El Nino Impacts: Weaker in the Past, Stronger in the Future?
DALLAS -- El Nino may have been less of a factor in Northern Hemisphere climate around 4000 B.C. than it is now, and global warming may be working to accentuate El Nino's current and future impacts. These conclusions by two scientists at the National Center for Atmospheric Research (NCAR) will be presented along with other research on the El Nino/Southern Oscillation (ENSO) on January 13 at the American Meteorological Society (AMS) annual meeting in Dallas, Texas.
Bette Otto-Bliesner used NCAR's recently developed climate system model (CSM) to study the behavior of ENSO, which affects much of the globe, and the North Atlantic Oscillation (NAO), a more regional cycle that alters temperatures and precipitation across the northwest Atlantic and Europe. She found that the impacts of ENSO on Northern Hemisphere winters (for instance, a general warming across Canada and the northern U.S. during El Nino) were up to 50% weaker 6,000 years ago. Her research was funded by the National Science Foundation (NSF), NCAR's primary sponsor.
To reproduce the climate of 4000 B.C., Otto-Bliesner altered the incoming solar radiation to account for the planet's greater tilt and a shift in perihelion (the earth's closest annual approach to the sun) at that time. The result is accentuated seasonality, with up to 6% more solar input during the Northern Hemisphere summer and a corresponding reduction in the winter. As a control run, Otto-Bliesner used the CSM to simulate present-day conditions. In both the 4,000 B.C. and present-day runs, the model captured the ebb and flow of El Nino and La Nina, but the impacts of ENSO were considerably weaker in the 4,000 B.C. run.
What about the future of El Nino? According to NCAR senior scientist Kevin Trenberth, ENSO's impacts may be enhanced by human-produced climate change. El Ninos have been unusually frequent since the mid- 1970s. The same period has seen a dramatic rise in global temperature. The year 1998 set a global record and was one of the United States' two warmest years since records began in the late 1800s.
Trenberth has found that the global mean temperature peaks three to four months after the peak in El Nino. "It is no coincidence that the exceptional warmth in the first seven months of 1998 occurred as the Pacific Ocean lost heat following the peak of the 1997-98 El Nino in December 1997," notes Trenberth. During El Ninos, warm waters spread across the tropical Pacific, evaporating large amounts of water vapor that release heat when the vapor condenses into clouds and rain. Thus, El Nino events tend to transfer heat from ocean to atmosphere, warming the globe about 0.1 degree Celsius for each standard deviation of departure from average temperatures in the Southern Oscillation index, according to Trenberth.
Trenberth theorizes that much of the additional heat trapped by increasing amounts of greenhouse gases may be going into the oceans. It is later released through El Ninos that are larger, more frequent, or less efficient in releasing the ocean-stored heat. The atmospheric warming induced by El Nino also helps to further dry out regions already prone to drought during El Nino, such as Indonesia, Australia, and parts of Africa and Brazil. "Naturally occurring droughts, such as from ENSO, are likely to set in quicker, plants will wilt sooner, and the droughts may become more extensive and last longer with global warming." Trenberth's work was sponsored by the National Oceanic and Atmospheric Administration's Office of Global Programs, the National Aeronautics and Space Administration, and NSF.
NCAR is sponsored by NSF and managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
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