink is currently changing, though the chain of cause and effect is obscure.
The other important thing is that, despite being based on data from only half a dozen ships, the team argues that its figure is accurate to within 10%. In the world of carbon sinks, that is impressively precise. If the same techniques were applied across the world, the size of the oceanic carbon sink, and its variation year by year, would be known considerably better than the size of the land sink. Indeed, it could help narrow down the alarmingly wide range seen in the estimates of the net amount of carbon taken up by plants on land. The amount of CO2 emitted by industry is reasonably well known. So is the amount in the atmosphere. If the amount being taken up by the oceans were also known accurately, then the quantity sucked up by the land could be deduced.
There is more to this than the satisfaction of good book-keeping. The possibility that the sinks are starting to fail is a real and worrying one. Corinne Le QuÃ©rÃ©, like Dr Watson, works at the University of East Anglia (and, like him, is not affiliated with the universityâ€™s controversial Climate Research Unit, which recently had vast amounts of its e-mail published on the internet, and whose head, Phil Jones, has just stepped down temporarily while the matter is investigated). In 2007 she published a study arguing that the uptake of carbon in the Southern Ocean had levelled out.
Again, climate was the reason, but this time suspicion fell on a human-induced change. One of the effects of ozone depletion is to strengthen the winds circling the south pole (see Southern bellwether), and global warming is expected to do something similar. The effect of these strengthened winds whipping across the sea surface is to bring deeper, more carbon-rich water up from below, thus reducing the surface layerâ€™s capacity to absorb more CO2 from the air.
Suffer the little children
In November Dr Le QuÃ©rÃ© and several colleagues from around the world published a paper in Nature Geoscience arguing that this and other climate effects might now be strong enough to slow down the sinks on a global scale, producing a positive feedback in which climate change reduces the Earthâ€™s capacity to absorb extra CO2, thus leading to yet more climate change. The problem with being sure of this, though, is that the data on the land sink are not reliable. Strong responses to â€œnormalâ€ climate variability such as that related to the El NiÃ±o/La NiÃ±a events of the southern Pacific make the picture very muddled.
Uncertainty about deforestation and subsequent regrowth of plants clouds this murky image still further. A study of the same problem by Wolfgang Knorr of Bristol University, published weeks earlier, in Geophysical Research Letters, used similar data to Dr Le QuÃ©rÃ©â€™s, but some different assumptions, to conclude that the sinks were still sucking up more or less the same percentage of emissions that they had done throughout the 20th century.
Dr Knorrâ€™s analysis is more sophisticated, says Chris Field, of Stanford University, who worked on an earlier study that suggested the sinks were slowing, and who is co-chair of the Intergovernmental Panel on Climate Changeâ€™s working group on impacts. To know what is really going on, though, requires better data. Time should help, but more concerted gathering of data is also needed. Analysis of changes in land use, using satellite data, could be improved. A worldwide shipboard system for monitoring ocean sinks, building on projects like that involving the Benguela Stream, would be a boon. But, as yet, there is no money promised for such a project.
The study published this week was merely a pilot programme. And the possibility of a satellite system for monitoring fluxes of CO2, which is not a panacea but would certainly help, was set back by the loss of the Orbiting Carbon Observatory, a spacecraft built by NASA, Americaâ€™s space agency, during its launch in February 2009. If delegates at Copenhagen, when not arguing over the hundreds of billions of dollars needed to avoid frightening levels of CO2 tomorrow, spared thought for the tens or at most hundreds of millions of dollars needed to work out where CO2 ends up today, it would be no bad thing.