The last time the Earth grew warmer than it is today, melting ice caused the oceans to rise very suddenly, a new study of fossil corals has found. The evidence backs up predictions that such dramatic sea level changes could soon happen again.
The rapid sea-level change in the study took place about 125,000 years ago at the end of the last ice age, said Paul Blanchon, one of the researchers who conducted the study published in Thursday's issue of Nature.
"Sea level must have [risen] at least two metres in 50 years," Blanchon estimated.
"We know there was less ice" at that time, compared with today, Blanchon told CBCNews.ca, "and we know sea level was higher. And we know that's what's going to happen in the future [if greenhouse gas emissions continue to grow]."
A 2007 report by the Intergovernmental Panel on Climate Change predicted a sea-level rise of 18 to 59 centimetres by the end of the century. In March, climate change scientists at a meeting in Copenhagen said new data suggests that the sea level could even rise more than a metre and is unlikely to rise less than 50 centimetres.
Blanchon said his study shows that kind of sea level rise has happened before in a very short period of time. He suggested such a jump in water levels that would affect the homes, drinking water and lives of people living along shorelines to an extent he describes as "catastrophic."
Blanchon, a geoscientist at the National Autonomous University of Mexico in Cancun, and three researchers at the Leibniz Institute of Marine Sciences in Kiel, Germany, came up with their estimates of sea level rise in the past by looking at a fossilized coral reef in the northeast Yucatan peninsula of Mexico.
Corals grow in layers at a rate of about one metre every 300 years. The highest part of the reef is called the crest, which is home to specialized reef-crest corals, and its position is an indicator of sea level, as corals cannot grow above the surface of the water.
The fossilized reef that the researchers examined included two separate reef crests — one higher and one lower — that were physically connected at their base, indicating that they grew from the same coral bed. Chemical dating measurements confirmed they grew around 125,000 years ago and suggested that the lower reef crest was older and the upper reef crest was younger, but were not precise enough to confirm their exact ages.
Rising seas made sediment swirl
By examining the growth of the two crests, the researchers determined that the corals grew toward the shore over time. At one point, the lower coral crest died suddenly and there was evidence that it was killed by stirred-up sediment. A rise in sea levels likely allowed the waves to crash over and past the reef, stirring up the sediment in the lagoon behind it, Blanchon suggested.
After the lower reef crest died, the upper coral crest began growing at a height of two to three metres above the previous sea level.
By looking at a common band of coral that stretched below both crests — using knowledge of coral growth rates, noting the change in coral types through the layers of the reef, and estimating the sea level from the height of the reef-crest corals — the researchers were able to estimate that a two-to-three-metre sea level change occurred in just 50 years.
Blanchon said researchers had been unable to find similar evidence of sudden sea level changes before because few fossil corals reefs have been found in areas stable enough that researchers can clearly read off a wide enough relevant period of their history. However, he is hopeful that similar fossil reefs can be found in other stable areas such as Western Australia and will show similar evidence.
"If this turns out to be the case, I think that we should be closely monitoring the events on the Antarctic and Greenland ice sheets, as well as doing all we can to reduce our greenhouse gas pollution, and fast," he said in a statement, "before the marine portions of these ice sheets start an irreversible slide into the ocean."