Back in 1992, as representatives from 154 countries gathered in Rio de Janeiro and agreed to keep greenhouse-gas emissions from getting out of hand, a US National Academy of Sciences report on climate change was receiving its final touches. The study was the consequence of congressional action, and drew on the knowledge of more than 50 experts on climate change, its impacts, mitigation and adaptation. About halfway though the nearly 1,000-page report, the mitigation panel considered a risky and uncertain endeavour to counteract climate change: the large-scale engineering of the Earth's environment.

Since then, there has been a steady stream of geoengineering proposals to manage climate change: iron-seeded oceans coaxed to absorb more carbon dioxide; an armour of mirrors for the Earth to reflect the sunlight into space; hazy mists of saltwater sprayed into the atmosphere to increase cloud formation over the oceans; the injection of sulphate aerosols into the troposphere to block incoming solar radiation. Yet most of the proposals — seemingly cribbed from the pages of science fiction — have rested quietly on the fringes of science for years.

Quick fix

Recently, though, geoengineering has been experiencing a renaissance1. Some of the Earth's carbon sinks, such as the Antarctica's Southern Ocean, are losing their ability to soak up carbon dioxide2, and as these carbon repositories near capacity more quickly than anticipated, technology may become more important in taming climate change. But some climatologists are cautious as they learn more about what will happen in a world that's cooler, but swathed in carbon dioxide.

Injecting sulphur dioxide into the atmosphere has become the most appealing scheme of the lot because it draws on natural phenomena. In 1991, the eruption of Mount Pinatubo spewed about 30 million tons of aerosols into the stratosphere. The tiny particles scattered some of the solar radiation back into space, cooling the Earth. This aerosol veil and its effects are well documented and considered to be a good natural analogue to injecting sulphur aerosols into the atmosphere.

Rapid response

If 50 years from now we found that climate warming was much larger than we anticipated, there is the potential for a quick geoengineered cooling to offset the warming. Damon Matthews

Sulphur aerosols are effective at blocking the sun's rays. In large quantities, they could offer a quick geoengineered solution to global warming. Credit: © Punchstock

Damon Matthews, of Concordia University in Montreal, and Ken Caldeira, of the Carnegie Institution of Washington recently tested whether a man-made version of the event would yield a similar response. Published last month in the Proceedings of the National Academy of Science3, their study, which modelled the climate response to a geoengineered sunshade, found that sulphur aerosols are effective at blocking the sun's rays and allow surface air temperatures to return to values found in the 1900s. The climate response was almost immediate, says Matthews. “If 50 years from now we found that climate warming was much larger than we anticipated, and there was the danger of imminent climate catastrophe, there is the potential for a quick geoengineered cooling to offset the warming,” he says.

Today, the ocean and terrestrial sinks each sop up about two billion tons of carbon per year. According to Matthews' and Caldeira's model, fifty years from now, even in the absence of geoengineering, those sinks are expected to double in response to high carbon dioxide levels. As levels increase, more is soaked up by plants (through faster growth), and by the oceans. But the warming that the Earth is experiencing, and that it is projected to experience in the future, counteracts the effect by suppressing the capacity of carbon sinks to sequester CO2.

Some of the Earth's major carbon sinks are losing their ability to soak up carbon dioxide Credit: © BAS

Conversely, the temperature drop induced by geoengineering would expand the Earth's carbon sinks to their full potential. In a geoengineered world, the oceans would be cooler and less stratified, says Caldeira, leading to greater carbon uptake. On land, decomposition rates would slow down and more carbon dioxide would accumulate in the terrestrial biosphere. But because neither oceanic nor land-based carbon sinks would be able to soak up carbon dioxide faster than it is emitted, CO2 levels would continue to rise, hitting around 800 p.p.m.v. by 2100.

Serious side effects

There would be other consequences too in a geoengineered world: acidified oceans and distorted precipitation patterns. Historically, ocean pH has been a slightly basic 8.16. It has crept down by 0.1 units over the last 200 years and may fall by as much as 0.4 units by 2100 if carbon dioxide emissions continue to rise at their current pace. The release of sulphate aerosols into the atmosphere could further increase ocean acidity as the oceanic carbon sink expands.

“There will be huge consequences for the entire marine ecosystem,” says Alan Robock, a climatologist at Rutgers University. The decline in pH hampers the ability of marine organisms to build shells and skeletons. Coral reefs may weaken or collapse, and pteropods, oysters, clams and mussels — all creatures with calcium carbonate shells — could be jeopardized, with the risk of changing the biodiversity of the oceans.

Geoengineering could disrupt global rainfall patterns, worsening regional drought. Credit: © Punchstock

Volcanic eruptions show that rainfall patterns across the globe would also be disrupted. Kevin Trenberth, an atmospheric scientist at NCAR, has been reconstructing rainfall and continental runoff patterns following the eruption of Mount Pinatubo in 1991. In the year after the incident, overland precipitation slowed down and the associated runoff and river discharge plummeted4. The timing coincided with widespread regions of moderate and severe drought.

Other studies of older eruptions show similar patterns. In September, Robock and his colleagues reported that the 1783 eruption of the Icelandic volcano Laki pumped over 100 million tons of sulphur dioxide into the atmosphere and weakened the monsoons in Africa and Asia5. Drought and famine soon followed in Egypt. Two other eruptions in the last 2,000 years have produced similar monsoon failure, says Robock.

Abrupt End

But within about a decade, temperatures return to where they would have been, resulting in very rapid rates of warming Damon Matthews

If the potential side effects aren't sufficiently dissuading, the consequences of the abrupt termination or failure of geoengineering schemes just may be. “Within about a decade, temperatures return to where they would have been, and that results in very rapid rates of warming,” says Matthews. The terrestrial biomass would begin to decompose and release carbon into the atmosphere, and the warmer oceans would absorb less. The annual temperature rise following failure could be as high as 4 °C per decade — about 20 times the current rate of global warming — and could have severe impacts on humans and the environment.

Among most scientists, cutting greenhouse-gas emissions remains the top choice for reducing global warming. Some say they're not totally against geoengineering, but all admit there are still many more modelling studies to do before it can even be tested.

Hannah Hoag is a freelance science writer