Without further emission cuts, global temperatures could reach 5˚C above pre-industrial levels by 2100 with devastating effects on oceans and species

A global warming increase of less than 2˚C could be enough to raise sea levels by six metres, research has shown.

Melting polar ice sheets have lifted ocean surfaces by this amount numerous times in the past three million years, scientists said.

And on each occasion temperatures had risen by only 1˚C to 2˚C – within the end-of-century target agreed by world leaders, which will require significant reductions in greenhouse gas emissions.

Without further emission cuts, global temperatures could reach 4˚C or even 5˚C above pre-industrial levels by 2100.

Sea levels six metres, or even three metres, higher than they are today could spell catastrophe for hundreds of millions of people living in highly-populated coastal regions of the world.

Peter Clark, one of the study authors from Oregon State University (OSU) in the US, said: “The ominous aspect to this is that carbon dioxide (CO2) levels are continuing to rise, so we are entering uncharted territory.

“What is not as certain is the timeframe, which is less well constrained.

“We could be talking many centuries to a few millennia to see the full impact of melting ice sheets.”

Study showed that changes in the earth’s climate and sea level were closely linked

The scientists pointed out that much of the US state of Florida was 15m or less above sea level and the city of Miami had an average elevation of just 1.8m. When Hurricane Katrina hit the US in 2005, parts of New Orleans and other areas of Louisiana were swamped by a surging Gulf of Mexico that could be six metres higher in the future.

Dhaka in Bangladesh, with a population of 14.4 million all living in low-lying areas, Tokyo and Singapore are also said to be extremely vulnerable to sea level rises.

Anders Carlson, another member of the OSU team whose findings are published in the journal Science, said: “The influence of rising oceans is even greater than the overall amount of sea level rise because of storm surge, erosion and inundation.

“The impact could be enormous.”

He said atmospheric carbon dioxide levels today were equivalent to what they were three million years ago when sea levels were at least six metres higher because the ice sheets were greatly reduced.

It takes time for the warming to whittle down the ice sheets, but it doesn’t take forever, Carlson said. There is evidence that we are likely seeing that transformation begin to take place now. The scientists analysed more than 30 years of research on past changes in ice sheet cover and sea level.

The study showed that changes in the earth’s climate and sea level were closely linked, with only small amounts of warming having a significant impact.

Ocean sediment records and ice sheet computer simulations were used to estimate Greenland and Antarctic ice sheet volumes.

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