By Paul Sussman for CNN
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(CNN) -- Another day, another apocalyptic global warming prediction.
In a persuasively-argued paper, published this week in the weighty scientific journal Philosophical Transactions of the Royal Society, six of the leading climatologists in the U.S. have warned that the future of human civilization itself is being imperiled by accelerating climate change.
"Recent greenhouse gas emissions place the Earth perilously close to dramatic climate change that could run out of control, with great dangers for humans and other creatures" warns the article.
Such dire predictions have been appearing with ever-increasing frequency over the last two decades, and have given ever-increasing impetus to efforts to slow down and reverse global warming.
Most of those efforts have, inevitably, focused on ways of limiting and reducing the causative greenhouse gas emissions.
At the same time, as the scale of the problem has become ever more apparent, so ever more elaborate technological solutions have been put forward to try to tackle the problem of a warming Earth.
These range from relatively mainstream ideas such as carbon capture and storage -- collecting the carbon dioxide produced by the burning of fossil fuels and pumping it into vast storage reservoirs deep underneath the earth -- to more outlandish proposals such as painting all the world's roads white to reflect sunlight back out into space and so lower the earth's temperature.
Between these two extremes lie a variety of suggestions that, while still bordering on the world of science fiction, are nonetheless attracting increasing attention as a means of heading-off the sort of environmental cataclysm predicted in this week's Royal Society paper.
Of all these "whacky-but-workable" theories the one that has perhaps generated the most interest, both in terms of scientific plausibility and practical implementation, is that of solar reflection.
The basic concept is the same as that underpinning the "white roads" idea -- the development of a technology that can reflect a proportion of the sun's radiation away from the earth's surface and back out into space, thereby cooling the atmosphere and slowing, or even reversing the damaging effects of global climate change.
Several models have been put forward as to how such a feat of "geo-engineering" might actually be achieved, most involving the positioning of a reflective shield of some sort out in space.
As early as 1989, for instance, in an article entitled "Space-based Solar Shield to Offset Greenhouse Effect," James T. Early of the Lawrence Livermore National Laboratory, California, proposed siting a vast, 2000 km-wide glass deflecting panel at the "inner Lagrange point" between the Earth and the sun, the latter diverting the sun's rays away from the Earth's atmosphere.
Similarly in 1992 the U.S. National Academy of Sciences (NAS) put forward the idea of launching 55,000 "solar sails" into orbit around the Earth, each with an area of 100 square kilometers, the sails collectively producing the same effect as Early's single glass panel.
Other "space shield" proposals have included the establishment of a cloud of miniature reflective parasols -- each just 60 cms in diameter -- to shade the Earth from the sun; and the creation of an artificial planetary ring around the Earth composed of passive particles.
While computer-modeling has shown all of these options to be essentially viable, they would nonetheless be technologically complex and hugely expensive.
The miniature parasols idea, for example, envisages at least 16 trillion such parasols, the latter requiring an estimated 20 million separate space missions to get them up into orbit.
Using particles to create an artificial ring around the earth, meanwhile, would cost an estimated $4-6 trillion.
Stratospheric sulphur shield
A more controversial variation on the solar shield theme, but one that is proving more and more popular due to the fact that it is both less costly and less complicated than space-based alternatives, is the creation of a stratospheric solar shield using sulphur.
The idea is inspired by the fact that sulphate particles unleashed into the stratosphere by large-scale volcanic eruptions have been observed to reflect radiation away from the Earth. This in turn lessens the amount of heat percolating downwards into the atmosphere, and so reduces the greenhouse effect.
The 1991 eruption of Mount Pinatubo in the Philippines, for example, the second most powerful volcanic event of the 20th Century, produced sulphur dioxide clouds that cooled the Earth by 0.5 degrees.
The sulphur shield proposal has been championed by a number of scientists, of whom the most high-profile is Paul Crutzen, winner of the 1995 Nobel Prize for Chemistry.
In a 2006 article in the journal Climactic Change Crutzen proposed deliberating pumping millions of tonnes of sulphur into the upper atmosphere where it would coalesce with other molecules to create a reflective barrier.
Such a barrier, Crutzen predicted, would reflect approximately one percent of the sun's heat back out into space, sufficient to counter the effects of current atmospheric levels of carbon dioxide.
Recent computer-modeling by Ken Caldeira of Washington's Carnegie Institute and Damon Matthews of Montreal's Concordia University has expanded Crutzen's work, demonstrating that such a sulphur shield would not only be relatively easy and cheap to implement, but should, within a decade of being deployed, lower global temperatures to early 20th Century levels.
Too good to be true
It all sounds a little too good to be true. And indeed it is.
"No-one knows exactly what the effect would be if you pumped this enormous amount of sulphur into the atmosphere," Dr. Benny Peiser of Liverpool's John Moores University told CNN. "It would be a huge and risky experiment."
Some of the possible results include debilitation of the ozone layer, acid rain and an increased incidence of human respiratory illnesses.
The sulphur would also not remain in the atmosphere permanently, gradually drifting back down to earth over a period of about two years. The shield would thus have to be constantly replenished.
Both Crutzen and Caldeira themselves, while pointing out the potential benefits of such a sulphur shield, are also candid about its drawbacks, with Crutzen saying that he hopes his "ugly baby" will never actually have to be deployed.
"Personally, as a citizen not a scientist, I don't like geo-engineering because of the high environmental risk," Caldeira said in a recent interview with New Scientist magazine.
"It is toying with poorly understood complex systems."
The general consensus within the scientific community is that the idea of creating a solar-shield -- whatever form it might take -- should not in any way be seen as an alternative to finding ways of reducing global greenhouse gas emissions.
At the same time, however, there is also a consensus that it is crucial to explore and develop such futuristic technologies in case the worst-case global warming scenarios should become reality.
"Despite their potential drawbacks we need to look at these geo-engineering fixes because we might need them at some point," says Dr. Benny Peiser. "It is always better to be prepared for any event.
"You insure your house against fire even though you know it probably won't be burnt down.
"That's what these sort of solar-shield technologies offer -- an insurance policy. It is unlikely we will ever have to use them, but it is good to know we could deploy them if necessary."
Many scientists believe that solar reflection could, as a last resort, help limit the damaging effects of global warming