By Chris Goodall. An extract from Ten Technologies to Save the World, pages 272-276, published in 2008.
If all else fails, can we avert global warming by emergency techniques to remove carbon from the atmosphere or block some of the sun’s radiation reaching the earth? Some environmentalists rail against such ‘geoengineering’ schemes, saying that they encourage the world to continue with rash and unsustainable consumption of fossil fuels. Nevertheless, rational governments and scientific institutions must carry out research into this topic. We need emergency fallbacks in case emissions reductions fail or we find that temperature increases begin to induce dangerous instability in our weather systems. If, for example, glacial and ice cap melting begins to speed up dramatically, which pal eoclimatic evidence suggests is a real risk, then even rapid emissions reductions will have no measurable impact. The thermal momentum of the ice means that melting will continue even if temperatures are stabilised. A large percentage of the world’s population will face disaster as sea levels rise and the summer flow of glacier-fed rivers declines sharply, causing severe water shortages for hundreds of millions of people. In these circumstances, the only appropriate action is likely to be an attempt to rapidly reduce global temperatures.
The simplest way to reduce temperature is to cut the total amount of the sun’s radiation reaching the earth’s surface. Blocking 1 or 2 per cent of the solar energy that would otherwise reach us would be enough to counterbalance the effects of the greenhouse gases added to the atmosphere since the Industrial Revolution began. Two apparently viable techniques are canvassed for achieving this reduction – increasing pollution in the upper regions of the earth’s atmosphere or inducing greater cloudiness in the lower regions of the air. We know from the eruption of Mount Pinatubo in 1991 that one violent volcanic eruption, blasting 20 million tonnes of sulphur dioxide 30 kilometres or more upwards, will provide enough of a solar umbrella to reduce temperatures by half a degree Celsius or more. In the case of Pinatubo, the effect lasted three or four years, changing weather patterns around the world, probably enhancing the drought in the Africa Sahel and causing excess rainfall in the US. Events on the scale of Pinatubo are rare: the twentieth century only saw one or two eruptions of equivalent size.
We could mimic the effect of large volcanic events by shooting sulphur compounds into the stratosphere. This idea has distinguished adherents, such as Nobel Prize winner Paul Crutzen, but most climate scientists are horrified by its potential side-effects. It might work at restraining temperature rises, but it would increase the rate of ozone depletion, change weather systems and increase acid rain. It would also do nothing to reduce the amount of carbon dioxide in the atmosphere, meaning that the oceans would continue to acidify as they absorbed increasing amounts of the gas. Among other effects, this will assist in the destruction of coral reefs and the gradual deadening of the seas arising from the loss of plankton and fish.
Another way of reflecting sunlight is to create more low-level clouds. Paradoxically, wispy high-level clouds tend to keep heat in but thick layers of cloudiness near the surface send light back into space. Probably the most plausible way of increasing low cloud cover would be to create a fine mist of salty ocean water and spray it upwards. If done on a large and increasing scale, this would tend to increase the amount of cloudiness over the seas and help to decrease temperatures. One variant of this scheme is proposed by Professor Step hen Salter, the inventor of one of the early devices for capturing wave energy mentioned in Chapter 2. His plan is to have hundreds of automatically controIIed ·wind-powered boats. shooting spray into the air, But, once again, even if the plan works it doesn’t reduce the bad effects of the increasing levels of carbon dioxide in the atmosphere. It simply masks more of the world s surface from the sun. Apparent y bizarre other schemes, such as shooting trillions of tiny mirrors out into space to reflect sunlight have similar flaws.
Others types of schemes for geoengineering try to increase the capacity of the seas to store carbon dioxide. The amount of carbon stored at the bottom of the oceans is many times what is in the air, the soil or trees. One idea for taking more carbon dioxide to the sea floor is to seed parts of the southern oceans with tiny iron filings. The theory is that the growth of plankton is held back by a shortage of iron, an important nutrient. Plankton absorb carbon dioxide in a photosynthesis-like process that produces calcium carbonate for their skeleton-like internal structure. When plankton die, they fall to the bottom of the ocean, carrying the carbon dioxide with them in the form of the carbonate. So increasing the number of plankton could help sequester carbon.
Experiments have shown that extra iron does indeed increase the growth of plankton. And since the plankton in the oceans have a total weight greater than all the trees and plants on the earth’s land, this is potentially extremely useful. However, excitement was tempered when it was discovered that a very small percentage of the extra plankton actually fell to the bottom of the sea, where the carbon dioxide would be safely sequestered. What actually seemed to have happened was that bigger sea creatures simply ate more plankton, and were themselves consumed by species further up the food chain. Most of the extra CO2 absorbed by the plankton eventually seems to have returned to the air. A similar scheme that involves sucking cold, nutrient-rich, deep-level water up to the surface in order to improve the rate of photosynthesis by tiny sea creatures may suffer from a similar problem.
Schemes such as these are widely derided by climate scientists, who tend to believe that geoengineering projects simply compound the original problem, rather than cure it. They correctly point out that mankind does not begin to comprehend many of the complexities of the world’s weather and climate systems. To them, the idea that we could cleanly counteract the consequences of increasing greenhouse-gas levels by simple techniques such as reflecting the sun’s energy is hubris of the worst sort. The great climate scientist Wally Broecker says that our increasing greenhouse emissions are having an effect on climate analogous to poking an angry beast with a sharp stick. Geoengineering may compound the risks by poking the animal with a second stick. It is far better to reduce emissions Or increase the rate of carbon capture in the soil, plants and trees or by safe underground injection. (As we saw in Chapter 8, Broecker himself supports a scheme to chemically capture carbon dioxide from ambient air and sequester it underground: an approach that shouldn’t have any climatic side-effects.)
But ruling out geoengineering entirely is surely a mistake. It is sensible contingency planning for a world that is only gradually waking up to the possible dangers of even modest further increases in atmospheric carbon dioxide levels. Although all geoengineering schemes will have risks, the possibility of unexpectedly rapid changes to the climate argues strongly for a sustained research effort. Even if we may never need to employ these techniques, we need to understand the best ways to reflect greater amounts of sunlight and improve the ability of the oceans to take up carbon dioxide without increasing acidification.