The last time carbon dioxide levels in the atmosphere were this high was around 3 million years ago - and the sea was 22 metres higher, the study suggests.
Even if we manage to limit global warming to 2°C, as the Intergovernmental Panel on Climate Change recommends, future generations could face sea levels 12 to 22 metres higher than present, according to new research.
The research was published today in the journal Geology, by Professor Ken Miller of Rutgers University (New Jersey) and an international team including New Zealander Professor Tim Naish from Victoria University of Wellington.
The researchers studied sediment cores in Virginia in the United States, Enewetak Atoll in the Pacific and the Whanganui region of New Zealand.
They investigated the late Pliocene epoch — 2.7 million to 3.2 million years ago — which is the last time the carbon dioxide level in the atmosphere was at its current level, and atmospheric temperatures were two degrees higher than they are now.
"We know that global sea levels at this time were higher than present, but estimates have varied from five to over 40 metres higher," says Professor Naish.
He says the team analysed the position of the sea level 3 million years ago and concluded that it was extremely likely — with 95 percent confidence — that sea level peaked 10 to 30 metres above present, with a best estimate of 22 metres.
"Whanganui holds one of the world’s best geological archives of global sea-level during the warm climate of the Pliocene and is a key data set in this new study," says Professor Naish, who has been conducting research there for the last 20 years.
Professor Naish also led an international team to Antarctica as part of the ANDRILL Project to drill beneath the floor of the Ross Sea in 2006 and discovered that the Antarctic ice sheets retreated significantly during the Pliocene epoch.
"What we’re seeing is that the evidence of Antarctic ice sheet collapse is consistent with evidence for sea-level rise in this new study," says Professor Naish.
Professor Ken Miller, who led the study, says that sea-level rise would take time.
"You don’t need to sell your beach real estate yet, because melting of these large ice sheets will take from centuries to a few thousand years," he says.
"The current trajectory for the 21st century global rise of sea level is 2 to 3 feet (0.8 to 1 metre) due to warming of the oceans, partial melting of mountain glaciers, and partial melting of Greenland and Antarctica."
Still, says Professor Naish, the study calls into question the sensitivity of the earth’s large ice sheets to temperature change and shows that the natural state of the earth under carbon dioxide already attained in the atmosphere is one with sea levels around 20 metres above present.
"If the present levels of carbon dioxide in the atmosphere are not abated, and humans were to disappear from the planet and return in 2,000 years, they would find a world where the oceans have risen 20 metres," says Professor Naish.
Editor's Note: Original news release can be found here.