Mount Pinatubo in the Philippines exploded in 1991 with terrifying effects. It was the second-largest volcanic eruption of the 20th century and by far the biggest in a densely populated area.
Advanced warning saved thousands of lives before avalanches of molten ash and mud roared down the flanks of the mountain, filling valleys with volcanic deposits up to 200 metres thick and displacing more than 200,000 people.
It also provided a glimpse of how geo-engineering might stem the effects of climate change.
Nearly 20 million metric tonnes of sulphur dioxide gas was injected into the upper atmosphere where it combined with other materials to form sulphuric acid particles. These sulphate aerosols acted as a giant sunshade, reflecting solar radiation back into space, thus cooling Earth's surface.
Dispersal of the aerosols around the world in the jet stream caused global temperatures to drop temporarily by 0.5°C for about two years, a level not far short of the increase at that time in the average land and sea surface temperature since the start of the Industrial Revolution.
International climate change negotiations are intended to prevent ever-larger amounts of greenhouse gas emissions, mainly from burning fossil fuels and clearing forests for agriculture, from spewing into the atmosphere and raising the mean global temperature by 2°C, a threshold for dangerous climate change adopted by the abortive United Nations conference in Copenhagen last December. But in subsequent negotiations there has been little progress on a binding agreement to take the costly steps needed to reduce global warming gases by preserving forests and deploying low-carbon energy sources, such as nuclear, hydro, wind and solar power, on a wide scale.
The scientific panel advising the UN has warned that without such cuts, the temperature could rise by as much as another 4°C by the end of the century, increasing the risk of dangerous, even catastrophic climate change because of the long-lasting effects of carbon dioxide, the main global warming gas from human activity, once it reaches the atmosphere.
With an effective accord on cutting greenhouse gases unlikely any time soon, an increasing number of scientists and officials are calling for research into "Plan G" - geo-engineering - to cool the planet. One of the latest signs of growing interest in controversial techniques to make large-scale alterations to the atmosphere, land or oceans to counter the effects of global warming was a meeting in September of climate scientists in Washington to debate "Geo-engineering: The Horrifying Idea whose Time has Come?"
The technologies fall into two main categories: One would reflect sunlight back into space, an approach known as solar radiation management. This mimics the global dimming and cooling associated with major volcanic eruptions. Another would remove carbon dioxide from the atmosphere on a scale far greater than anything attempted so far.
Solar radiation management includes using high-flying aircraft or projectiles to spread chemicals to make sulphate aerosols in the stratosphere. Aircraft can also inject seawater droplets or other cloud-condensation material into clouds over the ocean to whiten them, thereby reducing incoming solar radiation warming the ocean.
Among the most feasible of the carbon dioxide removal methods is to add iron, nitrogen or phosphate to selected regions of the ocean to increase phytoplankton growth and remove more carbon from air. These microscopic organisms already soak up huge quantities of carbon dioxide from the atmosphere.
Of course, geo-engineering raises thorny environmental, geopolitical and governance issues. Would it work as planned without dangerous side-effects? Interventions might reduce rainfall, deplete Earth's protective ozone layer, blur skies and reduce sunlight for solar power. Who would decide if and when to deploy geo-engineering techniques? Should the decision be in government or private hands?
The debate is set to intensify. On October 29, the US Congress received two reports on geo-engineering, both calling for more coordinated research. At about the same time in Nagoya, Japan, an intergovernmental conference to protect global biodiversity proposed a moratorium on large-scale geo-engineering experiments "until there is an adequate scientific basis on which to justify such activities and appropriate consideration of the associated risks".
The US space agency Nasa has said that geo-engineering could provide a way to slow global warming until carbon emissions can be reduced enough to prevent catastrophic climate change. Some companies have drawn up ocean-seeding plans to sequester carbon dioxide and generate valuable carbon credits.
Critics worry about the side-effects. Fertilising the seas could cause dead zones and toxic blooms that harm aquatic life. Adding large amounts of sulphate aerosols to the upper atmosphere could cool the planet, stop melting polar ice and slow sea-level rise. Interventions might also reduce Asian monsoon rainfall, deplete Earth's protective ozone layer and reduce sunlight for solar power.
Geo-engineering carries cost issues as well. At the lower end of the spectrum, a study by Rutgers University environmental scientists in 2009 estimated that it would cost $225 million (Dh826m) to put one million tonnes of sulphur gas a year into the atmosphere using nine American KC-10 tankers making three flights a day. European nations have aircraft that could perform a similar task, as do Russia and China.
Recent studies show that although the average global temperature could be lowered by seeding the stratosphere, it might leave some regions too warm and others too cool.
Apportioning the burden of cutting greenhouse gases still causes acrimonious debate among governments after years of negotiation. Putting Plan G into effect is also likely to be fraught with contention.
Michael Richardson is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore
©Yale Centre for the Study of Globalisation 2010