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Even if greenhouse gases emissions had been stabilised in the year 2000, we would still be unable to stop temperatures and sea levels from rising significantly, warn two studies in last week's issue of Science.

The results show the planet is already committed to climate change because of how slowly water warms up and cools down and the long lifespan of greenhouse gases already in the atmosphere.

"Many people don't realise we are committed right now to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere," says lead author of one of the studies, Gerald Meehl, of the US National Centre for Atmospheric Research.

"The longer we wait [before stabilising greenhouse gas emissions] the more climate change we are committed to in the future."

Experts have been warning that the planet could be facing unavoidable effects of climate change due to past emissions, but Meehl's team is the first to confirm this using models that take into account several of the Earth's climate variables.

Their results confirm the need to adapt to climate change as well as reduce emissions.

Water is denser than air so the sea takes longer to cool down and warm up. At the moment, the oceans are warming up, and expanding as they do so.

This 'thermal expansion' alone will cause sea levels to rise by 11 centimetres by 2100, predicts the study led by Meehl.

The authors warn that this rise in sea levels does not take into account melting ice sheets and glaciers — factors that could double the increase.

Meehl's team also estimates that even if no more greenhouse gases were emitted, global temperature would still rise by between 0.6 to 0.7 degrees Celsius before stabilising in 2100.

In a separate study, Tom Wigley, also of the US National Centre for Atmospheric Research, predicts that if the composition of the atmosphere were fixed today, global temperatures would rise by a minimum of one degree Celsius, and sea levels would rise by ten centimetres per century for "many centuries".

Wigley says these results are alarming "because they are based on a future scenario that is clearly impossible to achieve and so represent an extreme lower limit to climate change over the next few centuries".

Commenting on the studies, David Viner, senior research scientist at the University of East Anglia's Climatic Research Unit, United Kingdom, told SciDev.Net: "This study shows that whatever we do now to reduce greenhouse gases, we already have certain commitments that we cannot be rid of."

The predicted increase in sea levels would have significant long-lasting impacts on fragile ecosystems such as coral reefs and on communities in low-lying areas. Global warming is also predicted to lead to more extreme weather and storms.

"This will especially create problems for low-lying developing countries, which don't have the capacity to build sea-defences," says Viner.

Meehl's team also examined what would happen if greenhouse gases continued to be released at low, moderate or high levels. In the worst-case scenario, temperatures rose by 3.5 degrees Celsius by 2100 and sea levels by 30 centimetres.

Read more about climate change in SciDev.Net's climate change dossier.

Link to full paper by Meehl and colleagues in Science

Link to full paper by Wigley in Science

Science 307, 1769 (2005)
Science 307, 1766 (2005)