The return of geoengineering

This blog has mentioned the subject of so-called ‘geoengineering’ before – a series of proposed technologies designed to work on a planetary scale to slow or mitigate the effect of global warming. The reason it remains a topic of relevance to readers of this magazine is that, of all of the techniques proposed, those involving burning large volumes of sulphur to create sulphur dioxide in order to reflect sunlight at high altitudes seem to be the most feasible.
The cooling effect of high-level SO2 has long been documented. In 1991-2 planetary temperatures were cooled by about 0.6C by the eruption of Mount Pinatubo in the Philippines, which put an estimated 15-30 million tonnes of SO2 into the atmosphere. Similar effects were observed after the Krakatoa eruption in 1883. The argument is that if the emissions were more targeted, both in terms of geographical location and atmospheric height level, even greater changes could be achieved by smaller amounts of SO2. A study published in April in Atmospheric, Chemistry and Physics Discussions reported that modelling had indicated continuous injection of 5 million t/a of SO2 into the upper atmosphere might be able to achieve a 0.7C global temperature reduction (taking us back to pre-industrial levels), albeit with considerable regional variation.
Ironically, our success in reducing SO2 emissions from burning sulphur-containing fuels may have actually exacerbated the problem, as we used to put several tens of million tonnes of SO2 into the lower atmosphere every year. While this low level SO2 caused great damage to human health and the environment and caused the ‘acid rain’ phenomenon, enough reached the upper atmosphere to lead to a mild cooling effect which we no longer enjoy. It has on the other hand also provided us with stockpiles of tens of millions of tonnes of sulphur around the world for which we are continually searching for a use. Producing five million t/a of SO2 would require 2.5 million t/a of elemental sulphur – a readily achievable total on current production/storage levels.
While geoengineering was once an area that brought together a handful of ‘lateral thinkers’ with some of the stranger fringes of the global warming debate, as a global deal on reducing carbon emissions post-Kyoto has continued to prove elusive, so the topic has steadily become more mainstream as an item for discussion. A number of prominent scientists now argue that – in the absence of a comprehensive climate change deal - humanity needs some sort of ‘Plan B’. It is a planetary-scale Pascal’s Wager – the theory is that the scale of the risks and potential benefits make it imperative to at least put some serious money into researching the area, whatever our beliefs or viewpoint might be. In September last year the Royal Society in London said almost exactly this in a report on global warming. Other supporters have included such disparate personalities as Nobel chemistry laureate Paul Crutzen, Edward Teller the so-called “father of the hydrogen bomb”, economists and ‘Freakanomics’ authors Steven Levett and Stephen Dubner, Bill Gates and Richard Branson. Now in a report published just a few weeks ago, Dr David Keith at the University of Calgary has chimed in with some ideas on mirrored nanoparticles and sulphuric acid aerosols.
However, environmentalists have been loudly critical of the idea, most notably because they argue it is a seductive illusion, treating the symptoms rather than the disease, and allowing us to think we can engineer our way out of climate change without having to change our lifestyles. The effect on the ozone layer would be predictably adverse, while the long-term effect of high levels of CO2 and SO2 remains difficult to predict and could have any number of unplanned and unpleasant side effects. Even the study referred to above predicted potentially catastrophic changes to regional rainfall patterns, for example. But more than that, it is argued, once we had embarked down the road of geoengineering, we would be unable to stop. The underlying concentration of carbon dioxide would continue to rise, and any halt in sulphur injections, even for a year or two, could cause warming to rebound at a rate 10 to 20 times that of the recent past - a phenomenon referred to as the "termination problem". Once we start engineering the atmosphere we could be trapped, forever dependent on sulphur injections.
Carbon dioxide sequestration is of course itself a form of geoengineering, albeit one with more predictable consequences, and for that there is considerable government support, at least in Europe and North America. At the moment the debate still rages over SO2 injection between those who say we cannot afford to take the risk, and those who say we cannot afford not to, but it remains an open question as to how long will it be before a government somewhere decides that SO2 injection is a cheaper option.