The carbon dioxide released into the atmosphere by natural phenomena like volcanoes and human activities around burning fossil fuels has sent levels in the atmosphere soaring above what they were a century ago. Dumping iron dust in the seas or placing smoke and mirrors in the sky to dim the sun could help a world struggling to curb global warming, according to backers of extreme technologies like geoengineeringREUTERS

Scientists are revisiting controversial propositions to manipulate the atmosphere and slow down the increase in temperatures on Earth by reflecting more of the sun's energy back into space.

Researchers from Harvard School of Engineering and Applied Sciences (SEAS) are proposing a stratospheric perturbation experiment using small amounts of sulphuric acid to block sunlight.

This involves an amount equivalent to what a commercial aircraft releases in a few minutes of flying in the stratosphere, say scientists.

It can help understand how solar radiation management can reduce heating, concentration of water vapour in the stratosphere and study the movement of water vapour.

The potential risks of solar radiation management (SRM) are substantial and are among the reasons why experts have advised against such tinkering.

"The idea of conducting experiments to alter atmospheric processes is justifiably controversial, and our experiment, SCoPEx, is just a proposal," says David Keith, Gordon McKay Professor of Applied Physics at Harvard SEAS and a professor of public policy at Harvard Kennedy School.

"It will continue to evolve until it is funded, and we will only move ahead if the funding is substantially public, with a formal approval process and independent risk assessment."

Rubbishing the claim that geo-engineering results cannot be verified by small-scale tests, he says experiments like theirs will help reduce "unknown unknowns" in the picture.

Another paper co-authored by Keith and collaborators at the California Institute of Technology (CalTech) collects and reviews a number of other experimental methods, to demonstrate the diversity of possible approaches.

What is geoengineering?

Geoengineering looks at ways and means to reduce global warming by direct intervention that involves increasing capacity of planktons to take in more carbon dioxide as also blocking sunlight from hitting the surface using mirrors.

Opponents have looked at these as an escape way out of tougher mitigation methods that call for a low-carbon lifestyle.

Tinkering with the atmosphere or the oceans will only encourage nations and people to continue with the fossil-fuel based economy.

Recently, the International Renewable Energy Agency had shown that not only can renewable energy meet the rising demand but it can do so cheaply, and help limit global warming to the crucial two degrees during the century.

Attempts to buy time by fixing short-term pollutants like methane and sulphur dioxide will not work, a ETH Zurich study showed.

Carbon dioxide emitted in the burning of biomass and fossil fuels lasts thousands of years in the atmosphere, trapping heat and warming the planet.

The latest IPCC report on climate change has warned that unless the shift to renewable energy is complete by the end of the century, irreversible change will have devastating consequences on the planet.

In fact, it is the irreversible nature of climate change that can be addressed by geo-engineering, say advocates like Keith. Once the planet has crossed the threshold, there will be no going back for a 1000 years and it is then that forced cooling of the atmosphere will help, they argue.