Rising seas increased carbon intake
30 January 2009, by Tom Marshall
Rising sea levels since the last ice age have significantly increased the oceans' capacity to absorb CO2, according to new research.
Algal blooms like this one, off the coast of Ireland, form in continental shelf waters.
Global sea levels have risen some 130 metres since the height of the last ice age as glaciers and icecaps have melted. This flooded vast areas of land, equivalent to around twice the area of the USA, and increased the area of shallow 'continental shelf' sea by around 400 per cent.
The shallow bodies of water formed as the waters rose, such as the North Sea, can support large algal blooms each summer particularly in regions subject to thermal stratification - this is when the water column becomes separated into distinct layers of warm and cold water.
Researchers at the universities of Bangor and Kyushu have calculated that the increase in stratified conditions has greatly increased the total amount of carbon absorbed by the oceans. Understanding these effects is crucial to improving models to predict how the climate will change in response to human CO2 emissions.
Marine algae absorb large volumes of CO2 from the atmosphere as they photosynthesise, and much of this carbon finds its way into the deep oceans where it remains for thousands of years. The result is that while shallow, fully mixed coastal waters tend to be a weak source of CO2 to the atmosphere, the deeper thermally stratified areas act as a sink for atmospheric carbon.
Model visualisations of the growth of the northwest European shelf seas between the last ice age (22,000 years ago) and today.
At the moment about 80 per cent of the northwest European continental shelf seas become stratified in summer, and production of marine biomass in the area could be up to five times higher than in prehistoric times.
The researchers combined sophisticated tidal models with reconstructions of past landscapes to simulate the effect on CO2 absorption from growth of continental shelf seas in the 22,000 years since the height of the last ice age. Their results, which appeared in Geophysical Research Letters in December 2008, are consistent with the timing of changes in atmospheric CO2 concentration measured in ice cores taken from the Antarctic.
Until now scientists have mainly looked at this relationship from the other direction. 'We know a great deal about the effects of climate on sea level,' says Professor James Scourse, a marine geologist at Bangor University, Royal Society-Leverhulme Trust senior research fellow, and one of the paper's authors. 'But the feedback effects of changes in sea level on climate are less well understood, and these are what our paper explores in the context of the carbon cycle.'
It's an important question because the oceans have absorbed around half the CO2 released so far by burning fossil fuels. This has lessened the impact on the climate from human carbon emissions.
But it's not guaranteed the ability to absorb CO2 will keep pace as atmospheric levels of the gas rise at an unprecedented rate. For example, ocean acidification as atmospheric CO2 dissolves in the oceans to form a weak acid could harm populations of photosynthetic plankton by dissolving their calcite shells. Or the plankton could simply reach the limit of how much they can photosynthesise and absorb CO2.
'We are currently getting a 50% "discount" on the climatic impact of our fossil fuel emissions,' says Dr Tom Rippeth, an oceanographer at Bangor and another of the paper's authors. 'Unfortunately we have no guarantee that the 50% discount will continue, and if it disappears we will feel the full climatic brunt of our unrelenting emission of carbon dioxide from burning fossil fuels,' he adds.
The researchers say that more measurements of the exchange of CO2 between sea and air, and of the underlying physical and biogeochemical processes within the water column driving this exchange, are now needed to elucidate the respective roles of polar, temperate and tropical shelf seas in the global carbon cycle.
'Our research should let us better model CO2 fluxes between the sea and the atmosphere, which are a crucial component of the Earth system' says Scourse.
Scourse is also working on geological validation of the tidal model developed in the research to investigate other oceanographic phenomena, including the contribution of glacial to interglacial variations in abyssal tidal dissipation to the North Atlantic meridional overturning circulation - currently the focus of Mattias Green's NERC Advanced Research Fellowship at Bangor - and how sediment is transported by tides at the seabed in shallow coastal shelf waters which can used to predict sedimentary changes and the development of the bedform, or shape of the seabed.
At the time the research was conducted, Tom Rippeth was a NERC Advanced Research Fellow, and fellow author Stephanie McKeown was a NERC-funded M.Sc. student.
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