IF PEOPLE can warm the Earth, they can probably cool it too. That is the idea behind geo-engineering, which holds that besides cutting the rate at which it is turning fossil fuels into climate-changing carbon dioxide, humanity should also consider planet-wide engineering projects intended to reduce the side-effects of this combustion. All sorts of ideas have been proposed, from filling the stratosphere with reflective particles to giant space-borne parasols designed to shade the Earth from the sun. The idea of such a technological last chance, even if it sounds implausible, is a secret comfort to many of those frustrated by the lack of progress around the world in cutting emissions of greenhouse gases. Two papers published this week suggest, though, that those hopes may be misplaced.
The first, by a group of researchers from Britain's National Oceanography Centre, led by Raymond Pollard, looked at the idea of dumping iron in the oceans to promote huge blooms of phytoplankton--tiny algae that consume carbon dioxide as they grow. This approach has received a lot of interest. Though much of the carbon thus absorbed returns to the atmosphere when the plankton die, around 8-9% ends up locked away beneath the waves for decades or more. Dr Pollard's paper, which appeared in Nature, outlined the results of an experiment that looked at the effects of iron on the growth of phytoplankton near the Crozet Islands in the Southern Ocean.
Every year, as the days lengthen, the seas near these islands produce an enormous bloom of plankton, roughly the size of Ireland. Erosion of the islands dumps large quantities of iron in the water. The prevailing currents then carry most of this iron north, leaving the waters to the south with less. That makes the seas around the Crozet Islands an ideal natural laboratory. Dr Pollard and his colleagues found, as geo-engineers would hope, that phytoplankton blooms in the waters north of the islands were bigger and longer-lived than those to the south. More plankton means more carbon dioxide sucked from the air.
The researchers reckon that the extra iron boosted growth rates and carbon-dioxide consumption between two- and threefold. They also found the first evidence of a similar boost in the amount of carbon locked away in the deep ocean. But while the theory of iron fertilisation seems sound, the practice may be tricky: the team's results suggest that geo-engineers have overestimated the amount of carbon removed per tonne of iron by between 15 and 50 times.
Similarly sobering conclusions are reached in the second paper, by Tim Lenton of the University of East Anglia and Naomi Vaughan of the Tyndall Centre for Climate Change Research, and published in Atmospheric Chemistry and Physics Discussions. This study attempts to rank the likely effectiveness of various geo-engineering proposals.
Because geo-engineering is a new field, the researchers chose to ignore trifles such as cost or practicality, and focused instead on the sheer physical limits on what can be done and how much good the different schemes would do. Top of the list is a solar shade, a gigantic umbrella in space that would shield the Earth from the sun's rays. This is both the most effective option and the most 'scalable', since a hotter Earth would simply require a bigger or more opaque parasol.
The other ideas suffer from fundamental limits that stop them being scaled up indefinitely. Injecting sulphate particles into the stratosphere, for instance, would cool the Earth by reflecting more sunlight into space. (Nature has already shown that this concept can work, since volcanic eruptions that send sulphur-rich plumes into the stratosphere can temporarily alter the world's climate.) However, Dr Lenton notes that the method becomes less effective as the atmosphere becomes more saturated with particles. At most, he reckons, the sulphate-injection approach could counteract half of the warming the world is expected to suffer over the next 100 years if carbon-dioxide emissions continue to rise unchecked.
Other options seem even less effective. Encouraging cloud formation over the oceans by spraying seawater into the air would be roughly as helpful as pumping the stratosphere full of particles. Its effects, though, would be geographically patchy. And dumping nutrients such as iron into the sea would be only one-sixth as effective as either sulphate injection or promoting the formation of clouds.
Moreover, effectiveness is only one way to rank the ideas. In theory, a solar shade could provide any amount of cooling, but the researchers estimate that it would have to have an area of 4.1m square kilometres (half the size of Brazil) to offset half the warming expected over the next century, assuming no cuts in carbon-dioxide emissions occur. A polite critic of such a plan might describe it as 'ambitious'. Sulphate particles, meanwhile, survive only a few years in the atmosphere. For the sulphate-injection method to remain effective they would need to be replenished constantly for centuries--longer than the lifetime of most countries.
Then there are the side-effects. Any geo-engineering project would necessarily be enormous, and would therefore cause plenty of disruption to ecosystems. Plans to suck carbon from the air by growing trees conflict with the need to grow crops. Creating gigantic algal blooms risks using up all the oxygen in large parts of the ocean, killing anything else that lives there. These sorts of things are unlikely to go down well with environmentalists. Indeed, a group of researchers hoping to conduct more experiments on ocean fertilisation was recently ordered to stop by the German government. Ministers were worried that the experiment might have been illegal under international laws designed to protect marine wildlife, although they eventually gave the go-ahead. When it comes to the environment, there are no perfect answers.