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Dr. Jay Zwally is a glaciologist with NASA based at the Goddard Space Flight Center in Greenbelt, Maryland. He studies the growth and shrinkage of the Greenland and Antarctic ice sheets and polar sea ice and the effects of changing climate. Zwally is Project Scientist for NASA's ICESat satellite, which uses lasers to measure changes in the thickness of ice to fractions of an inch. In the region of the Jakobshavn Isbrae glacier in Greenland, he led the discovery of how the ice flow accelerates in summer as the melting has been increasing. 
Dr. Konrad Steffen is Director of the Cooperative Institute for Research in Environmental Studies (CIRES) at the University of Colorado Boulder, where he is also a Professor and Fellow. A climatologist, he has spent an average of seven weeks in the high Arctic every summer for the past three decades. He studies processes related to climate and cryosphere interaction in polar and alpine regions based on in-situ and satellite measurements, and he uses climate system modeling to study their sensitivity. |
From their base camp at Africa's Mt. Kilimanjaro, where they were doing fieldwork, Jay Zwally and Koni Steffen answered selected viewer questions about Greenland's Jakobshavn Isbrae glacier on August 1, 2005. Please note we are no longer accepting questions, but please see our links and books section for additional information on Jakobshavn and other glaciers. Below, read answers to a wide range of questions viewers have e-mailed. Q: How much water does it take to move a glacier? A: Glaciers can actually move without any water underneath, but water between the ice and the ground makes them move even faster. We believe only a few inches of water is needed to make them slide faster across the ground. Q: Is the additional fresh meltwater from the accelerated melting of the Jakobshavn glacier enough to affect the thermohaline circulation in the next 25 to 100 years? Would it be enough to bring about abrupt climate change? Thank you. A: Probably no. But if you include the ice from all the outlet glaciers of Greenland and the water from surface melting, perhaps yes. Ice core records show that abrupt climate changes have occurred, but their causes are not well explained. Currently, the Jakobshavn glacier is the only outlet glacier we have sufficient data and observations on to conclude that the motion has accelerated. More outlet glaciers should be monitored in the future. Q: Is there any assessment as to when places of low sea level like parts of Florida, Holland, and the like will be affected by this great influx of water to the seas? A: As we mentioned above, the effect on coastlines is already serious and will become worse, causing more coastal erosion and flooding. Costs are estimated in the 10s to 100s of billions of dollars. In the reports of the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC), which are updated every five years, the international scientific community estimates the current rate of sea-level rise and attempts to predict the rise for the next 50 to 100 years. They are now working on a new report to be published in about a year. The last report predicted that sea level will rise about 50 cm ± 50 cm over the next century. You see there is a big uncertainty. Sea level has risen about 15 cm over the last century and now is rising at the rate of about 30 cm/100 years (i.e., 3 mm/yr). About 0.7 mm/yr is believed to be coming from mountain glaciers and small ice caps around the world. Maybe only 0.2 mm/yr is presently coming from the ice sheets. Some is coming from thermal expansion of the oceans as their temperature rises, but the water source for the rest of the rise is not known. In the future, the ice sheets will contribute more or less depending on whether increased melting and faster ice flow will be greater than the effect of increased snowfall. Q: Does the Jakobshavn glacier have any special properties that make it different from other glaciers? A: The Jakobshavn glacier flows through a deep channel eroded in the bedrock. The lower part of the glacier has warmer and softer ice that allows the upper part to flow faster. About 8 percent of the freshwater discharge (iceberg calving) for the entire Greenland ice sheet occurs at the margin of the Jakobshavn terminus. But there are a lot of things we do not know, like what will happen when the floating part disappears and the ice flows directly from the grounded part into the ocean. Will it continue to speed up or slow down? Q: Would the melting of glaciers affect the overall salt content of the ocean (I assume over time the sea becomes less salty, especially around the poles)? Wouldn't that tend to mean the polar seas would be more likely to refreeze in winter? A: Yes, but the change is very small. If all the glaciers and all of Antarctic and Greenland ice melted, sea level would rise roughly 100 m. The average depth of the oceans is roughly 2,000 m. So even such a drastic change in ice melting would only cause about 5 percent change in salinity if the ocean water would be well mixed from top to bottom. However, freshwater usually remains in the upper levels of the ocean, and hence the salinity decrease would be larger at the poles and in the upper part of the ocean until the freshwater spread around the world. Q: What can be done to stop glaciers from melting? A: On a small scale, people in Switzerland and Austria are experimenting with covering the lower parts of glaciers in ski areas with plastic to slow the summer melting. But there is no practical way to stop them worldwide. However, the rate of melting can be slowed if the world's nations (including the U.S.) could agree on reducing the emission of greenhouse gases. In the 2001 report of the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC), the consensus of the international scientific community was that humans are causing climate to warm from the worldwide emission of carbon dioxide and other greenhouse gases. The primary source is the burning of coal, oil, and gas. We should increase our energy efficiency, develop alternative energy sources, and reduce our dependency on oil and coal. If we do not develop new energy-efficient technology (e.g., cars, air conditioners, wind and solar power), other countries will, and then the U.S. will buy new technology from them instead of selling it. Q: I am 72 years old and enjoyed your program concerning the Jakobshavn glacier. On the broadcast, it was mentioned that you believe that the cause for the glacier speeding up and melting is humans. I seem to remember from reading some books that the complete history of the Earth is filled with intermittent periods of warming and then freezing to glaciers. If indeed this is true and has been a natural occurring event over millions of years, then why today is the Jakobshavn glacier's rapid melting the result of "humans" rather than some natural occurring event set up in Earth's "clock"? I am not doubting the melting rather just questioning the "cause" and "effect." I do really enjoy the NOVA programs and always watch them in hopes of learning something new and interesting. Thank you.
A: We appreciate your question and are still searching for some of the answers. By the way, I (Jay) at 66.5 years old just came down from the peak of Mt. Kilimanjaro studying the shrinking glaciers there with Koni and Austrian colleagues. In the last report of the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC), the consensus of the international scientific community was that humans are causing climate to warm from the worldwide emission of carbon dioxide and other greenhouse gases. The primary source is the burning of coal, oil, and gas. You are right about "intermittent periods of warming and freezing." The last ice age ended about 8,000 to 15,000 years ago because of gradual periodic changes over tens of thousands of years in the way the Earth orbits the sun (i.e., inclination of the Earth's axis, eccentricity of the orbit, and the time of the year when the Earth is closest to the sun). However, man is now beginning to cause temperature changes as large as going from an ice age to a warm interglacial period. This man-induced change is imposed on top of any natural changes that may be taking place. The best scientific climate models have for the last 15 years predicted that the warming from increased greenhouse gases would be greatest in the Arctic. The shrinkage of the Arctic sea ice, the melting of Alaskan glaciers, and the increased melting on Greenland are just some of the signs of the model predictions being correct. Interestingly, the last interglacial period was actually warmer than this one, because of differences in the Earth's orbit then. There is good evidence suggesting that much of the Greenland ice sheet melted then. If we do not take action to stop or slow the man-induced warming, we believe that over the next century the Greenland ice sheet will be put on an irreversible path of melting as it seems to have done 120,000 years ago. Q: Why do glaciers move? A: Glaciers move because the weight of the ice (from gravity) causes the ice to bend and in many places to also slide over the ground. Ice is soft compared to rock or steel, and it takes less than 15 pounds per square inch to make the upper layers of ice slide over the lower layers in a glacier. Also, where the bottom of the glacier is too warm to freeze to the ground, the glacier slides over the ground and the rock underneath the ice. Water between the ice and ground also makes glaciers move even faster, kind of like slipping on a wet floor. Q: Would the shrinking glaciers put less pressure on the Earth's crust, causing Earth movements? A: Yes. The Earth's crust is still adjusting to removal of ice sheets at the end of the last ice age 8,000 to 15,000 years ago. For example, the center of Canada is rising about 1 cm/yr, and due to bending of the crust Baltimore is sinking a little. Today, the Greenland ice sheet's bottom is at or below sea level. The weight of the ice (1.8 miles thick at its center) has pushed the plate down. Without this ice, the landmasses of Greenland would be well above the ocean surface. Changes in ice mass also affect the speed of rotation (i.e., length of day) and other parameters such as the tilt of the Earth's axis. Q: Is it likely that as we seek to cool ourselves in response to rising temperatures the "exhaust" we generate from air conditioners, for instance, will collectively cause an accelerating rise in temperature? A: Yes, but not because air conditioners exhaust heat. It is because they use electricity from the burning of fossil fuels that produces carbon dioxide in the atmosphere. Carbon dioxide allows heat from the sun to pass through to the Earth, but traps heat emitted from the Earth because of differences in the temperatures of the sun and the Earth and the respective wavelengths of the heat. This trapping is called the greenhouse effect, which is much stronger than the direct heat generated from making and using of electricity. Q: As the NOVA program said, the meltwater forming lakes on top of the glaciers finds its way down to the bottom of the glaciers and spreads out. With up to 4,000 feet of ice on top of this water, wouldn't the weight and pressure of all that ice on the water cause the waters temperature to rise and speed up the melting from the underside of the glaciers? A: Good idea, but no. Water and ice are nearly incompressible, so they do not heat up much when put under pressure. However, the water from the surface is warmer than the cold ice deep in the glacier, so the water can warm the deep ice and make the glacier flow faster. Also, water is heavier then ice and therefore the surface meltwater can penetrate (and melt its way) to the bottom of the ice sheet. Q: To those who believe that the actions of man, not the Earth itself, are warming the planet, I have to ask, what ended the Ice Age? Man and his machines were not around to warm the Earth, yet the ice that covered my home melted. A: Yes, the last ice age ended because of gradual periodic changes over tens of thousands of years in the way the Earth orbits the sun (i.e., inclination of the Earth's axis, eccentricity of the orbit, and the time of the year when the Earth is closest to the sun). But now, man is beginning to cause temperature changes as large as going from an ice age to a warm interglacial period. These changes from greenhouse warming are imposed on top of any natural changes that might be taking place. Even without the problem of climate warming, it makes good economic sense to increase our energy efficiency, develop alternative energy sources, and reduce our dependency on oil and coal. If we do not develop new energy-efficient technology (e.g., cars, air conditioners, wind and solar power) other countries will, and then the U.S. will buy new technology from them instead of selling it. Q: Why should we be concerned about the sea level rising from a melting glacier if the floating glacier already has displaced its own weight in water (Archimedes' principle, which states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases)? A: You are right about buoyancy, but most of the Greenland ice sheet is sitting on land and not floating. Part of the Jakobshavn outlet glacier has been floating as it flows into ocean in a deep fjord. The floating part is almost gone now, and the speed of the ice into the ocean has doubled. When the floating part is totally gone, it may allow more ice to come out from the main ice sheet. The difference between total ice input (snow accumulation) and output (surface melt, iceberg calving, and melting at the underside of large floating ice tongues) determines how much the Greenland ice sheet contributes to sea-level rise. At higher elevations, snowfall adds ice to the ice sheet. Along the edges some ice melts and the water goes into the ocean along with the ice through Jakobshavn and other outlet glaciers. We are working on determining which is greater, the input or output. Q: Is it fun to look at glaciers? A: Yes it is, but "exciting" is probably a better word than "fun." Like looking at the icecaps that Dr. Steffen and I just visited on Mt. Kilimanjaro with Austrian colleagues last week. Exciting because of the remaining scientific mysteries about why they are there and how they are changing, and because of the adventure of working on them. |
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