Melting Ice, Rising Seas
Visual: Aerial views of fragmenting ice sheets floating on the ocean.
Speaker: Jonathan Overpeck, Institute for the Study of Planet Earth, University of Arizona
On Earth, we have a lot of water locked up as ice. Particularly at the poles, you get really big piles of ice, and we call those ice sheets. These ice sheets are great indicators of what's going on with our climate. So, there's a lot of investigation: are they melting, are they not melting?
Visual: Jonathan Overpeck in office
Because if they do melt, they raise sea level, and that affects everybody in big ways.
Title: Melting Ice: Rising Seas
Visual: An aircraft propeller as seen through a plane’s window. An ice sheet seen through a plane’s cockpit window.
Speaker: Aircraft crewman
That’s the edge of the glacier, right here. Co, go straight over the glacier right here.
Visual: The 109th Airlift Wing flight to Greenland. An airman sits in the aircraft with noise-reducing headphones on. Bob Hawley walks along the edge of a glacier.
Speaker: Bob Hawley, Department of Earth and Space Sciences, University of Washington
We're at the Russell Glacier, which is a small outlet glacier on the edge of the Greenland Ice Sheet.
Visual: The edge of the Russell Glacier.
A glacier is ice in motion. It’s flowing under its own weight.
Visual: Bob Hawley at the edge of the Russell Glacier
If you were to stand here with a camera, day after day, what you would see is the ice marching towards us and falling off, and falling off.
Visual: The edge of a glacier by the ocean. Large chunks tumble into the sea.
When we say the Greenland Ice Sheet is melting, the way that happens is the outlet glaciers speed up.
Visual: A large portion of the glacier crashes into the sea. Smaller chunks of ice float in the ocean.
And when the outlet glaciers speed up, more ice breaks off into the fjords.
Visual: An aerial view of the Greenland Ice Sheet
It’s not as if the entire surface of the Greenland Ice Sheet is melting. The actual melting of the ice, most of that takes place at the coast.
Visual: Large chunks of ice floating in the ocean.
Speaker: Jonathan Overpeck
Yeah, for Greenland, what we're seeing is quite a bit of change and I think a lot of the change is surprising a great number of scientists.
Visual: Map of Earth, with glaciers locations highlighted.
You know, all glaciers around the world are melting, but they don’t hold all that much water.
Visual: A close-up of Greenland, with its borders shrinking, representing glacial melt.
Greenland has about 7 meters of sea level equivalence. So, if that ice sheet melts, all of it, we'll get 7 meters of sea level rise. That's over 20 feet.
Visual: A close-up of Antarctica, with the borders of the West Antarctic Ice Sheet shrinking.
If the West Antarctic Ice Sheet—the one down in the Southern Hemisphere that we're worried about—melts, that could give us 5 meters of sea level.
Visual: Jonathan Overpeck in office.
So, up to about 40 feet of sea level could result if we melt both of these ice sheets.
Visual: Aerial view of a town on the Greenland coast. Daniel Muhs walks in a quarry, cleared among lush plant life.
It's pretty important to look back at the last time sea level was higher than present day, because you want to figure out, could we in the future go across that threshold?
Speaker: Daniel Muhs, Research Geologist, U.S. Geological Survey
Visual: Daniel Muhs stands by a rock wall in the quarry.
We’re standing here in one of my favorite places in the world, which is Windley Key Quarry, Florida, in the Florida Keys. And if we had been here 125,000 years or so ago, we’d be underwater by several meters.
Visual: Daniel Muhs gestures toward the quarry walls. A close-up of the structures in the walls.
What you see here in the quarry walls on either side of us here is a rock called the Key Largo formation.
Visual: The coral-like structures in the rock face.
It’s a limestone, and about 30 percent of it is composed of corals that were once part of a living coral reef that formed about 125,000 years or so ago during a period we call the last interglacial.
Visual: Daniel Muhs points out brain coral in the rock.
This is a remarkable example of a brain coral in its original growth position. Diploria, or brain corals, are one the most common constituents in the modern reef off Florida today, and here in the Key Largo limestone.
Visual: Daniel Muhs continues to point out features in the rock.
The top of the coral head is here, and you can trace this along the sides, down here to the base of the coral. The top of the quarry here, and very close to the top of this brain coral, are about 5 meters above sea level. And brain corals need to grow in water at least 3 meters deep or so. So that means that sea level has to have been probably anywhere from 6 to 8 meters higher than present during the last interglacial period.
Visual: Close-ups of various coral structures in the rock.
One of the most powerful things about the geologic record of sea level change is it does answer the question, is such a thing possible?
Visual: The rock wall of the quarry.
We know it’s possible because it’s happened in the past.
Visual: Waves crash on a rocky shore. A sailboat drifts on a body of water in front of a wooded mountain.
Speaker: Jonathan Overpeck
Visual: Buildings at the shore of a waterfront.
Well, it’s not hard to take what we know about the height of coastlines, to get maps showing, as sea level rises, which areas will get flooded, where.
Visual: Jonathan Overpeck at computer
Here’s a visualization showing the Earth and the areas in red that would be submerged with 6 meters of sea level rise.
Visual: Map images of England and Florida. Shaded red areas indicate varying levels of sea level rise. Map images of New York City and India with shaded red areas indicating sea level rise.
So 6 meters of sea level doesn’t sound like very much, but it’ll affect a very large part of the world’s population and landmass.
Visual: A satellite image of Florida
When we zoom in to finer special scales, it allows us to put a more human face on the sea level rise that could occur in the future.
Visual: A close-up satellite image of a beachfront community in Florida. Individual buildings are visible.
We can actually see individual communities that will be submerged by sea level rise.
Visual: A zoomed out map of Florida, the Caribbean, the United States, then Earth, with red areas indicating sea level rise.
Science is never certain. What science is all about is trying to figure out what’s probable, what’s not probable, and to be really open with what the uncertainties are.
Visual: A lake
In this case, we know the sea level was higher in the past.
Visual: Daniel Muhs walks in the quarry.
And we know that was because the Earth was warmer, particularly in the Northern Hemisphere over the Greenland Ice Sheet.
Visual: Daniel Muhs examines a rock wall in the quarry closely with a lens.
The big question is, how much warmer was it?
Visual: Jonathan Overpeck in office.
And both our climate modeling and our paleoclimatic reconstructions suggest that somewhere between three and five degrees Celsius is the temperature by which a lot of Greenland will be melting fast.
Visual: A glacier in Greenland
But to put a little perspective, three to five degrees, how long will it take us to get there?
Visual: A river of meltwater by a glacier’s edge. A traffic jam in the United States. Smokestacks pump out smoke.
Just in the last 50 years, we've warmed up the Arctic by over two degrees.
Visual: Ice chunks float in the ocean.
So, we're more than halfway to three to five degrees.
Visual: An industrial tower against the sky. Electrical towers tangle a grassy hill.
And sometime later in this century we'll cross that mark.
Visual: A crowded city sidewalk. The 109th Airlift Wing exits its transport plane, landed on the ice.
In our work we don’t know for sure where the threshold is beyond which the ice melting is inevitable.
Visual: A glacier face bordered by meltwater.
But we know it’s coming.
Visual: A person and dog walk along a shoreline. A man with waders fishes among crashing waves in the ocean. A traffic jam. A mother and child on the beach.
And if we don’t do something about greenhouse gas emissions soon, we’re going to cross that threshold, and future generations are going to
have to deal with that big sea level rise.
The rising temperatures of global climate change are melting the world’s ice. Most notable are the shrinking ice sheets of Greenland and west Antarctica, which have shown dramatic loss in recent years. Travel to the glaciers of Greenland and to fossilized coral reefs of the Florida Keys, where earth scientists are studying geologic records of past warming to predict future ice loss and associated sea level rise.
Classroom discussion activity for use with the video.
Read this related article.
The Fate of a Frozen Land
Bob Hawley faces the steep terminus of the Russell glacier, which is exiting the southwestern edge of the Greenland Ice Sheet. It's not big, as glaciers go, so it's not the most dramatic yardstick of what's occurring in Greenland right now. But the face of this small, slow-flowing river of ice is changing, visibly, constantly. "This is the sort of place you don't want to stand very close to," says Hawley, a glaciologist with the University of Washington and the University of Cambridge's Scott Polar Research Institute. "A piece might fall off."
As the glacier emerges from the constraints of the continental ice sheet, it is free to disintegrate. In the warm month of August, chunks of glacier fall, or calve, from Russell once or twice an hour. They collapse like mini-avalanches and splash into the ice-strewn water that flows readily from the glacier's base. These chunks will join the ocean, adding to its volume.
Greenland and the western and eastern sides of Antarctica are Earth's three ice sheets, where most of our world's freshwater resides. Both Greenland and western Antarctica are rapidly losing volume. In the last 50 years, humans have warmed the Arctic by over 2 degrees Celsius. This is hastening a number of melt mechanisms that affect the world's frozen water, including the speed at which Greenland's glaciers flow and crumble. A sobering example is the flow rate at the massive Jakobshaven glacier, which increased 95 percent from 1996 to 2005.
Were the Greenland Ice Sheet to disappear completely, global sea level would rise by about 7 meters (23 feet). "This doesn't seem like much," says Hawley, "until you look at places like the Mississippi Delta, New Orleans, the Netherlands, or the Philippines." Such a sea-level rise would drive out the great many people who live in these low-lying coastal areas. Could Greenland really unfreeze? And what might happen if it does? Observations of current conditions, paleoclimatic research, and computer models are providing some answers.
More and More Melt
Greenland's climate is particularly sensitive to global warming. As temperatures increase and the surface of snow and ice gives way to land and ocean, Greenland's reflectivity, or albedo, decreases. Lower albedo means more sunlight is absorbed rather than reflected back into space. This melts the remaining ice even faster, decreasing the elevation of the ice sheet. Because lower elevations are warmer than higher elevations, this thinning exposes the ice sheet's surface to even higher temperatures.
Most computer models that gauge Greenland's melt (and make predictions of when all its ice could disappear) account for albedo and ice-sheet thinning. But they don't account for the recent rapid ice loss from glacial calving, despite the fact that glaciers are moving faster than ever before. As ice sheets melt, the water seeps through cracks and crevasses to the bedrock, then eventually flows to the ocean. This meltwater percolating downward might be contributing to faster glaciers. "It acts as a lubricant to their flow," says Hawley.
The new projection of the 2007 Intergovernmental Panel on Climate Change (IPCC) is one of those that fails to account for increased glacial flow. The IPCC predicts that the Greenland Ice Sheet could disappear in "millennia" given the current rate of warming. But to Greenland-focused climatologists such as the University of Arizona's Jonathan Overpeck, the outlook is much bleaker. "There are a couple of answers to the question of what might happen in the future," he says. "One is the conservative answer--that we'll continue to get sea-level-rise acceleration, primarily due to expansion of the ocean as it continues to warm, and sometime after the turn of the century, we'll start to get a larger and larger contribution from the big ice sheets. Our work indicates that the future change could be a little more quick and large-scale. And that's because we actually looked at the last time this may have happened."
The last time sea level was higher than the present day--by up to 6 meters (20 feet)was 115,000 to 129,000 years ago. This period is called the Last Interglacial, and the legacy of its sea levels is visible in the geological record. One of the best places to see it is an abandoned limestone quarry in southern Florida's Windley Key State Park. Much of the limestone here was formed from a coral reef community that lived during the Last Interglacial. The corals' calcium carbonate structures are now fossilized as rock, the traces of boulder-star and brain corals frozen in time in their original growth positions.
Boulder-star and brain coral are still among the most common reef species in the Caribbean and western Atlantic. Scientists observe that brain corals grow only in water about two or three meters deeper than they are high. The top of the quarry – where the coral heads stop – is about 5 meters high. Therefore, sea level at Windley Key was perhaps 6 to 8 meters higher than its present level some 130,000 years ago.
Models for the Future
The Windley Key corals show a sea-level rise of about 7 meters in the Last Interglacial. That's how much oceans would rise if the Greenland Ice Sheet melted completely. Coincidence? Not according to Overpeck. His research and that of a number of his colleagues indicate that Greenland was a major contributor to high sea levels at that time, and that the Antarctic may have also been an important contributor.
Higher concentrations of carbon dioxide in the atmosphere are causing much of Earth's current global warming. Yet during the Last Interglacial, carbon dioxide levels in the atmosphere were not elevated much above pre-industrial levels. Instead, the Earth's northern hemisphere tilted more toward the Sun in summer than it does now. Although some of the Western Antarctic Ice Sheet melted, most of the warming was over the Northern Hemisphere. So more – in fact much more – of Greenland's ice sheet melted. To find out how much this tilt drove up temperatures over the Arctic, Overpeck and his colleagues ran a state-of-the-art computer climate system model from the National Center for Atmospheric Research. "We found that you don't need a lot of warming to get large-scale Greenland and Antarctic melting," says Overpeck. The Last Interglacial, it seems, was only 3 to 5 degrees Celsius above present-day temperatures in the Arctic, and much less on a global scale.
"With our current 2 degrees of Arctic warming, we're already about halfway to 5 degrees Celsius," says Overpeck. He predicts that sometime later in this century – not in millennia – we'll cross the threshold beyond which the bulk of the Greenland Ice Sheet could melt. Although climate scientists understand that such predictions are extremely uncertain, a 7-meter rise in sea level would change the face of our planet, flooding vast stretches of what are now shallow coastal areas. The entire southern third of Florida including Windley Key's ancient coral reefswould again disappear into the blue. Just a 1-meter rise could put whole nations with unique cultures, such as Tuvalu and the Maldives, under water.
"The good news is that humans have a lot of control in how much more warming there will be," says Overpeck hopefully. "Our whole purpose of digging into what warmth is necessary to melt ice sheets is to give policy makers and citizens the chance to make a choice."