Q+A: How great is the threat from melting ice sheets?

(Reuters) - The U.N. Climate Panel says seas could rise by 18-59 cms (7-24 inches) by 2100, without taking account the possible acceleration of a melt of ice sheets in Antarctica or Greenland.

Even a small thaw of Antarctica and Greenland would affect sea levels since together they lock up enough ice to raise sea levels by about 65 meters (215 feet) if they all melted.

Following are responses to questions from Reuters by a leading glaciologist as part of an ad-hoc global series of top climate change scientists, policy makers and academics.

Ian Allison is leader of the Australian Antarctic Division's Ice, Ocean, Atmosphere and Climate program and a researcher within the Antarctic Climate and Ecosystems Cooperative Research Center.

He has been involved in Antarctic science for over 40 years.

HOW GREAT IS THE THREAT FROM ICE SHEETS MELTING?

"I think it is now unequivocal that warming of the world is occurring and I think the last IPCC (Intergovernmental Panel on Climate Change) conclusively showed that a major cause of warming is greenhouse gas emissions from mankind.

We now know that the ice sheets are contributing to sea level rise and for the Arctic, at least, this is because the warming of this region is much greater than in other places on Earth.

We also know that glaciers in mountain areas are undergoing a very rapid retreat and they're a major contributor of sea level rise, too.

WHICH IS OF MORE CONCERN? GREENLAND OR WEST ANTARCTICA?

"Greenland is of more concern because of the warming of the Arctic. Greenland is at lower latitude than much of Antarctica and we've seen the direct effect of the melting.

We still don't understand many things about the dynamic response of the ice sheets but we do see direct melt exceeding snowfall in Greenland.

This might not mean a runaway effect but it does mean Greenland is contributing to sea level rise and will continue to add to sea levels at the present temperatures for many hundreds of years."

EXPLAIN THE THREAT FROM WEST ANTARCTICA

"Ice shelves and floating ice tongues can buttress the flow of grounded ice from the interior of the ice sheets. We've seen examples in both Greenland and Antarctica of floating ice disappearing, and the ice that sits on the land then flowing more quickly into the ocean.

"In addition, the West Antarctic may be inherently unstable. The West Antarctic forms what is called the marine ice shelf. The ice is resting on bedrock but that bedrock is below sea level. It's like if you load too many ice cubes in your gin and tonic, the bottom one touches the bottom of the glass even though it's well below the water level.

Where the bedrock under a marine ice sheet slopes down toward the interior, such as under parts of West Antarctica, the ice sheet may be unstable. If it thins, it will start to float at the edges, becoming an ice shelf.

For a bedrock that slopes backwards and becomes deeper further in, continued retreat of the grounded ice sheet may proceed very rapidly. A small retreat could in theory destabilize the entire West Antarctica ice sheet, leading to rapid disintegration."

WHAT ARE THE MAIN GAPS IN OUR KNOWLEDGE?

"There two areas. One, we need to improve our mathematical models of ice streams, ice sheets and ice shelves to be able to better project future changes. We also need more detailed measurements of how deep the bedrock is under the ice sheets to use in the models.

The other major gap in our understanding is what is happening at the bed of the ice sheets; how they react with liquid water at the base, what role water may have in sliding processes and the role of gravels and slurry at the base.

We now know there is a lot of liquid water under the ice sheets. But we don't really know how changes in this may affect the ice flow. Knowing what's under the ice sheets we really need to measure that with radar systems."

WHAT ARE YOUR MAIN MESSAGES TO POLICY MAKERS?

"The main thing is monitoring what's actually happening with sea level rise and the ice sheets. We've now got tools that can do that, we can improve those and make sure they keep going, particularly satellite-based systems.

We need better predictive tools to know just what is likely in the next 100 years. I don't think we should be rushing into building up coastal defenses until we know what we could be defending against. So our biggest requirement is to be able to refine our projections for what may happen in the future."

(Editing by David Fox)