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Last Updated: Tuesday, 20 February 2007, 08:53 GMT
Creating a 'sulphur screen'
Launching rockets to create a sulphur screen high in the stratosphere is one way to counter global warming explored in a new BBC documentary, Five Ways To Save The World.

Computer generated image of sun's rays being reflected back into space by sulphur blanket
A 'sulphur blanket' would stop the Sun's rays from reaching us

In 1995, Professor Paul Crutzen won the Nobel Prize for helping to explain how the ozone layer is formed and depleted.

Partly as a result of his work, world governments took action and banned the chlorofluorocarbons (CFCs) - chemicals used in fridges and aerosols - that were thinning the ozone's presence over Antarctica.

Now the chemistry professor has a solution to mitigate global warming.

He believes that sulphur particles similar to those erupting from volcanoes could act as a natural cooling device for the planet, by creating a "blanket" that would stop the Sun's rays from reaching the Earth.

Perfect model

In 1991, there was a massive eruption in South East Asia.

Mount Pinatubo ejected about 10 million tonnes of sulphur into the stratosphere at about 10-40km above the Earth's surface.

Professor Paul Crutzen
I am prepared to lose some bit of ozone if we can prevent major increases of temperature in the future
Professor Paul Crutzen
Scientists like Professor Crutzen could measure how much sulphur dioxide was injected into the stratosphere, where it was injected and what happened to it over time.

"After the injection at high altitude, it started to move around the globe with the air motions; first in an east-west direction, but also with time in a north-south direction. After about a year, the initial input of pollutants in the stratosphere by the volcano had spread rather evenly around the world," the Nobel Laureate said.

For two years after Pinatubo erupted, the average temperature across the Earth decreased by 0.6C.

The volcano's location close to the equator helped make Pinatubo the perfect model for explaining how sulphur in the stratosphere could reduce global warming.

Computer generated image of sulphur rockets being launched
Hundreds of sulphur rockets could launch into the stratosphere
But Professor Crutzen does not want to wait for another volcano.

Instead, controversially, he wants to duplicate the effects of volcanic eruptions and create a man-made sulphur screen in the sky.

His solution would see hundreds of rockets filled with sulphur launched into the stratosphere. He envisages one million tonnes of sulphur to create his cooling blanket.

"Hydrocarbons are burnt to lift the rocket material, and the rocket then goes into the stratosphere. In the stratosphere, hydrogen sulphide is burnt, and the sulphate particles reflect solar radiation," he explains.

Devastating effects

But at low altitudes within the Earth's atmosphere, sulphur has been known to create a lot of damage.

Since the industrial revolution began over 200 years ago, the combustion of fossil fuels has put just over a trillion tonnes of carbon dioxide, as well as sulphur, into the atmosphere.

By the mid-1950s, the effects of sulphur were killing thousands of people through respiratory disease. It also caused acid rain and had devastating effects on plants and animals.

To combat this, clean air acts were introduced and filters were put in place to reduce sulphur emissions.

The chemistry professor finds it ironic that prior to these clean air acts, filthy factories actually shielded us from the Sun.

He explains the paradox: "We want to clean up the environment because air pollution is unhealthy. But this pollution also cools the Earth by reflecting solar radiation into space."

Professor Crutzen is not proposing a return to the bad old days; rather, he wants to avoid the previous problems by making sure the sulphur is injected into the atmosphere at high altitude.

Unknown consequences

Nevertheless, the consequences of putting gargantuan quantities of sulphur into the atmosphere as he proposes are unknown.

Regarded as the second layer of Earth's atmosphere
Above the troposphere but below the mesosphere
Positioned at 10-50 km altitude above the Earth's surface
It could increase acid rain, or even damage the ozone layer - the very thing Professor Crutzen has dedicated his life to protecting.

Neither does his solution tackle - or offer a way of reducing - the increasing amount of CO2 that is still being emitted.

But Professor Crutzen believes global warming may reach such critical levels within the next 30 years that a radical strategy will be needed.

He thinks we should at least test his plan, so we know now what the risks might be if we face a catastrophic situation in the future.

"I am prepared to lose some bit of ozone if we can prevent major increases of temperature in the future, say beyond two degrees or three degrees," he says.

Whether other scientists agree that a sulphur screen is a viable solution remains to be seen.

Five Ways To Save The World was broadcast on Monday 19 February at 2100 GMT on BBC Two.

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