A German-led team of scientists has given the thumbs down to an idea that would help tackle global warming by pumping clean water from the ocean floor up to the surface, effectively drawing down carbon dioxide (CO2) from the atmosphere. Computer simulations generated by the researchers show that the benefits of implementing the ambitious geo-engineering scheme are minor and the risks too high. Their findings are published in the journal Geophysical Research Letters.
Large-scale geo-engineering schemes are being touted as 'Plan Bs' in the event that governments around the world fail to meet agreements and reduce their CO2 emissions.
One approach would see nutrient-rich water driven to the surface via vertical plastic tubes (several hundred metres in length) to stimulate the growth of phytoplankton, the marine algae key to biological production. An increase in phytoplankton at the water's surface equates to an increase in draw-down of atmospheric CO2 by photosynthesis. Deep in the ocean, some of the carbon would be removed from the system for centuries or even millennia to come.
The scheme was put to the test by a team of scientists from Australia, Germany and the UK. Led by Professor Andreas Oschlies of the Leibniz Institute of Marine Sciences in Germany, the team was able to show through computer model simulation that even under the best-case scenario only 3 gigatonnes of CO2 could be captured per year. This represents only one tenth of the current 36 gigatonnes of annual anthropogenic CO2 emissions produced across the globe.
Professor Oschlies said: 'Besides the technical feasibility of such devices on large space and time scales, this method has, similar to other suggested approaches, a very limited sequestration potential and a risk of substantial side effects.'
One of the potential side effects observed by the team is a steep increase in CO2 when the pumps stop working. When this happens, 'atmospheric CO2 concentrations and surface temperatures increase rapidly to values that are even higher than those of a control simulation that never employed the artificial pumps', Professor Oschlies explained. For the scheme to be valid, the pumps must therefore never cease to work.
The paradox is cleverly reflected in the title of the team's paper, 'Climate engineering by artificial ocean upwelling - channelling the sorcerer's apprentice'. The authors link it to the Goethe poem, 'The Sorcerer's Apprentice', in which the title's namesake draws on a magical broom to wash down the sorcerer's workshop for him; unable to stop the broom and control the magic, the workshop is soon flooded with water.
Another significant feature of the team's simulations was that the main effect of the scheme took place on land instead of the ocean. The cold water brought up to the surface cooled down both the atmosphere and land surface, shifting a large amount of CO2 that was removed back to land.
'The model predicts that about 80% of the carbon sequestered is stored on land, as a result of reduced respiration at lower air temperatures brought about by upwelling of cold waters,' the researchers explain in their paper. 'This remote and distributed carbon sequestration would make monitoring and verification particularly challenging.'
'Even though not all interactions are known and correctly modelled, we do not see this method as a solution of our CO2 problem,' Professor Oschlies concluded.
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Document Reference: Oschlies, A., et al. (2010), 'Climate engineering by artificial ocean upwelling - channelling the sorcerer's apprentice'. Geophysical Research Letters, 37, L04701, (published online 16 February). DOI: 10.1029/2009GL041961.
Subject Index: Climate change & Carbon cycle research; Coordination, Cooperation; Scientific Research; Resources of the Sea, Fisheries