Researchers from Europe and China warn that little can be done to stop dangerous increases in the global sea level, as it will rise between 30 to 70 centimetres (cm) by 2100 even if all but the most aggressive geo-engineering schemes are undertaken to mitigate the effects of global warming and stringently control greenhouse gas emissions. Such changes are likely to cause devastation for the 150 million people living in low-lying coastal areas including inhabitants of some of the world's largest cities. The study's findings were published in the Proceedings of the National Academy of Sciences (PNAS) journal. Some scientists have proposed ways of geo-engineering the Earth to tackle global warming, thereby reducing its impact on both the main contributors of sea level rise: thermal expansion of ocean water and the melting of glaciers and ice sheets. However, Dr Svetlana Jevrejeva from the UK's National Oceanography Centre, Professor John Moore from Beijing Normal University in China and Dr Aslak Grinsted from Copenhagen University in Denmark believe that only the most ambitious of these schemes would have any effect on sea levels and that they could provoke their own problems. The researchers modelled sea levels over the course of the 21st century under various geo-engineering schemes and carbon dioxide (CO2) emission scenarios. 'We used 300 years of tide gauge measurements to reconstruct how sea level responded historically to changes in the amount of heat reaching the Earth from the Sun, the cooling effects of volcanic eruptions, and past human activities,' explained Dr Jevrejeva. 'We then used this information to simulate sea level under geo-engineering schemes over the next 100 years.' Changes in temperature predicted to result from increased atmospheric CO2 or geo-engineering are large compared with those caused by volcanism over the last 100,000 years or by changes in the amount of the Sun's energy reaching the Earth over the last 8,000 years. Dr Jevrejeva's simulations show that extreme geo-engineering projects could have some effect on stabilising sea levels, but she questioned the impact they could have on the planet. For example, she suggested that injections of sulphur dioxide (SO2) particles into the upper atmosphere, equivalent to a major volcanic eruption such as that of Mount Pinatubo in the Philippines every 18 months, would reduce temperature and delay sea-level rise by 40-80 years. 'Maintaining such an aerosol cloak could keep sea level close to what it was in 1990,' said the researchers. However, use of SO2 injection would be costly and risky because its effects on ecosystems and the climate system are poorly understood. Similarly, large mirrors orbiting the Earth could deflect more of the Sun's energy back out to space, reducing temperatures and helping control sea level, but the logistics and engineering challenges of such a scheme are daunting. 'We simply do not know how the Earth system would deal with such large-scale geo-engineering action,' cautioned Dr Jevrejeva. The researchers argued that perhaps the least risky and most desirable way of limiting sea-level rise was via bioenergy with carbon storage (BECS). Biofuel crops could be grown on a large-scale, the CO2 released during their combustion or fermentation could be captured, and the carbon stored as biochar in the soil or in geological storage sites, according to them. '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,' said the scientists. But Dr Jevrejeva warned that any extreme geo-engineering scheme posed potential serious problems. 'Substituting geo-engineering for greenhouse emission control would be to burden future generations with enormous risk,' she concluded.