Coral extracted from a remote central Pacific island has helped scientists at Scripps Institution of Oceanography at the University of California, San Diego, construct a valuable new record of climate conditions during the 20th century. The record, which allowed the researchers to trace sea surface conditions over a 112-year-period, may hold implications for long-range climate forecasting and predictability due to the central tropical Pacific’s key influence on climate conditions around the world.
With samples drawn from a tiny Pacific atoll called Palmyra, Kim Cobb and Christopher Charles’s new coral record shows that a 12- to 13-year cyclical pattern of temperatures emerges in the Pacific that is related to similar patterns in the Atlantic and Indian oceans.
"Several important implications arise from these findings, including the possibility that the observed global climate variability on decadal time scales reflects a ‘teleconnected’ response to changing conditions in the central tropical Pacific Ocean," said Kim Cobb, lead author of the study.
The report, co-authored by David Hunter, a former Scripps graduate student, is published in the June 1 issue of Geophysical Research Letters.
Climate researchers have studied the tropical Pacific’s role in powerful, far-reaching phenomena such as El Niño, which operates over a 3- to 7-year period.
But, says Charles, associate professor of geology in the Geosciences Research Division at Scripps: "This new, highly accurate coral record shows that there are processes that connect these ocean basins on time scales longer than El Niño. We know from intensive monitoring of the El Niño that the ocean basins act in concert to a certain extent. But this record is showing without a doubt that there are processes, many of which are analogous to El Niño processes, that are operating on 12- to 13-year cycles. So this shows that connections exist on much longer time scales, which is important for long-range forecasting and predictability."
Climate scientists have developed models that outline several scenarios for air-sea interactions that operate on cycles described by Cobb and Charles, known as "decadal variability." However, proof from the field, or instrumental records, have been sparse. Prior to World War II, significant gaps existed in critical regions of the ocean, especially from the vital tropics regions.
Early in 1998, Cobb and Charles began compiling a list of locations that could deliver such important records. Targeting the Line Island chain for a 1998 cruise, Cobb and her colleagues found the long-lived, healthy corals necessary for the study on Palmyra Island. Over two weeks, Cobb and a small team used portable drills to extract more than 70 samples from coral heads above and below sea level.
With samples in hand, the following year was dedicated to laboratory analysis. Using mass spectrometry analysis, Cobb measured tiny differences in the ratio of oxygen isotopes in the coral cores. These differences allowed Cobb to determine precisely how the monthly sea water temperatures changed, thus becoming a detailed climate record for the tropical Pacific.
"It’s definitely the highest-quality and longest record from that area," said Cobb, a fifth-year graduate student at Scripps. "We now have a well-proven coral record taken from a very sensitive area."
Although it is not addressed in the current study, the coral also represents a new record for analyzing changes over the last century that may have been caused by human-induced global warming. In fact, the coral reveals a rapid warming in the tropical Pacific over the last 30 years.
Cobb and Charles are in the process of extending the coral-based climate record from Palmyra beyond the 112-year period covered in the current paper. Analysis of fossil corals collected at Palmyra allows them to trace climatological phenomena such as El Niño and decadal variability throughout the last millennium.
The study was funded by the National Science Foundation Graduate Fellowship program and the National Oceanic and Atmospheric Administration Consortium on Ocean’s Role in Climate. Additional support was provided by The Nature Conservancy and HRH Prince Khaled Bin Sultan Bin Abdulaziz.