The ocean is something of a martyr when it comes to mankind's continuing CO2 emissions. Like a giant sponge, it soaks up much of the greenhouse gas coming from planes, factories and automobiles. That takes the edge off of the atmosphere's rising concentration of CO2, but that same carbon dioxide, being dutifully dissolved into the sea, makes the ocean itself more acidic - and so less able to support marine life. But will the seas be able to continue with those valiant efforts, as the global warming proceeds? That's a question partially answered by a new study of three-decades worth of data on the Atlantic, published in today's Nature Geoscience.
Scientists have been uncertain as to how the oceans will respond to a warming world,ever since the threat of global warming was recognized. Will they carry on soaking up the same fraction of our emissions? Or will they find it increasingly difficult to play catch up to our higher CO2 output - leaving the atmosphere saddled with more of the greenhouse gas burden?
That problem of the great oceanic 'carbon buffer' is one that has troubled climate and oceanic scientists for many years. Although the basic mechanics of how CO2 gets from the air into the sea are well-known, a deep understanding has eluded them, because of the complexity of carbon's flow through the oceans. Water temperature, 'sea roughness', the changing patterns of oceanic circulation, and the use of carbon by marine creatures - all of these factors play up against one another.
And the direct measurement of the CO2 being soaked up in the oceans is complicated by natural fluctuations. The atmosphere and the oceans pulse with decade-long rhythms, that are only just beginning to be teased out. What is needed is enough data to spot those ongoing cycles, so that they can be taken out of the equation. Then the background changes in CO2 can be seen more clearly. This is the approach taken by this international team, supported by NASA and led by Galen McKinley, from the University of Wisconsin-Madison.
'Because the ocean is so variable, we need at least 25 years' worth of data to really see the effect of carbon accumulation in the atmosphere,' she says. 'This is a big issue in many branches of climate science - what is natural variability, and what is climate change?' The team pooled together data collected from a huge range of sources - from 1981 to 2009 - across the North Atlantic, weaving them together to form single time-line.
What they found was that there are signs that the warmest parts of the North Atlantic are indeed struggling to take up CO2. 'The ocean is taking up less carbon because of the warming caused by the carbon in the atmosphere,' said McKinley. 'We are already seeing this in the North Atlantic subtropical gyre, and this is some of the first evidence for climate damping the ocean's ability to take up carbon from the atmosphere.' That could mean that our climate change problem just got a little harder - but more work is needed on other oceans. Some previous studies have suggested that climate change is aiding CO2 absorption there. Understanding how these affects balance out, across the oceans, will be critical for understanding the kind of future we are driving the planet towards.
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