Paleoclimatologists do not have the luxury of measuring past climates directly; instead, they use proxy signals that follow climatic signals closely yet indirectly. d18O, the ratio of
heavy oxygen (18O) to light oxygen (16O), is a crucial proxy signal and the most important result of laboratory analysis. This ratio is determined by the ambient water conditions (i.e. sea surface temperature and, in some locations, fresh water influx
and precipitation) at the time when a given portion of coral skeleton was formed.
This graph illustrates the close correspondence between the record of
obtained from coral cores from Punta Pitt, Galapagos and instrumental
measurements of sea
surface temperatures (SST) from Puerto Chicama, Peru (8S, 79W). Notice
that red spikes in the d18O
record match up with red spikes in the SST record and with the yellow
zones that indicate ENSO warm phases. These red areas indicate periods
when water temperatures were above the average. Remember that in the Galapagos,
high water temperatures indicate the eastward movement of the Pacific
warm pool and the reduction of upwelling activity in the eastern Pacific.
As this graph shows, coral d18O
data is nearly as accurate as instrumental data; moreover, coral records
can cover the past 500-800 years of climate
change, while instrumental records are only available for the last 50-100
years in many tropical areas.
NOAA Paleoclimatology Program
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