Sea levels are likely to rise by 30 to 70 centimetres by 2100, even if the most aggressive geo-engineering schemes are undertaken to curb global warming and greenhouse emissions.

Sea levels to rise upto 70 cm by 2100

Sea levels are likely to rise by 30 to 70 centimetres by 2100, even if the most aggressive geo-engineering schemes are undertaken to curb global warming and greenhouse emissions.

"Rising sea levels caused by global warming are likely to affect around 150 million people living in low-lying coastal areas, including some of the world's largest cities," warned Svetlana Jevrejeva of the National Oceanography Centre in Britain.

Geo-engineering is an artificial mode of counteracting increasing carbon dioxide levels in the air or screening out sunlight with orbiting mirrors, among others.

Scientists from Britain, Denmark and China have proposed ways of 'geo-engineering' the earth system to tackle global warming, reports the Proceedings of the National Academy of Sciences.

Jevrejeva and her colleagues have modelled sea level over the 21st century under various geo-engineering schemes and carbon dioxide (CO2) emission scenarios, according to a statement of the National Oceanography Centre.

Their simulations show that injections of sulphur dioxide particles into the upper atmosphere, equivalent to a major volcanic eruption such as that of Mt Pinatubo every 18 months, would reduce temperature and delay sea-level rise by 40 to 80 years.

Maintaining such an aerosol cloak could keep sea level close to what it was in 1990.

However, use of sulphur dioxide injection would be costly and also risky because its effects on ecosystems and the climate system are poorly understood.

"We simply do not know how the Earth system would deal with such large-scale geo-engineering action," Jevrejeva said.

Large mirrors orbiting the Earth could deflect more of the Sun's energy back out to space, reducing temperatures and help control sea level, but the logistics and engineering challenges of such a scheme are daunting.

The researchers argue that perhaps the least risky and most desirable way of limiting sea-level rise is bioenergy with carbon storage (BECS).

Biofuel crops could be grown on a large-scale and CO2 released during their combustion or fermentation could be captured and the carbon stored as biochar in the soil or in geological storage sites.

BECS has some advantages over chemical capture of CO2 from the atmosphere, which requires an energy source, although both approaches could eventually reduce atmospheric CO2 levels to pre-industrial level, according to the new simulations.