When you can't do it all with mirrors
[This is the unedited text of my recent article for muse@nature.com.]

A new proposal and costing for a technofix to global warming shows that there are probably better ways to spend the money

The leading economist Nicholas Stern has just handed us, in advance, the bill for the impacts of climate change: close to $4 trillion by the end of this century [1].

And with perfect timing, astronomer Roger Angel of the University of Arizona has delivered the equivalent of a builder's estimate for patching up the problem using a cosmic sunshade [2]. It will set us back by… well, let's make it a nice round figure of $4 trillion by the end of the century.

Both figures can be criticized – after all, when costs add up to a significant fraction of global GDP, no one can expect them to be very accurate. But this happy conflux of estimates puts some perspective on the hope that global warming can be addressed with high-tech mega-engineering projects.

From a pragmatic point of view, the sunshade solution looks like a bad bargain. If a builder told you that the cost of fixing a problem with your roof was likely to be about the same as the cost of not fixing it, except that the fix was untested and might not work at all, and in any event you know the work is likely to run over budget and probably over schedule – well, what would you do?

One could argue, however, that in this case the 'problem' involves the potential suffering of millions of people, who could be killed by disease or flood or drought, displaced from their homes, or caught up in conflict as a result of climate change – in which case you might conclude that investing in a risky technofix can be justified on humanitarian grounds.

But Stern's report, commissioned by the UK government and hailed by many other economists as the most definitive study of its sort to date, doesn't just tot up the costs of inaction over climate change. It makes some estimate of the likely costs of tackling it using existing approaches and technologies – and the answer looks cheaper and a whole lot more attainable than Angel's sunshade.

That doesn't mean Angel's proposal is without value. On the contrary, it performs the service of showing just what would be involved in pursuing one of the favourite ideas of those who believe technofixes could save us from rising world temperatures.

A space shade that reduces the amount of sunlight reaching the Earth has been debated for decades. Many of these schemes invoke a screen that would be unfolded or assembled in space, like a gigantic sail. But as James Early of the Lawrence Livermore National Laboratory in California pointed out in 1989 [3], a sail is precisely what it would be: radiation pressure would push against the sunshade, and it would therefore need to be kept actively in position.

Angel has found inventive ways of coping with all the challenges while keeping costs down. To minimize radiation pressure, the screen would deflect sunlight through only a small angle, just enough to miss the Earth. To keep it in line between the Earth and Sun, it would be placed at the so-called Lagrange point L1, a point in space 1.5 million km away that orbits the Sun with the same 1-year period as our planet.

The size of the screen would be mind-boggling: about 4-6 million square km, around half the area of China. But to avoid complicated space-assembly problems, and to simplify the launching and increase the screen's versatility, Angel proposes that it should consist of a vast swarm of 1-m disks, made from lightweight, microscopically perforated and laminated films of ceramics. Each of these 'flyers' is manoeuvrable thanks to tiny solar sails placed on tabs at the rim, powered by solar cells.

As usual, science fiction got there first. In a short story by Brenda Cooper and Larry Niven published in 2001, an alien species wipes out another by deploying a fleet of tiny mirrors around their planet, plunging it into an ice age [4] – a reminder, perhaps, that we'd better not overdo the shadowing.

Angel's flyers would be launched in stacks, like piles of Brobdingnagian dinner plates, packaged into canisters and fired into space from electromagnetic guns more than a kilometre long. Twenty such cannons would fire 1-ton payloads every five minutes for ten years. Once in space, the flyers make their way to the Lagrange point using fuel-efficient ion thrusters, where they spread out into a cloud as wide as the Earth and 100,000 km long.

And the bill, please? Estimating the costs of materials and launch facility, launch energy, and manufacturing, Angel says it could be done for less than $5 trillion.

All this sounds a long way from the sober accounting of the Stern report. But if you take the report seriously – and as a former chief economist of the World Bank, Stern apparently has the right credentials, although his conclusions have proved predictably controversial – it is similarly mind-boggling.

For example, Stern says that the impacts of climate change could end up costing the world up to 20% of its annual GDP. He compares the effect to that of the world wars or the Great Depression. The "radical change in the physical geography of the world" that climate change would produce, he says, "must lead to major changes in the human geography – where people live and how they live their lives".

Mitigating this potential crisis would require equally drastic measures. Stern does not consider technofixes like the space sunshade, but dwells instead on the far less sexy measure of reducing greenhouse-gas emissions. Gordon Brown, the UK's Chancellor of the Exchequer, who commissioned the report, has called for cuts of 30% by 2020 and 60% by 2050.

Stern's solutions involve energy-saving and improvements in energy efficiency, stopping deforestation, and switching to non-fossil-fuel energy sources. That will work only if the effort is international, he says (which is one reason why sceptics have scoffed), and it will incur a substantial cost: 1% of global GDP over the next 50 years, an amount that Stern calls "significant but manageable", and which squares with some previous estimates.

Whether the targets can be reached by putting solar cells on roofs, turning out lights, banning SUVs and building wind farms, or whether this will require more substantial measures such as new nuclear power stations, extensive carbon capture and sequestration, and fierce taxation of air travel, is a question that environmentalists, industrialists and politicians will continue to debate, no doubt as dogmatically as ever.

But as well as sketching an essay in ingenuity, Angel has done us the great favour of showing that there is probably never going to be the option of conducting business as usual under the shelter of a gigantic technofix.

Reference
1. http://www.hm-treasury.gov.uk/independent_reviews/stern_review_economics_climate_change/stern_review_report.cfm
2. Angel, R. Proc. Natl Acad. Sci. USA in press (2006) [doi:10.1073/pnas.0608163103]
3. Early, J. T. J. Brit. Interplanet. Soc. 42, 567-569 (1989)
4. Cooper, B. & Niven, L. "Ice and Mirrors", in Asimov's Science Fiction, February 2001.