Mark McCaffrey, CIRES Outreach and Education
In April of 1997, paleoclimatologist Brad Linsley along with several colleagues traveled to the island of Rarotonga in the Cook Islands of the western Pacific. Using scuba diving gear and equipped with a special drill, Linsley and his team drilled into a species of hard coral which was known to have annual bands that serve as a record for chemical changes caused by fluctuations in sea surface temperature over the course of the year. They pulled a core from the coral that represented over 270 years of growth in the coral. Taking the core to a laboratory, Linsley and his team analyzed the chemical make-up of the core, specifically the ratio of the elements strontium to calcium, and, comparing with modern records of sea surface temperature in the region, confirmed that the ratio accurately mirrored the surface temperature of the ocean. The data show an overall cooling trend in the region over the past several centuries. The data are helpful to researchers in understanding what they know (and don't know) about the long-term dynamics of decade-scale (ENSO or El Niñ
o Southern Oscillation) and multi-decade-scale (PDO or Pacific Decadal Oscillation) climate processes in the Pacific.
Braddock Linsley, Gerard Wellington, Daniel Schrag
Research into past climate variations by examining the annual bands in coral reefs which capture information about the sea surface temperature.
In 18 meters of water off the coast of southwest side of Rarotonga in the Cook Islands of the Western Pacific.
April, 1997; the data collected covers over 270 years of past climate variations in the region.
Drilling into a hard coral of the genus Porites using a special drill, Brad Linsley and his colleagues took the core of coral back to the laboratory where, using an atomic emission spectrophotometer, they were able to compare the strontium (Sr) and calcium (Ca) ratio, which is acknowledged in scientific circles to be an accurate means of reconstructing past changes in sea surface temperature.
In order to understand the complexities of the Earth system, including climate processes in the Pacific Ocean such as ENSO (El Nino Southern Oscillation) it is important to use paleoclimate (past climate) reconstructions to gain insight into the range and processes of natural climate variability.
A plot of the data indicates a cooling trend in the region, which is located in the South Pacific gyre, with sea surface temperatures ranging from a high over 29°
C around 1748 to a low of around 22°
C around 1912. Cooling around 1815 may be related to the Tambora volcanic eruption in April of that year.
Is there still a cooling trend in the region? (Remote sensing data should provide data to answer this question.) Did coral bleaching occur during this period? (Coral bleaching can be caused by wide swings in sea surface temperature, especially warming events.) What causes the wide swings in sea surface temperature or SST? How does this data set compare with other regional and global data sets for a similar time span?