Scientists rank climate cooling schemes

28 January 2009

Giant sun shades in space, painting roofs white and fertilising the oceans with iron have all been proposed as ways to slow or even stop global warming.

Space mirror

Giant sun shades in space would cool the planet but the economic and environmental costs may be too steep

But which ideas are worth further investigation and which are pure science fiction?

Scientists from the University of East Anglia publish today the first assessment of the climate cooling potential of various geoengineering schemes.

The research, partly funded by the Natural Environment Research Council, examined several ideas including: erecting sunshades in space; injecting tiny sulfate particles (aerosols) high into the atmosphere; covering deserts, roofs and roads in reflective material; growing shinier crops to reflect more sunlight; planting more trees; seeding clouds; and ramping up biochar production - turning plants into a type of charcoal in soil where it boosts crop productivity.

The paper, published in the journal Atmospheric Chemistry & Physics Discussions, states, "On the face of it some encouraging results emerge from our analysis, but they come with very large caveats."

Lead author Professor Tim Lenton says, "We found geoengineering options could usefully complement mitigation, and together they could cool the climate, but geoengineering alone cannot solve the climate problem."

Schematic of geoengineering proposals

Some geoengineering solutions on offer

The research, based on simple calculations of the effects of each solution, ranked the options in order of effectiveness by 2050, 2100 and then much longer time scales.

The biggest and most immediate cooling effect comes from putting sunshades in space or injecting stratospheric aerosols. But these solutions also carry the greatest risks. Failure to replenish aerosols continually, for example, could lead to 'extremely rapid warming'.

An order of magnitude below that come chemical and biological methods of removing carbon dioxide from the air and shiny deserts, followed by planting more trees, biochar and changing the albedo, or reflectance, of cropland. Ocean fertilisation by iron, phosphorus or nitrogen has some merits, while other suggestions, on paper at least, look like a poor bet.

Lenton says the most surprising result is that inadvertently adding phosphorus to the oceans, which societies already do on a large scale, may remove more carbon dioxide in the long term than the much-touted iron fertilisation.


Though inadvertent, phosphorus, which is found in fertilisers and sewage, is leading to some removal of carbon dioxide from the atmosphere, but has many ecological drawbacks

Phosphorus is put on to soils as a fertiliser and is present in sewage. Eventually it runs into rivers and out to sea where it again acts as a fertiliser for tiny marine plants that absorb carbon dioxide from the atmosphere.

The paper says "If our estimates are even remotely accurate, recent interest in ocean carbon cycle engineering is misplaced because even the more promising options are only worth considering on a millennial time scale."

Striking a balance

The basic calculations looked at the heat coming into the planet, and the heat leaving - the Earth's energy balance. The sun pumps 342 Watts per metre squared of solar energy into the planet. Burning fossil fuels, cement production, deforestation and land use change - human activities in short - are responsible for an additional 1·6W/m2 right now. But this is a shifting figure. It is expected to soar to 3·7W/m2 when CO2 in the atmosphere doubles to about 560 parts per million from its pre-industrial level of 280ppm. It is currently at 385ppm.

"Even the most optimistic emissions reductions scenario still means we will top out at 3·1W/m2 by 2100," says Lenton. Without efforts to curb emissions or adopt geoengineering techniques this could reach 7W/m2 by 2100. "Mitigation could tackle a little more than half the problem, but we've got this remaining 3W/m2," adds Lenton. Few of the options even come close to counteracting this.

The paper says, "Placing sunshades in space or injecting stratospheric aerosols appear to be the only options that can achieve a reasonably uniform -3W/m2."

"These measures need to be continually replenished to maintain their effects."

The proposal to erect sun shades in space would be a monumental engineering achievement. It would involve creating a cloud of tiny spacecraft, each 60 centimetres in diameter, 1·5 million kilometres from Earth towards the sun.

'Space flyers'

The cloud would need to shield an area roughly 16 times the size of Britain just to offset current human-induced warming. Initial calculations show that an additional area of 31,000 square kilometres would need to be added each year, requiring 135,000 launches, each carrying 800,000 'space flyers'.

By comparison, adding sulfate to the upper atmosphere may seem a more practical solution. "Sulfates have a life time in the atmosphere of a year or two," says Lenton. "We know this from monitoring volcanic eruptions such as Mount Pinatubo [Indonesia, 1991]."

"Once you start on this track you are committed to year on year renewal for centuries to balance the global radiation budget."

"But there are big caveats. We know it affects the global water cycle and it accelerates ozone depletion."

The report says planting new forests and biochar would give -0·8W/m2. But they have a greater short-term cooling potential than ocean fertilisation. Iron, nitrogen and phosphorus fertilisation of the oceans combined would only achieve -0·45W/m2.

Over thousands of years, phosphorus addition comes out as one of the best solutions, purely in terms of cooling. "This was my surprise result," says Lenton. "It is generating a considerable carbon sink already."

"I looked at how it is likely to increase this century. It outstrips iron fertilisation of the oceans, suggesting there is too much hype and attention given to iron fertilisation."

While this looks like an efficient carbon sink, other environmental effects, such as large-scale toxic algal blooms may prove unacceptable to ecologists.

Capturing carbon dioxide by growing more plants or using chemicals such as sodium hydroxide to 'scrub' CO2 out of the atmosphere could achieve a -2W/m2 by 2100. "Afforestation and biochar, plus bio-energy capture and storage look pretty promising to me," says Lenton. "They can't solve the problem this century, but are realistic and useful in the long term."

"Biochar in particular seems to have hit a media tipping point: suddenly lots of people have come to the same conclusion and everyone is talking about it."

Globally, the discarded parts of the plants we grow produce a significant amount of waste. Burning this waste in the absence of oxygen converts it to charcoal. This could be used to generate energy, store carbon beneath ground or act as a fertiliser by boosting nutrient retention and so increasing crop productivity.

The 'very large caveats' the researchers allude to are the subject of another paper due out shortly by the same team. It will attempt to quantify the known side effects and environmental risks.

Storm gathers over Indo-German ocean experiment

Geoengineering is not without its critics, as the controversy surrounding an iron fertilisation experiment in the Southern Ocean has shown this week. The Indo-German research experiment to fertilise 300 square kilometres of ocean with six tons of dissolved iron was postponed following complaints from environmental groups.

The complaints prompted the German government to order further evaluations of potential environmental risks. These have now been completed and confirm the results of the original risk assessment. The project has been given the green light, but the incident has highlighted how contentious this area of research could become.

'The radiative forcing potential of different climate geo-engineering solutions', Atmospheric Chemistry & Physics Discussions, 2009.