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Fall 2000

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Link found between El Niño and Bangladeshi cholera outbreaks

About 11 months after the start of an El Niño event in the equatorial Pacific, hospitals thousands of miles away in Bangladesh can expect a surge of cholera cases, according to the first mathematical model to link climatic cycles with subsequent cholera outbreaks. Details of the climate-disease model were reported last month in Science by ecologists at Cornell University and the Universities of Barcelona, Maryland, and London.

"So far we aren't seeing a return to the time when cholera was such a scourge of humanity," says Stephen Ellner of Cornell. "But we are getting an explanation for outbreaks of cholera and diarrheal diseases in South America and the recent, higher-than- historic levels of cholera in South America and Asia." Ellner provided the model for the study. His coauthors are Mercedes Pascual (University of Maryland), Xavier Rodo (University of Barcelona), Rita Colwell (director of NSF and professor at the University of Maryland), and Menno Bouma (University of London).

Cholera is caused by a bacterium that lives among zooplankton in brackish waters and in estuaries and infects humans through contaminated water. Colwell previously had proposed a link between cholera outbreaks and El Niño–Southern Oscillation (ENSO) events involving increased sea-surface temperatures and higher numbers of bacteria-bearing zooplankton.

Data on cholera incidence, which normally can rise and fall twice each year with local influences such as monsoons and seasonal temperature changes, came from a hospital in Bangladesh that had tested all incoming patients for cholera from 1980 to 1998. The Ellner model took into consideration recent frequencies of cholera cases, an ENSO index based on sea-surface temperatures in the Pacific, and seasonal variation in local climates.

Peaks in cholera incidence at the Bangladesh hospital were found to occur every 3.7 years—exactly the same frequency as of ENSO events between 1980 and 1998. A separate analysis of climate variables by co-authors Pascual and Rodo—including humidity in the troposphere, cloud cover, and the amount of absorbed solar radiation—suggested that the 11-month lag breaks down into a 6-month lag between an ENSO and increases in sea surface temperatures off the coast of Bangladesh plus a 5-month lag between increased sea surface temperatures and a peak in cholera.

This discovery comes at a time when some ecologists are predicting major increases in disease and death as global climate change provides ideal conditions for disease-causing organisms. Although public-health authorities will now have an 11-month advance morning beginning when an El Niño starts, Ellner says, "this model will be more useful when somebody figures out how to predict El Niño."

The study was supported in part by grants and fellowships from the James S. McDonnell Foundation, the Knut and Alice Wallenberg Foundation, and the Mellon Foundation.

Cornell University, University of Maryland, University of Barcelona, University of London

Carbon dioxide could replace hydrofluorocarbons as refrigerant

Researchers are making progress in perfecting automotive and portable air-conditioning systems that use carbon dioxide as a refrigerant instead of conventional, synthetic chemicals. CO2 was the refrigerant of choice during the early 20th century. Now it may be on the verge of a comeback, thanks to technological advances that include the manufacture of extremely thin yet strong aluminum tubing.

Although CO2 is a greenhouse gas, conventional refrigerants called hydrofluorocarbons cause about 1,400 times more global warming than the same quantity of CO2. Also, the tiny quantities of CO2 that would be released from air conditioners would be insignificant compared to the huge amounts produced from burning fossil fuels for energy and transportation, says Eckhard Groll (Purdue University). This summer, Groll chaired the Gustav Lorentzen Conference on Natural Working Fluids, at which Purdue engineers discussed their progress on designing and assessing a portable CO2-based air conditioner.

CO2 is promising for systems that must be small and lightweight, such as automotive or portable air conditioners. CO2 systems must be operated at high pressures—up to five times as high as commonly seen in current technology. The high operating pressure required for CO2 systems enables the refrigerant to flow through small-diameter tubing, which allows engineers to design more compact air conditioners. In the past, however, heavy steel tubing had to be used. Now, extremely thin yet strong aluminum tubing can be manufactured, reducing the weight of the unit.

Environmental regulations now require that refrigerants removed during the maintenance and repair of air conditioners be captured with special equipment, instead of being released into the atmosphere as they have been in the past. The new "recovery" equipment is expensive and will require more training to operate, important considerations for the U.S. Army and Air Force, which together use about 40,000 portable field air conditioners. The units, which could be likened to large residential window-unit air conditioners, are hauled into the field for a variety of purposes, such as cooling troops and electronic equipment.

"For every [conventional] unit they buy, they will need to buy a recovery unit," Groll says. "That's a significant cost because the recovery unit is almost as expensive as the original unit. Another problem is training. It can be done, but it's much more difficult than using carbon dioxide, where you could just open a valve and release it to the atmosphere" since it is a natural, comparatively benign gas.

Groll estimates that CO2 systems will take another 5 to 10 years to perfect. His work is funded by the U.S. Army, U.S. Air Force, and the American Society of Heating, Refrigerating and Air- Conditioning Engineers, as well as the Air Conditioning and Refrigeration Technology Institute.

Purdue University

Pacific Decadal Oscillation packs a one-two punch

About five years ago, scientists at the University of Washington discovered that every 15 to 20 years the Pacific Ocean undergoes an El Niño–like shift in temperature known as the Pacific Decadal Oscillation. New research shows shows there may be a second, much longer, PDO pattern that lasts about 70 years.

Yi Chao (NASA Jet PropulsionLaboratory), Michael Ghil, and James McWilliams (both of the University of California, Los Angeles) have found evidence of the PDO's two-part structure in a study of the past 92-year record of sea-surface temperatures in the North and South Pacific. Their results appeared in August in Geophysical Research Letters.

Compared with El Niño, "the PDO is larger, longer, and more difficult to visualize," said Chao. "An explanation might be that it isn't just one thing; it's potentially two big events going on."

In their study, the scientists clearly saw large-scale temperature oscillations of 1–2°C taking place in the Pacific basin approximately every 15 to 20 years. In addition to this regular and relatively short fluctuation in the Pacific basin's temperature, they found evidence of another temperature shift that appears to take place on a scale of about 70 years. At the beginning of this century, sea surface temperatures seem to gently drop to a low in the 1930s, gradually rise again until the 1970s, and then begin a similarly paced decline to the present. "While we were only able to see one cycle in our data, tree-ring records, which go back 200 to 300 years, and fishery data show a similar time-scale shift," Chao explained.

The PDO also reveals striking symmetry between the northern and southern Pacific. In its "cool" phase, the PDO is a giant, horseshoe-shaped arc of warmer-than-normal water off the coast of Japan, enclosing a wedge of cooler-than-normal water near the equator. In the new study, this pattern appears around 1976, 1957, 1941, and 1924.

"What's striking is that the PDO pattern is similar in both the North and South Pacific and covers a huge area from the Aleutian Islands to the South Pacific," said Chao. "No computer models developed so far have been able to reproduce this symmetric pattern across the equator. This symmetry is a key to understanding what creates the PDO."

The research was supported by NASA.

University of California, Los Angeles

Greenland ice cores show natural changes; Himalayan cores show human influence

Almost three years ago, Curt Davis (University of Missouri–Columbia) discovered that some areas of the southern Greenland ice sheet varied dramatically in elevation over a ten- year period. An extensive study by a team of scientists, including Davis, has now found that weather, not long-term climate change, is the cause of these variations. The findings were reported in Nature in August.

"When we released our original findings, they were somewhat controversial," said Davis, who has been using satellite data since 1990 to study changes in the ice sheet. "Our data indicated that overall, the ice sheet was maintaining a constant elevation, but we found great variability over short distances, with substantial thickening in some areas and strong thinning in other areas." After his study was released, Davis and a group of researchers led by Joe McConnell (Desert Research Institute) joined together to investigate the cause of the variability in elevation. Other universities who participated in the study include the University of Washington, Ohio State University, the University of Arizona, and the University of Nebraska. Funding was provided by grants from NASA and NSF.

Using ice cores from 45 to 400 feet (15 to 130 meters) deep that were collected from 12 locations around the southern Greenland ice sheet above 6,000 feet (X meters) in elevation, the researchers measured variations in the concentrations of dust and chemical compounds such as hydrogen peroxide, calcium, and ammonium. The researchers used this analysis to determine the amount of snow that accumulated each year over the time span of the cores. Ice core analysis and modeling revealed that areas where elevation changed dramatically had a corresponding variation in snowfall during the study period. Further analysis indicated these snowfall variations were consistent with natural fluctuations over decades.

A separate study by Lonnie Thompson (Ohio State University) used ice cores drilled through a glacier more than four miles up in the Himalayan Mountains. According to these cores, which give a highly detailed record of the last 1,000 years of climate in the Tibetan Plateau, both the last decade and the last 50 years were the warmest in that entire period.

"This is the highest climate record ever retrieved," Thompson said, "and it clearly shows a serious warming during the late 20th century, one that was caused, at least in part, by human activity."

The cores also revealed periodic failures of the South Asian Monsoon. In 1790, the monsoon cycle changed, and drought took hold on the plateau, a condition that cotninued for seven years until 1796, when the monsoons returned.

"That event was major," Thompson said. "It killed more than 600,000 people in one region of India alone. And that was at a time when global populations were much less than they are today [an estimated 980 million in 1800]. If a similar event occurred today, the social and economic disruptions would be horrendous." The ice-core record showed other serious monsson failures and ensuing droughts in 1876–77 and around 1640, 1590, 1530, 1330, 1280, and 1230, though none was as devasting as the 1790 event. Thompson's paper on the research, published last month in Science, offered no indications of what might have triggered the monsoon failures.

The data, however, do seem to point to the impact human activities have had on the region's climate. Core samples covering the last century reveal a fourfold increase in dust trapped in the ice and a doubling of chloride concentrations, suggesting an increase in both drying and desertification in the region.

The core-drilling expedition was supported by NSF.

Ohio State University

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Last revised: Wed Dec 13 17:24:16 MST 2000