Sea levels rising twice as fast

Much of the rise in sea levels over the past century has been due to the thermal expansion of the oceans caused by rising temperatures, writes Steve Connor

SEA levels are rising twice as fast as they were 150 years ago and man-made greenhouse emissions are the prime cause, according to a US study. Tide lines around the world are climbing by about two millimetres a year on average, compared to one millimetre a year in 1850, said Kenneth Miller, professor of geology at Rutgers University in New Jersey.

The rate at which sea levels are increasing is probably greater than at any time for several thousand years, suggesting that greenhouse emissions are accelerating the pace of climate change, he said.

"Without reliable information on how sea levels had changed before we had our new measures, we couldn’t be sure the current rate wasn’t happening all along," said Professor Miller.

"Now with solid historical data, we know it is definitely a recent phenomenon," he said.

The study was based on analysing the sediment of five core samples drilled to a depth of 500 metres off the New Jersey coast between Cape May and Sandy Hook.

Analysing various fossils, variations in radioactive isotopes and other chemical elements allowed the scientists to make accurate estimates of sea levels at different times over the past 100 million years.

This is the most reliable and comprehensive record of sea levels for this period of time, and is better than previous core samples drilled for commercial purposes, Professor Miller said.

The analysis, published in the journal Science, established that there was a steady millimetre-per-year rise in sea levels from about 5,000 years ago until about 200 years ago.

More recent measurements from tidal gauges and satellites show that the rate of increase in sea levels has doubled since about 1850, he said.

"The main thing that’s changed since the 19th century and the beginning of the modern observation has been the widespread increase in fossil fuel use and more greenhouse gases," Professor Miller said. "Our record therefore provides a new and reliable baseline to use in addressing global warming," he said.

Much of the rise in sea levels over the past century has been due to the thermal expansion of the oceans caused by rising sea temperatures because water increases slightly in volume when warmed.

However, melted water from mountain glaciers and the massive ice sheets of Antarctica and Greenland could cause a further dramatic increase in sea levels that would be big enough to inundate most of the world’s coastal cities, including those on tidal rivers, such as London.

A separate study published in Science has found further evidence to show that greenhouse gases such as carbon dioxide and methane are at their highest levels in the atmosphere for many thousands of years.

The scientists analysed tiny air bubbles trapped in ice cores drilled from the Antarctic ice sheet, which can reveal the composition of the atmosphere going back over 650,000 years, said Ed Brook, professor of geosciences at Oregon State University.

"The levels of primary greenhouse gases such as methane, carbon dioxide and nitrous oxide are up dramatically since the Industrial Revolution, at a speed and magnitude that the Earth has not seen in hundreds of thousands of years. There is now no question this is due to human influence," Professor Brook said.

Ice cores drilled from the ice sheets of Greenland and Antarctica are providing a valuable source of information on the rate at which climate has changed in the past and how unusual is the present change in the composition of the atmosphere.

"We predict, for instance, that rising levels of greenhouse gases will warm our climate. There’s evidence that this is happening right now, and it would be interesting to find out if the same thing has happened at times in the distant past," Professor Brook said.

— By arrangement with The Independent

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