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Summer 1999

Science Bits

Arctic snow chemistry is more complex than previously thought

Purdue University and Michigan Technological University research teams studying natural processes that affect ozone in the Arctic atmosphere have discovered that snowpacks not only absorb chemicals from the atmosphere, but also can help produce them. The findings, published in two papers--one in Nature and the other in Geophysical Research Letters--cast a new light on scientists' perceptions of how atmospheric gases are processed, says Paul Shepson, professor of atmospheric chemistry at Purdue.

The new findings also may affect the way that scientists view data from ice core studies, because researchers have assumed that the air trapped in ice provided representative samples of atmospheric conditions at the time the ice was formed. "Ice core studies designed to look at reactive species such as nitrates may have to be revisited, as the air bubbles found in these ice cores may not be the mirrors of atmospheric composition that we suspected they were," Shepson says. This is not a concern for more stable greenhouse gases such as carbon dioxide and methane, which have been extensively studied in ice cores, because those gases are less likely to react with other compounds in snow or ice.

Shepson led a research group to the Canadian Arctic last spring to observe how sunlight interacts with various gases in the atmosphere to reduce near-surface ozone levels. (For a report on NCAR's participation in this project, see the Fall 1998 UCAR Quarterly. In the Arctic spring, after several months of total darkness, atmospheric ozone may be completely depleted at times. From the Environment Canada research site at the Canadian Forces base at Alert, the group tracked levels of formaldehyde and other atmospheric compounds. Formaldehyde is an important part of the atmosphere's self-cleaning mechanism because when it absorbs light, it breaks apart to produce hydroxyl radicals.

Previous studies of formaldehyde in the Arctic had shown concentrations up to ten times higher than expected. The Purdue team's measurements of formaldehyde in the snowpack and in the atmosphere, published in the Nature article, suggest that the compound is produced through photochemical reactions at the snow surface. "The data account for much of the discrepancy between the high concentrations of formaldehyde found in the Arctic and the amounts predicted by our models," Shepson says.

The paper in Geophysical Research Letters reports on studies at an ice core site at Summit, Greenland, led by Richard Honrath of Michigan Technological University, in which the Purdue group also participated. That team found that concentrations of nitric oxide and nitrogen dioxide--collectively known as NOx--were actually higher within the snowpack than in the atmosphere. The findings suggest that nitrate ions in the snow can interact with sunlight to produce NOx, a pollutant derived largely from the combustion of fossil fuels and a critical precursor to the production of ozone in the atmosphere.

"This observation changes the way we look at atmospheric chemistry in a fundamental way, in that deposition of nitric acid to the snow was previously regarded as the final fate of NOx," Shepson says. "Now it appears that nitric acid in the snow can be reprocessed by interactions with light, causing re-release of a variety of pollutants back into the atmosphere."

In addition to forcing a re-evaluation of data from ice core studies, the new findings call into question model treatments of the interaction of gases with surfaces, Shepson says. "Although we are starting to do better with atmospheric particles, it is important to remember that a potentially important atmospheric surface is the surface of the earth."

Purdue University, Michigan Technological University

Antarctic ice shelves breaking up due to decades of higher temperatures

Two ice shelves on the Antarctic Peninsula known as the Larsen B and Wilkins have lost nearly 3,000 square kilometers (1,200 square miles) of their total area in the last year, according to researchers at the University of Colorado at Boulder's National Snow and Ice Data Center (NSIDC) and the British Antarctic Survey. The scientists attribute the retreats to a regional warming trend that has caused the annual melt season to increase by two to three weeks over the last 20 years.

"We have evidence that the shelves in this area have been in retreat for 50 years, but those losses amounted to only about 7,000 square kilometers," said David Vaughan, a researcher with the Ice and Climate Division of the British Antarctic Survey. "To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation. Within a few years, much of the Wilkins ice shelf will likely be gone." The Larsen B shelf is currently about 7,000 km2, and the Wilkins shelf is about twice that large.

Satellite photos monitored by NSIDC show that the Larsen B ice shelf has continued to crumble after an initial small retreat in early 1998. In a series of events that began in November 1998, an additional 1,714 km2 of shelf area caved away. On the opposite side of the peninsula, the Wilkins Ice Shelf retreated nearly 1,100 km2 in early March of last year.

Scientists looking at weather satellite imagery at that time suspected a breakup was under way and had their suspicions confirmed by radar satellite images. "The radar images showed a large area of completely shattered ice, indicating an ice front 35 kilometers back from its previous extent," said Ted Scambos of NSIDC. "The sudden appearance of thousands of small icebergs suggests that the shelves are essentially broken up in place and then flushed out by storms or currents afterward."

The British Antarctic Survey scientists had predicted one of these retreats, using computer models to demonstrate that the Larsen B was nearing its stability limit. Where ice shelves are supported by islands and sheltering coastline, they can become stable, long-term features. Surface features on the Larsen B indicate that it has existed for at least 400 years. But with the small breakup observed last spring, the shelf had already retreated too far to continue to be supported by adjacent islands and shorelines.

The British researchers, who have monitored the peninsula's climate warming for decades, report an increase in mean annual temperature of about 2.5°C or roughly 4.5°F since the 1940s. Average summertime temperatures have inched to just above 0°C. Both groups of scientists concur that ice shelf breakup is a direct result of local climate warming.

Images of the Larsen B and Wilkins ice sheets are available on the Web.

University of Colorado, British Antarctic Survey

When waffling on flood control is a good thing

North Dakota's Red River of the North, which flooded catastrophically two years ago, is the site of a proposal to protect an entire river basin by using existing road networks to create temporary floodwater storage instead of spending millions of dollars to build more dikes.

"There are two types of dikes: Those that have been topped and those that will be topped," says Gerald Groenewold, director of the University of North Dakota (UND) Energy & Environmental Research Center (EERC) in Grand Forks. "The waffle is a cost-effective option that doesn't rely on new permanent structures to control flooding. It would also benefit everyone living within a drainage basin during times of droughts and floods."

The idea behind waffle flood control is to use existing, raised road networks that crisscross rural areas--resembling the grid pattern on a waffle--as temporary microstorage basins. Just as each individual square on a waffle can hold syrup, each area enclosed by roads is capable of storing water. During major flooding, the water would be held for several days and then released gradually, lowering river crests by an estimated 30%.

In times of drought, water captured from winter snow could be used for irrigation, to recharge depleted aquifers, or to improve topsoil and subsoil moisture on farm land.

Implementing waffle control would require gathering highly accurate location coordinates and elevations of road crests, drainage structures, bridges, and land surface in areas of the basin thought to have potential value as floodwater storage areas. Using a combination of global positioning system and geographic information system technologies, a three-dimensional picture of the drainage basin would be created. The data would be used to model the water storage potential and characteristics of the basin. If model simulations showed that the idea was feasible, a floodwater storage and release plan would be developed for use by policy-making organizations in the Red River Valley

Groenewold says the idea of using the waffle in the Red River Valley is receiving strong support in the United States and Canada from farmers, farm groups, water management organizations, local and state elected officials, urban and rural residents, and federal agencies. In the 1999 North Dakota legislature, an amendment authorizing $2.25 million in state funding to study the waffle concept has been added to a bill containing a variety of water and flood control projects throughout the state.

Leon Osborne, director of the Regional Weather Information Center at UND, says that based on studies of regional climate cycles, it appears that the potential for major flooding in the Red River Valley could continue through 2005. An EERC study on climate cycles based on geological evidence indicates that the northern plains region is in a wet cycle that occurs about every 160 to 170 years. But "we have done virtually nothing to prepare ourselves for a truly great flood," according to Groenewold.

"We can't continue to address this issue on a town-by-town basis," says Groenewold. "The consensus is growing for meaningful, basinwide flood protection. This requires the development of strategic partnerships--local, state, national, and international--within the Red River drainage basin."

University of North Dakota

Changes in El Niño show up in the Nile

Hydrologists at the Massachusetts Institute of Technology have used records of the Nile River's height to put recent occurrences of El Niño into historical perspective. The researchers, led by associate professor Elfatih Eltahir, compared records from 1872 to 1997 of both the Nile River--indicating years of flooding or drought--and Pacific Ocean sea-surface temperatures--indicating El Niño years. They found that 30% of the natural variability in the Nile's water-level fluctuations could be linked to El Niño. Based on that information, they analyzed records of Nile water levels for the past 1,000 years. These levels have been measured since 622 A.D. using a simple gauge, the nilometer.

Using the Nile's height as an indicator of El Niño years, the researchers determined that El Niño has occurred more often and with longer duration in the past two decades than in most similar periods during the last millennium. Continuation of this trend for a few more decades would indicate a shift in global climate, but Eltahir cannot say whether that shift is the consequence of human activity. The research, funded by NSF, NASA, and the Alliance for Global Sustainability, was published in Geophysical Research Letters.

Massachusetts Institute of Technology

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 15:10:57 MDT 2000