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Carbon uptake of world’s oceans widely underestimated

Image credit: Petergridin/Dreamstime

The world’s oceans soak up more carbon than most scientific models suggest, according to a new study.

Previous estimates of the movement of carbon (known as “flux”) between the atmosphere and oceans have not accounted for temperature differences at the water’s surface and a few metres below. The new study includes this – and finds a significantly higher net flux of carbon into the oceans.

The study, led by the University of Exeter, calculated CO2 fluxes from 1992 to 2018. Over this period, they found up to twice as much net flux in certain times and locations compared to uncorrected models.

“Half of the carbon dioxide we emit doesn’t stay in the atmosphere, but is taken up by the oceans and land vegetation ‘sinks’,” said Professor Andrew Watson, of Exeter’s Global Systems Institute. “Researchers have assembled a large database of near-surface carbon dioxide measurements – the 'Surface Ocean Carbon Atlas' – that can be used to calculate the flux of CO2 from the atmosphere into the ocean.”

Watson said that while previous studies have used this method, they have ignored the small temperature differences between the surface of the ocean and the depth of a few metres where the measurements are made. He stressed such differences are important as carbon dioxide solubility depends very strongly on temperature.

“We used satellite data to correct for these temperature differences and when we do that it makes a big difference: we get a substantially larger flux going into the ocean,” he explained. “The difference in ocean uptake we calculate amounts to about 10 per cent of global fossil fuel emissions.”

“Our revised estimate agrees much better than previously with an independent method of calculating how much carbon dioxide is being taken up by the ocean,” said Dr Jamie Shutler, of the Centre for Geography and Environmental Science on Exeter’s Penryn Campus in Cornwall.

He said that the method makes use of a global ocean survey by research ships over decades, to calculate how the inventory of carbon in the ocean has increased. “These two ‘big data’ estimates of the ocean sink for CO2 now agree pretty well, which gives us added confidence in them,” he added.

In April, scientists said that ‘biological carbon pump’ (BCP) in the Earth’s oceans are capturing twice as much carbon dioxide (CO2) from the atmosphere as previously thought. Meanwhile, in November 2019, the researchers at Exeter called for the creation of a “robust network” of satellites to help monitor the levels of carbon in the ocean.

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