January 4, 2013
Coral Fossils Indicate 20th-Century El Nino Was Intense But Not Unusual
Watch the video “Examining Fossil Corals”
April Flowers for redOrbit.com - Your Universe OnlineScientists have dramatically expanded the amount of information available on the El Nino-Southern Oscillation (ENSO), a Pacific Ocean climate cycle that affects weather patterns worldwide, by examining a set of 7,000-year-old fossilized corals. The teams says that their study will help assess the accuracy of climate model predictions for twenty-first-century climate change in the tropical Pacific.
According to research on the coral fossils, twentieth-century ENSO climate cycles were significantly more intense than the ENSO variations captured in the fossil corals. The new data also revealed large natural variations in past ENSO strength, making it difficult to attribute the twentieth-century changes in weather patterns to rising carbon dioxide levels alone. Additionally, multi-century climate model simulations also show large natural fluctuations in ENSO activity.
"We looked at the long-term variability of ENSO in the climate models and asked how it compares to the long-term variability of ENSO in the real world," said Kim Cobb, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.
"We show that they actually match fairly well. This project sets the stage for conducting more detailed data-model comparisons from specific time intervals to test the accuracy of ENSO characteristics in the various models."
ENSO extremes accelerate changes in global temperatures and precipitation patterns every two to seven years. The team, which included members from the Scripps Institution of Oceanography and the University of Minnesota, found that those variations are more pronounced in the central tropical Pacific where the fossil corals were collected. The scientists gathered information about ancient ENSO-related temperature and rainfall variations from the fossilized corals by analyzing the ratio of specific oxygen isotopes in the coral skeletons.
"Fossil corals are the kings of El Nino reconstruction," explained Cobb in an official statement. "Corals grow in the heart of the El Nino region, and with monthly-resolved records, they provide a very high level of detail."
To obtain the samples, the team drilled into massive coral "rocks" rolled onto Pacific island beaches by strong storms or tsunamis. They studied 17 cores of varying lengths and ages recovered from beaches on Christmas and Fanning islands — part of the Line Island chain in the mid-Pacific.
Experts say that the Line Islands are perfect for collecting records of past ENSO activity since they are close enough to the source region for ENSO to be affected by the variations in temperature and precipitation, but not close enough that the islands' corals are destroyed or bleached by large temperature increases during strong El Nino warming events.
Each core sample study began with careful dating by analyzing the ratio of the elements uranium to thorium in the fossils. Once the age of each core had been determined, the researchers chose a subset of the collection to study in detail.
Each core was sawed in half and the cross section was then X-rayed to reveal the growth direction of each coral. The researchers then drilled out small samples of coral powder at intervals of one millimeter into the core sample. These samples were then analyzed using mass spectrometers to determine the ration of oxygen isotopes.
Temperature and rainfall abundance can change the isotope ratio of the coral skeleton, providing detailed information about environmental conditions during each period of the coral's growth. As many as 20 samples are analyzed for each year of the coral's lifetime.
"We are able to count back in time, following the seasonal cycles locked in the coral skeleton, as long as the core will allow us," Cobb explained.
The massive study started in 2005 and analyzed over 15,000 samples to add 650 years of monthly-resolved information about ENSO variations across nearly 7,000 years.
The researchers have detected a modest, but statistically-significant increase in twentieth-century ENSO strength that could potentially be due to man-made climate change. The coral reconstruction, however, revealed an even higher level of ENSO strength 400 years ago, though it was of a shorter duration than its twentieth-century counterpart.
"The level of ENSO variability we see in the twentieth century is not unprecedented," Cobb said. "But the twentieth century does stand out, statistically, as being higher than the fossil coral baseline."
ENSO variation information is important for climate scientists because the cycle drives other aspects of global climate change.
"El Nino is something that people want to know about when they reconstruct past climate changes at a specific site," Cobb said. "Our data will provide a reference for the magnitude of ENSO-related changes that may have occurred, and allow researchers to probe the causes of past climate changes evident in other paleoclimate records and in model simulations of past climates."
The findings of this study have already called into question the long-held belief that the effects of ENSO began to taper off some 6,000 years ago. Cobb said that some climate models support that belief, but fossil data from that period simply doesn't support a decrease in ENSO strength.
The team believes that their work will provide a critical comparison to help test the accuracy of future climate model predictions. A report of their study was published in a recent issue of the journal Science.