We’ve overshot the ‘safe’ level of atmospheric greenhouse gases are are going to be forced to look for ways to artificially cool the planet, or alternatively, to rapidly drawn down free-air CO2.
Most geoengineering solutions look to do the former. For instance, the ‘simulated volcano’ (stratospheric sulphates) or ‘sunshade world’ (mirrors in space) are intended to reduce total solar irradiance. They do nothing to reduce the enhanced greenhouse effect.
I recently made some comments to COSMOS Magazine on the risks of sulphate cooling:
Barry Brook, director of the Research Institute for Climate Change and Sustainability at the University of Adelaide in South Australia, said that the sulphate aerosol scheme might also lead to acid rain and droughts. “[This study] points out another mechanism by which these geoengineering schemes can go wrong,” said Brook, who was not one of the study’s authors. “There is always a chance of unintended side effects.”
But that doesn’t mean I don’t think it needs to be carefully considered, risky as it may be. If, for instance, relatively few aerosols are released – about 10% of what is currently injected into the trophosphere by coal-fired power plants – then we could get sufficient cooling without any major disruption to rainfall or ozone formation.
I also made some comments on a recent GCM simulation of the ‘sunshade world’ scenario on the Faculty of 1000, here:
Can ‘geoengineering’ protect ecosystems and humanity from climate change impacts, should global warming start to run out of control? Well, not really, say Lunt and co-authors, in a fascinating application of a Global Climate Model (HadCM3L) that poses the ultimate ‘what-if?’ scenario for a human response to climate change. The situation they model is the speculative idea that we could install roughly a trillion 1m diameter reflective mirrors between the Earth and the Sun, to reduce incoming solar radiation by 2-5%.
Costs and logistics aside, would this mitigate climate change impacts? The answer is complex, but the upshot is that such a geoengineering solution-of-last-resort would seem to create as many problems as it solves. The tropics would cool, which might spare rain forest biomes or cause them to revert to savanna, but polar amplification of the warming is predicted to continue, leading to the elimination of Arctic sea ice and the probable continued destabilisation of land-based polar ice sheets. This solution could avoid major heat waves that threaten coral reef systems with bleaching. The global hydrological cycle would likely become less intense, with the atmosphere being drier overall. However, ocean acidifiction due to high CO2 would be unaffected by this geoengineering, and this impact alone is likely to be catastrophic for species such as corals, forams and pteropods that secrete a calcite or aragonite skeleton, potentially disrupting entire strands of the marine food web.
Interestingly, the authors speculate that the sort of conditions implied by this scenario (lower total solar irradiance and high atmospheric CO2 concentration) would have the side effect of re-creating a world similar to the Cambrian period, 500 million years ago – the dawn of the Phanerozoic, when visible life first became abundant.
This was a commentary on the following paper:
Lunt DJ, Ridgwell A, Valdes PJ, Seale A 2008 “Sunshade World”: A fully coupled GCM evaluation of the climatic impacts of geoengineering. Geophysical Research Letters 35:L12710
Recently, a new solution has been proposed. It’s highly speculative, but its great appeal is that if achieveable, it would solve both the heating and ocean acidification problems. It involves adding huge amounts of lime (from limestone, such as is found in abundance in the Nullarbor Plain of Australia) to the oceans, in order to precipitate out the carbonic acid. The trick is to extract the lime using an energy efficient process (it would have to be powered by renewable energy sources such as solar thermal), and to spread it in a sufficiently dilute form to avoid ocean alkalinification! Think it can’t be done? Read the story at COSMOS and decide yourself. Here was my 2c to the reporter:
Barry Brook, director of the Research Institute for Climate Change and Sustainability at the University of Adelaide in South Australia, agreed that, “there are risks, but we need to be trailing such ideas.” He added that “we are already in the danger zone, and may need to geo-engineer our way out of it,” in addition to drastically cutting greenhouse gas emissions. Brook himself is a supporter of the idea to bury CO2 in the deep oceans.
Barry W. Brook