BBC News  [Printer-friendly version]
March 2, 2006


By Molly Bentley

Could giant shades in space curb climate change on Earth?

Any cook who has tasted a dish after tossing in too much salt or hot
pepper knows the panic of trying to undo the damage.

You raid the spice rack to mask the glut of cayenne, but oops -- now
is swimming in oregano. It is hard to restore the balance once you
overdo it.

We have experienced this with the Earth's atmosphere for some time
now; concentrations of greenhouse gases continue to grow even as we
adopt international protocols and clean technologies in an attempt to
bring them down.

It is apparent our energy-saving tricks are not enough to prevent
perhaps catastrophic climate change. But there may be a "plan B"; and
while scientists hope it will not be needed, they are considering it

It would be plucked from an array of bold and contentious proposals
which go under the heading "geoengineering".

Feasible or far-out?

"Humans are changing the Earth, and it's a big effect we're having,"
says Mike MacCracken, chief scientist for climate change projects at
the Climate Institute in Washington DC.

"To really stop climate change in its tracks, you have to go to
virtually zero emissions in the next two decades.

"So the question is, is there a silver bullet that can help us to
limit the amount of climate change?"

Some such "silver bullets" aim to scrub carbon dioxide (CO2) out of
the atmosphere, some to cool Earth directly by veiling it; others are
yet more radical.

While most are confined to computer models or scribbling on the backs
of envelopes, a few have been tried cautiously.

Scientists have sprinkled iron in patches of the Southern Ocean to
increase absorption of carbon dioxide from the atmosphere, and are
testing the feasibility of sequestering carbon in saltwater aquifers
or rock.

But what distinguishes geoengineering from localised tinkering is the
scope; this would be manipulation on a global scale.

Earth in the shade

Consider the notion of shading the planet with mirrors. The US
National Academy of Sciences found that 55,000 orbiting mirrors would
reflect enough sunlight to counter about half the doubling of carbon

But each mirror must be 100 sq km; any larger and you would need a
manufacturing plant on the Moon, says Dr MacCracken. The price tag of
space-based fixes makes them prohibitive -- for now.

By contrast, the "human-volcano" approach is on terra firma and less
costly. Inspired by studies of the Mt Pinatubo eruption of 1991 and
the cooling effect of its sulphur plume, one proposal suggests that
naval guns shoot sulphur pellets into the air to increase Earth's
albedo, or reflectivity.

We know that blocking sunlight can counter global warming, but can we
get the balance right?

"I don't think we can get it right," says Ken Caldeira from the
Carnegie Institution Department of Global Ecology at Stanford
University in California.

"One of the problems of putting sulphate particles in the stratosphere
is that it would destroy the ozone layer; so you might solve the
global warming problem, but then we'd all die of that."

And this from a man whose work supports the idea of dimming the Sun.

A few years ago, Dr Caldeira set out to disprove an idea put forward
by Livermore physicists Lowell Wood and Edward Teller to cool the
Earth with a sheet of superfine reflective mesh -- similar in concept
to orbiting mirrors.

In a computer model, Dr Caldeira and colleague Bala Govindasamy
simulated the effects of diminished solar radiation.

"We were originally trying to show that this is a bad idea, that there
would be residual regional and global climate effects," explains Dr

"Much to our chagrin, it worked really well."

Acts of hostility

The simulation showed that blocking even a small percent of sunlight
balanced out the doubling of atmospheric CO2. But in their published
paper, the scientists caution against the environmental risks of

A broad simulation cannot account for all feedbacks lurking in the
system, and Dr Caldeira does not recommend building an Earth parasol
based on the results; current computer models are not up to the task
of predicting the consequences of large-scale plans such as Earth

The knowledge that we maybe could engineer our way out of climate
problems inevitably lessens the political will to reduce emissions

Dr Caldeira did the numbers. He found it would require the energy of
five thousand, million, million hydrogen bombs to move Earth's orbit
1.5 million km out, which would compensate for doubling CO2 in the

If geoengineering seems like a "what if?" diversion for the science
fiction crowd, scientists take it seriously, even if they are set
against it.

"I should say right up front, I am not at all in favour of
geoengineering," says Richard Somerville, a climate researcher at
Scripps Institution of Oceanography in California.

"I think it's inherently unethical. I don't see how you decide on the
basis of all humanity how to change the planet. But I think it's
irresponsible, in a way, not to study it."

Aside from its feasibility, says Dr Somerville, geoengineering raises
many issues that scientists have only begun to list.

There are ethical questions of whether we commit children to a planet
that requires constant tinkering, and of who ultimately decides to
release a supertanker full of iron into the oceans.

There may be legal questions as well, says Dr MacCracken. Prompted by
US cloud-seeding attempts in Vietnam, a 1976 international convention
outlaws the hostile use of environmental modification techniques.

"That would normally be in the case of war, but 'hostile' is not a
word that's easily defined," says Dr MacCracken.

Perhaps some countries would consider re-calibrating the Earth's
reflectivity -- let alone its orbit -- as a hostile act.

Diversionary tactic?

While humans have a long history of wanting to control weather and
climate -- cloud seeding is an example -- this incarnation of
geoengineering is such a hot potato that scientists cannot even agree
whether it should be discussed publicly.

"The knowledge that we maybe could engineer our way out of climate
problems inevitably lessens the political will to begin reducing
carbon dioxide emissions," observes David Keith from the University of
Calgary in Canada.

Meanwhile, we might reconsider our investment priorities, says Dr
MacCracken. For the enormous sum it would take to launch an orbiting
mirror, we could develop energy alternatives.

"If I'm going to put satellites in orbit, why not put energy
generating satellites that capture solar energy and beam it down to
Earth?" he asks.

"Rather than blasting sulphur into the stratosphere, why not invest in
other kinds of energy systems, such as wind energy or ocean energy,
that don't cause these kinds of problems? There's a lot to do well
before you get to geoengineering."

Ken Caldeira agrees that geoengineering is, for the moment, a tempting
but illusory quick fix to an intricate system; a much less problematic
solution, he says, would be to change our lifestyles by reducing
energy consumption and CO2 emissions.

"I think the Earth's system is so complicated that our interfering
with it is very likely to screw things up and very unlikely to improve
things," he says. "And this is the only planet we have."