By stitching together valuable sea temperature information from an obscure Pacific Ocean island, researchers at Scripps Institution of Oceanography at the University of California, San Diego, have constructed an important new record of climate change over the last 1,100 years.
Using coral fossils from tiny Palmyra atoll, a remote central Pacific island, and several laboratory analysis techniques, Kim Cobb and Christopher Charles developed a unique window into climate variability over the last millennium. Their results are published in the July 17 issue of the journal Nature.
Among its most striking revelations, the fossil-coral record shows a clear trend of increasingly warmer and wetter climate in the central tropical Pacific in the late 20th century, particularly since 1976, a change unprecedented in the last millennium. The record also depicts a range of new information about the El Niño phenomenon, including the first glimpses of El Niños in the 17th century that matched or exceeded recent powerful El Niños in the 1980s and 1990s.
"The first thing that leapt out at us was the trend towards a warmer, wetter climate in the 20th century," said Cobb, who received a doctoral degree from Scripps Institution last year. "You don't need a microscope to see the dramatic changes in recent decades."
Charles, an associate professor in the Geosciences Research Division at Scripps, says the records are the first direct evidence for the behavior of El Niño in the pre-industrial period. He believes the results will be important for future studies investigating
human activities and their influence on El Niño events. Some have speculated that El Niños have become stronger as a result of such activities.
"To the contrary," said Charles. "These results show that there were episodes during the height of the little ice age when El Niño was just as strong, if not stronger, than the 1997 and 1982 El Niño events. The study also found episodes when El Niño activity jumped abruptly from one regime to another-a strong suggestion that El Niño is essentially a chaotic phenomenon, as opposed to one that responds regularly to some external forcing."
The coral-fossil record indicates wide fluctuations of El Niño activity, revealing the most intense events during the mid-17th century. More than five years ago, Cobb and Charles identified Palmyra and other parts of the Pacific's Long Island chain as potential sources of new climate change information. In 1998 and 2000, Cobb led expeditions there and used portable drills to extract well-preserved core samples from a variety of coral heads above and below sea level.
Laboratory analyses, including mass spectrometry and uranium and thorium geochemistry, were used to determine precise sea surface temperature and climate fluctuations at the atoll. These were carefully spliced together month by month over several centuries of the last 1,100 years.
"This study provides one of the most direct reconstructions of tropical Pacific climate over the last millennium," said Cobb, currently a post-doctoral researcher at the California Institute of Technology in Pasadena, Calif. "Such data from the tropical oceans never existed so it gives us a new baseline of where we have been and where we might be going."
The clear warming trend seen during the late 20th century, the paper notes, in all probability relates to the rise in greenhouse gases. The paper concludes that the Palmyra
corals could provide critical tests of climate models charged with predicting future
Coauthors of the study were Hai Cheng and R. Lawrence Edwards of the University of Minnesota.
The research was supported by the Khaled bin Sultan Living Ocean Foundation, the Nature Conservancy, the National Science Foundation, and the National Oceanic and Atmospheric Administration.