If current temperature trends continue to the end of the century, Earth's climate will be warm enough to cause a massive melting of Greenland's ice sheet and a partial collapse of the Antarctic ice sheet, resulting in a global sea level rise of six metres from the torrent of melt water, according to two new research papers.
The jump in ocean levels would be enough to inundate many low-lying coastal areas around the world, place dozens of major cities under water and become irreversible at some point later this century ”unless something is done to dramatically reduce human emissions of greenhouse-gas pollution,” warns Jonathan Overpeck, a professor at the University of Arizona and one of the authors of the studies.
He said that if serious efforts to limit global warming aren't taken soon, ”we're committed to four to six metres of sea level rise in the future.”
The research is some of the most alarming to date on the possible impact of global warming.
It was based on computer models that recreated the climate during the last really warm period in Earth's history before the present era. An interglacial heat wave that began about 130,000 years ago caused northern regions of the world to be bathed in an warm spell that endured nearly 12,000 years.
At that time, temperatures in Arctic regions were about three to five degrees warmer than they are now – the same levels they are projected to attain later this century because of emissions of carbon dioxide and other greenhouse gases from human activity.
If what happened in the past is any kind of prologue to what might happen in the future, scientists believe there will be serious consequences.
”Although the focus of our work is polar, the implications are global,” said Bette Otto-Bliesner of the National Center for Atmospheric Research in Boulder, Colo., and another one of the authors. ”These ice sheets melted before, and sea levels rose. The warmth needed isn't that much above present conditions.”
The new research could have major significance in the debate about the effects of global warming, because it suggests that current projections on the rise in sea levels because of the increase in Earth's surface temperatures may be far too conservative.
The UN's Intergovernmental Panel on Climate Change, the main body of scientists assessing the threat from global warming, has been estimating that sea levels will rise anywhere from only about 10 centimetres to just under a metre by the year 2100. The new research concluded ”that the melting could be faster and hence more challenging for society.”
That earlier warming spell caused about half of Greenland's ice cap to melt, the boreal forest to march northward to the Arctic Ocean in most areas of the north, and the disappearance of ice sheets from almost all the areas of the Canadian high Arctic islands, according to the two papers, which are being published simultaneously in the current issue of the journal Science.
Greenland even started to live up to the literal meaning of its name, at least in southern part of the island, which at the time was covered in tundra similar to what is now found in much of Northern Canada.
The melting in the Arctic caused only about half of the rise in sea levels calculated to have occurred at the time, and the researchers concluded that some unknown process linked to this warming in the Northern Hemisphere then prompted a massive chuck of Antarctic's ice sheet to float into the ocean, accounting for the rest of the sea level change.
It is the first time studies have linked melting in the Arctic and Antarctic during the last interglacial period, and it presents a worrisome possible new feedback loop if the world's climate changes because of global warming, as many scientists project. If melting of ice in the Arctic is able to trigger the disintegration of Antarctic ice, even more sea level rise than is now expected could occur.
Investigations of the last interglacial period are a fascinating research topic for climate scientists trying to figure out what might happen as global warming takes place.
That last warm spell was unlike the current one, which is attributable to human causes, such as burning fossil fuels and clearing forests.
About 130,000 years ago, there as a slight change in Earth's orbit and tilt, causing more intense sunlight to fall on Arctic regions. This sunlight caused temperatures to remain above freezing for longer each year, setting in motion the vast melting that raised ocean levels for about 12,000 years.
