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Trending science: Removing carbon dioxide from atmosphere may have only limited impact on oceans

A study led by researchers in Germany finds that carbon dioxide removal (CDR) would not significantly counteract the effect of past CO2 emissions in the marine environment.
Trending science: Removing carbon dioxide from atmosphere may have only limited impact on oceans
The deadline for taking meaningful action to avert catastrophic climate change draws nearer every day. Many are looking to science, specifically geoengineering, for a miracle solution that will lead us out of the quagmire, or the very at least buy us some time. However the results of a recent study suggest that carbon dioxide removal (CDR) – a geoengineering technique whereby CO2 is sucked out of the atmosphere – may actually have only a limited impact on the problem.

The study, led by Sabine Mathesius from the Potsdam Institute for Climate Impact Research in Germany, explored the long-term response of oceans to CO2 removal from the atmosphere. Focusing on pH, temperature and dissolved oxygen, it found that even after several centuries of CDR deployment, past CO2 emissions would still ‘leave a substantial legacy in the marine environment’.

Published this week in Nature Climate Change, the study does not explore the technologies that would be used but rather tests the potential impact of CDR. The team simulated the effect of two massive CDR interventions with different CO2 extraction rates – removing either 5 gigatons or 25 gigatons of carbon dioxide from the atmosphere each year under a high emissions scenario (Representative Concentration Pathway (RCP) 8.5). The team falsified two hypotheses: the first being that CDR can restore pre-industrial conditions in the ocean by reducing the atmospheric CO2 concentration back to its pre-industrial level, and the second being that high CO2 emissions rates followed by CDR have long-term oceanic consequences that are similar to those of a low emissions scenario (RCP2.6).

When 5 gigatons were removed each year in the simultations, scientists found the environmental benefits were surprisingly small. In particular, CDR struggled to significantly reverse the acidification of the ocean. Science magazine reports, ‘Without the CDR the surface pH was reduced by 0.75 units by 2200; with CDR the acidification was reduced 0.7 units.’

When the scientists simulated what would happen if 25 gigatons of carbon dioxide were removed from the atmosphere each year starting in 2150, they found that surface ocean pH was restored, but only by 2300. Lead author Mathesius told Science, ‘I expected CDR would have a bigger effect.’

Science magazine explains that the main reason for this disappointing response rate is that CDR removes CO2 from the sky, but it cannot act on the carbon locked in the deep ocean, which takes thousands of years to return to the surface. Science adds, ‘Simulations in the paper show that massive and immediate cuts to carbon emissions would have a more potent impact, limiting acidification to only 0.2 units by 2100.’

Co-author, Ken Caldeira, who is normally based at the Carnegie Institution in the US told the Guardian, ‘Interestingly, it turns out that after “business as usual” until 2150, even taking such enormous amounts of CO2 from the atmosphere wouldn’t help the deep ocean that much – after the acidified water has been transported by large-scale ocean circulation to great depths, it is out of reach for many centuries, no matter how much CO2 is removed from the atmosphere.’

The Guardian also quotes co-author Hans Joachim Schellnhuber who offered a stark warning to those who are still holding out hope for a technological miracle in lieu of immediate CO2 emissions reductions: ‘In the deep ocean, the chemical echo of this century’s CO2 pollution will reverberate for thousands of years. If we do not implement emissions reductions measures in line with the 2 degrees Celsius target in time, we will not be able to preserve ocean life as we know it.’

For further information, please visit:
http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2729.html

Source: Based on a study published in Nature Climate Change and media reports.

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