By Mark Kinver
Science and environment reporter, BBC News
Changes to the oceanic carbon sinks can have a wider impact, say researchers
There are substantial variations in the amount of carbon being absorbed by the North Atlantic Ocean, a study shows.
Writing in Science, an international team of researchers said the ocean's uptake of carbon varied by as much as 10% over the space of a few years.
The data set, described as the largest of its kind, was gathered by devices fitted to a fleet of commercial ships.
The world's oceans are believed to absorb about half of the total carbon emissions from human activities.
"Out of all the carbon dioxide released into the atmosphere, about half of it does not stay there," said lead author Andrew Watson, a researcher from the University of East Anglia's School of Environmental Sciences.
"It is taken up by the natural world; half of it is absorbed on land, and half of it ends up in the oceans."
In the carbon cycle, natural fluxes are the biggest, accounting for about 330 gigatonnes per year, and are in near equilibrium
The roughly 7.5 gigatonnes coming from all human sources may be sufficient to tip this system out of balance, warming the Earth
Professor Watson said that it had been assumed that the amount of CO2 absorbed by the oceans remained constant.
"What we seem to be seeing is that it appears to be changing over a period of several years," he told BBC News.
"We are talking about a variability (in the North Atlantic Ocean) that is in the order of about 0.2bn tonnes of carbon each year.
"Over a few years, the uptake is changing by at least 3% of the total production of CO2 by all human activities."
Professor Watson explained that climate modellers had attempted to assess how much variability there was in the overall carbon cycle.
Devices on a network of commercial vessels gathered the data
"They had some difficulty because they simply did not have a sufficient amount of basic data," he said.
Writing in the paper, the international team of researchers said that they overcame the historical problem of sparse observations by using a network of commercial vessels.
"We began this work in the mid-90s when we fitted one automated device," Professor Watson recalled.
"We then realised that if we got a few devices to cover a large region, we could map an ocean with this technique."
Researchers from a number of nations, including Spain, Denmark and the UK, established a co-ordinated network in 2005 that placed the instruments on volunteer observing ships (VOS) that made regular journeys across the Atlantic.
Climate models suggest that carbon sinks will weaken over time as the climate changes, and Professor Watson hopes his team's research will shed light on the dynamics of ocean sinks.
"It is important because the natural system takes up so much carbon, and we do have a suspicion that this uptake will change," he commented.
"Some people are quite nervous about that."
He added that the monitoring system developed by the team could be rolled out in other regions.
"We don't know what is happening elsewhere because we do not have the network in place.
"We could not cover all of the world's oceans because there are not many vessels in the Southern Ocean, for example.
But he said that there was enough traffic across the North Pacific and South Atlantic to establish feasible networks.
"It would be relatively cheap and it would be a huge advance in our understanding of the carbon cycle and where carbon is going."