Greenland ice melting faster than expected

When scientists make a prediction they usually make several: A conservative estimate and a seemingly wilder number which is frequently more accurate. The conservative estimate is really more the press and the naysayers, and the more other number is either scary or delightfully good news, depending on the situation.

So when a group of scientists comes forward and says that Greenland's ice sheet is melting faster than expected, you have to ask... were they comparing it to the conservative estimate or the supposedly-more-accurate one? As you will see below scientists don't agree all the time.

A group of NASA and university scientists are warning the steady loss of the Greenland ice sheet could raise sea levels three times higher than estimated. In a report in the journal Nature Geoscience, the study challenges current predictions about the rate at which the massive ice sheet is predicted to melt over the next century as greenhouse gases rise and temperatures warm.

The report's authors say the loss of the ice mass could raise global sea levels by up to five millimetres a year – almost three times the current estimates set by an international authority on the issue. (Basically its one group of scientists saying the other group was using a really conservative estimate.)

"We're showing that the geologic record shows that in the past, ice sheets have melted much faster than we're predicting at the end of this century," Anders Carlson, a geologist at the University of Wisconsin-Madison.

The team of researchers, including scientists from NASA and the University of British Columbia, used geologic data to study the Laurentide ice sheet, the last massive ice dome to cover much of the northern hemisphere.

Faron Anslow, a glaciologist at UBC in Vancouver, said they studied marine and terrestrial records to determine how fast the Laurentide sheet melted and if it might predict the fate of the Greenland sheet.

The team discovered that the ancient ice cap, which spanned 1.7 million square kilometres, went through two periods of rapid melting. The first occurred about 9,000 years ago and again about 7,600 years ago, when there was increased solar radiation.

"The ice sheet was existing in a pretty warm climate and what we show is that that sunlight was enough to melt the ice sheet away very rapidly," he said.

They also found that the melt led to a speedy rise in sea levels of almost two centimetres a year.

Anslow said the temperatures at the time of the Laurentide melt are similar to what's expected for Greenland by the end of this century, suggesting it could undergo an equally rapid melt.

The Laurentide sheet, which was almost twice the size of its Greenland cousin, was at its largest about 22,000 years ago when it began its slow decline due to warming temperatures.

It virtually disappeared about 6,000 years ago.

Carlson says that if the Greenland sheet completely disappeared, it would raise sea levels by seven metres, adding that even the slightest increases could threaten hundreds of millions of people in coastal communities.

"The word 'glacial' used to imply that something was very slow," co-author Allegra LeGrande of the New York-based NASA Goddard Institute for Space Studies wrote in the report. "This new evidence ... indicates that 'glacial' is anything but slow.

"This finding shows the potential for ice to disappear quickly, given the right push."

The Intergovernmental Panel on Climate Change predicts a maximum sea level rise over the next 100 years of up to 10 centimetres, based mainly on the expansion of the oceans through warming.

Anders said it doesn't take into account contributions from ice sheet melt.

"They now predict a half-metre sea level rise with most of that coming from the expansion of the ocean due to warming and very little of that coming from ice sheet melting," he said.

"Ice sheet melting could be a much bigger component, so these values should be seen as low estimates."

Anders said science hasn't been able to get an accurate picture of how fast ice sheets melt as a result of climate change until now. The scientific team used sophisticated computer modelling and terrestrial records to track the sheet's disappearance, linking it in time to warming temperatures.

In 2006 a huge iceshelf snapped in northern Canada, surprising scientists at the sheer speed it disappeared. In 2007 the Greenland ice sheet retreated by a record amount and that record will likely be broken again in the future.