Thinning Sea Ice Stokes Debate on Climate DebateBy William K. Stevens, The New York Times, Nov. 17, 1999
The great ice cover that stretches across the top of the globe has become about 40 percent thinner than it was two to four decades ago, scientists have found after analyzing data collected by nuclear submarines plying the Arctic Ocean.
Since the ice is already floating in the water, no rise in sea level has accompanied its melting, as happens when land-based glaciers shrink.
But the finding raises anew the question of whether climatic changes observed in recent decades are the result of the atmosphere's natural behavior, or a global warming caused by human activity, or some combination of the two.
The scientists found in a new study that from 1958 through 1976, the average thickness of the Arctic sea ice was about 10 feet.
From 1993 through 1997, it was about six feet.
In the 1990's, say the researchers, the thinning appeared to be continuing at a rate of about four inches a year.
"It's a startling result," said Dr. D. A. Rothrock, an oceanographer at the University of Washington in Seattle and the leader of the study, which is being published in the December issue of the journal Geophysical Research Letters.
The research involved measurements of sea ice thickness made by upward-looking sonar aboard naval submarines operating under the ice sheet.
The first period of data began in 1958 with the first nuclear submarine, the United States' Nautilus, and concluded with a cruise by H.M.S. Sovereign in 1976. The second data set was collected by American vessels from 1993 to 1997.
Dr. Rothrock and two colleagues, Y. Yu and G. A. Maykut of the University of Washington, compared data from the two periods at 29 points where the courses of submarines in the 1990's intersected with the courses of those in the earlier period.
There is substantial evidence that the climate of the Arctic and sub-Arctic region is warming, at least in some seasons. The area covered by sea ice has diminished and the duration of the cover has shortened in many places.
Mountain glaciers in Alaska have shrunk, as has the Greenland ice cap. The average surface temperature of the earth has risen by about 1 degree Fahrenheit or a little more over the last century and by several times that amount in northern regions.
Mainstream scientists expect that the global average will rise by about an additional 3.5 degrees by the year 2100 if emissions of heat-trapping gases like carbon dioxide continue to be produced by human activity at present levels.
By comparison, the earth's surface temperature has risen by 5 to 9 degrees since the depths of the last ice age, 18,000 to 20,000 years ago.
Dr. Rothrock believes that a shift in prevailing natural patterns of atmospheric circulation in the Arctic may be responsible for the warmer North and the thinning sea ice.
Other scientists say that the shift in natural patterns may have been touched off or enhanced by global warming, and Dr. Rothrock agrees that it is possible a warming climate could be related in some way.
The key is a seesaw sort of winter circulation pattern in which prevailing westerly winds in the Arctic, sub-Arctic and North Atlantic vary between two basic states, one stronger and one weaker.
In this Arctic oscillation, as it is called, one pattern or another has been dominant for decades at a time before switching to the opposite mode.
In one mode, which has been dominant since about the mid-1970's, stronger westerly winds blow surface water away from the Arctic and toward the southeast.
This, said Dr. Rothrock, could carry cold surface water away from the Arctic, bearing chunks of sea ice with it.
Other observations have established this.
As the chunks depart, areas of open water enlarge, presenting more ice edges to melt should a source of warming appear.
One source, said Dr. Drew Shindell, an atmospheric physicist at the NASA Goddard Center for Space Studies in New York City, is warm, deep water that flows upward to replace the departing surface water that has been chilled by the winter.
In addition, said Dr. Rothrock, the stronger winds may also bring warmer air to the Arctic, furthering the melting.
Computer modeling studies, said Dr. Shindell, indicate that the present situation is "not a natural thing." According to the models, the buildup of heat-trapping greenhouse gases is largely responsible.
The reason, he said, is apparently that one effect of global warming is to increase the temperature contrast between the tropics and polar regions at critical altitudes, thereby strengthening the westerly winds.
He suggests that this pattern will prevail for the next 30 or 40 years.
Whether the present state will persist or the Arctic oscillation will swing back to its alternate condition is going to be "extremely interesting," Dr. Rothrock said.
Melting Arctic Ice Linked to Human Combustion of Oil and Gas
By Cat Lazaroff (ENS)
GREENBELT, Maryland, December 3, 1999 (ENS) - Enormous swaths of Arctic sea ice are vanishing each year, and humans are the likely culprits, a team of scientists announced Thursday. Using ground based and satellite data, the researchers estimate there is a 98 percent chance that the melt is due, at least in part, to global warming caused by human activities.
For the first time, scientists placed space based observations of Arctic sea ice retreats into a much longer term context, and have examined the likelihood that the sea ice decreases are in part because of human caused climate change.
The results seem to support doomsayers' predictions that climate change is the reason for the melting of the Arctic ice. Most scientists agree that the climate warms as heat-trapping gases emitted by burning oil and gas accumulate in the upper atmosphere.
Sea ice is a sensitive indicator of climate change - if the global climate is warming, then the amount of permanent sea ice in the Arctic should decrease.
Satellite measurements and surface observations have shown that the area of Northern Hemisphere sea ice has, in fact, been decreasing for almost 50 years. Two reports in today. s issue of the journal "Science" published by the American Association for the Advancement of Science present new measurements and analyze changes in Arctic sea ice.
One team, led by Konstantin Vinnikov of the University of Maryland, used computer climate models to examine whether the decreases observed in the ice cover of the Arctic over the past few decades are the result primarily of natural climate changes or might also be influenced by human induced global warming, says study co-author Dr. Claire Parkinson, a National Air and Space Administration (NASA) climatologist at the Goddard Space Flight Center in Greenbelt.
Their model suggests that the probability of this change being caused by natural variability is less than two percent for the last 20 years. They predict the decreases will continue during the next century.
Vinnikov and Parkinson conclude that the decrease in Northern Hemisphere sea ice is probably a result of human driven global warming.
The team combined five separate data sets to measure sea ice retreats. Scientists used two independent computer climate models to simulate how much ice there would be without human added carbon dioxide in the atmosphere.
"Satellite data from November 1978 through March 1998 (19.4 years) reveal that the Arctic ice extent overall has shown a downward trend of 37,000 square kilometers per year, meaning a loss each year of an ice area well exceeding the combined areas of the states of Maryland and Delaware. The total loss over 19.4 years would be an area exceeding the size of Texas," said Parkinson.
Taking the long-range view, the team used a 5,000 year run in a global climate model of the National Oceanic and Atmospheric Administration. s (NOAA) Geophysical Fluid Dynamics Laboratory at Princeton University. They found the probability of getting a negative trend over 19.4 years as large as that found from the satellite data was less than two percent if no human caused emissions were factored in.
The team then examined outputs from computer simulations that include human created increases in greenhouse gas emissions that tend to warm the atmosphere, and increases of smoke and dust that tend to cool the atmosphere. This model shows atmospheric warming that much more nearly matches the actual observations of Arctic sea ice shrinkage.
Vinnikov says that the results suggest that melting Arctic sea ice is probably related to human induced global warming. "We only have satellite data for a relatively short period of time," says Parkinson. "It was interesting to be able to put the satellite data into a longer term context by using the model simulations."
The study was done by a team of meteorologists, physicists, and climatologists from the University of Maryland, Rutgers University, NOAA, the University of Illinois, NASA, the Hadley Center in Great Britain, and the Arctic and Antarctic Research Institute in Russia.
A second study also points to dramatic changes in Arctic sea ice. Research by a team from Norway and Russia measured the extent of the perennial Arctic sea ice pack, and found that this area of multiyear ice has decreased by more than twice as much as that of total sea ice.
This change in ice cover has been accompanied by a decrease in ice thickness. The Russian/Norwegian team says this suggests the heat and fresh water balances of the Arctic may be changing in significant ways.
The team, led by Ola Johannessen of the Geophysical Institute at the University of Bergen, Norway, used microwave satellite remote sensing data to measure changes in the composition of ice cover. They found a reduction of about 14 percent in the area of multiyear ice in winters from 1978 to 1998.
If this apparent transformation continues, the team predicts, it may lead to a markedly different ice regime in the Arctic, altering heat and mass exchanges as well as ocean stratification.
As "Science" writer Richard Kerr warns, also in today. s issue, "If global warming is at fault, the entire ice pack will eventually disappear, with drastic climate implications for the Northern Hemisphere."
More evidence of human caused global warming comes from a new comparison of climate in the past millennium with projected climate for the new millennium.
The New York based conservation organization Environmental Defense Fund (EDF) prepared charts from existing models of global temperature change, sea level rise, and concentrations of the greenhouse gas carbon dioxide. They show that fairly stable historic patterns are now beginning to change quickly.
EDF. s report, released Wednesday, reveals the rapid onset of global warming and the potential for unprecedented warming in the next thousand years.
Without action to cut greenhouse gas emissions, the climate of the coming millennium is projected to be much warmer than in the past. Global temperature may rise to 10 degrees Fahrenheit (F) above the recent average by 2200, in the worst case shown.
If action to lower fossil fuel emissions does not start until 2100, it could result in a six degree F temperature increase by 2200.
Immediate action to cut fossil fuel use, however, may result in a less threatening increase of no more than three degrees F in the coming millennium.
"Humanity is creating a new climate for the new millennium. Once this climate change occurs, there's no going back," said Dr. Michael Oppenheimer, EDF chief scientist. "Actions taken now to cut greenhouse gas pollution will determine if adjustment to future climate will be manageable or impossible."
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