Accelerated rise in sea levels blamed on global warming

Sea levels are rising twice as fast as they were 150 years ago and man-made greenhouse emissions are the prime cause, a study by scientists in America has found.

Tide lines worldwide are rising by about 2 millimetres a year, compared to 1 millimetre a year in 1850, said Kenneth Miller, professor of geology at Rutgers University in New Jersey.

The rate at which sea levels are rising is probably greater than at any time for thousands of years, suggesting that greenhouse emissions are accelerating 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."

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 fossils, variations in radioactive isotopes and other chemical elements allowed 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, in the journal Science, showed there was a steady rise of about one millimetre a year in sea levels from 5,000 years ago until about 200 years ago. Recent measurements from tidal gauges and satellites show that the rate of increase in sea levels has doubled since 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," the professor said. "Our record therefore provides a new and reliable baseline to use in addressing global warming."

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. Melt water from mountain glaciers and the 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 worlds' coastal cities, including those on tidal rivers such as London.

A separate study published in Science has found further evidence to show that greenhouse gasses are at their highest levels in the atmosphere for thousands of years. Scientists analysed tiny air bubbles trapped in ice cores drilled into the Antarctic ice sheet, which can reveal the composition of the atmosphere going back more than 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," he said.

Ice cores drilled from the ice sheets of Greenland and Antarctica are providing a valuable source of information on the rate of climate change. "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 in the distant past," Professor Brook said.