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global warming in 21st century inevitable Global warming and rise in sea levels would have continued even if all greenhouse gases had been stabilised by 2000, according to a new US study.

This new research quantifies the relative rates of sea level rise and global temperature increase that already began in the 20th century, Xinhua said.

Even if no more greenhouse gases were added to the atmosphere, globally averaged surface air temperatures would rise about a half degree Celsius and global sea levels would rise another 11 cm from thermal expansion alone by 2100, according to the researchers.

The research was conducted by a team of climate modellers at the National Centre for Atmospheric Research (NCAR) here and is published in this week's issue of the journal Science.

The study is the first to quantify future committed climate change using "coupled" global three-dimensional climate models. Coupled models link major components of the earth's climate in ways that allow them to interact with each other.

The researchers ran the same scenario a number of times and averaged the results to create ensemble simulations from each of two global climate models. Then they compared the results from each model.

They also compared possible climate scenarios in the two models during the 21st century in which greenhouse gases continue to build in the atmosphere at low, moderate, or high rates.

The worst-case scenario projects an average temperature rise of 3.5 degree Celsius and sea level rise from thermal expansion of 30 cm by 2100.

"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," said lead author Gerald Meehl.

"Even if we stabilise greenhouse gas concentrations, the climate will continue to warm, and there will be proportionately even more sea level rise. The longer we wait, the more climate change we are committed to in the future."

The half-degree temperature rise is similar to that observed at the end of the 20th century, but the projected sea level rise is more than twice the five cm rise that occurred during the latter half of 20th century.

These numbers do not take into account fresh water from melting ice sheets and glaciers, which could at least double the sea level rise caused by thermal expansion alone.

Though temperature rise showed signs of levelling off 100 years after stabilisation in the study, ocean waters continued to warm and expand, causing global sea level to rise unabated.

"With the ongoing increase in concentrations of GHGs (greenhouse gases), every day we commit to more climate change in the future. When and how we stabilise concentrations will dictate, on the time scale of a century or so, how much more warming we will experience. But we are already committed to ongoing large sea level rise, even if concentrations of GHGs could be stabilised," the researchers concluded in the paper.

The inevitability of the climate changes described in the study is the result of thermal inertia, mainly from the oceans, and the long lifetime of carbon dioxide and other greenhouse gases in the atmosphere. Thermal inertia refers to the process by which water heats and cools more slowly than air because it is denser than air.