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NASA Tree-Ring Study Reveals Long History of El Niño
December 9, 2002NASA TREE-RING STUDY REVEALS LONG HISTORY OF EL NIÑO
El Niño is not a new weather phenomenon, according to a recent NASA study that looks 750 years into the past using tree-ring records.
Utilizing special computer techniques, a NASA scientist has linked tree-ring widths — a natural record of local and regional climate conditions — with sea surface temperatures (SST) to compile a record that looks back three-quarters of a millennium, indicating that El Niño caused heavy rain in some places in South America and droughts in other areas.
“We feed the computer model with past tree-ring data, and this model ‘hind casts’ past sea surface temperatures,” said Hector D’Antoni, a scientist at NASA Ames Research Center in California’s Silicon Valley. “We can go back in time and reconstruct some of the factors controlling ecosystems.” An ecosystem is a system composed of living organisms and their environment.
“The hypothesis I had all along was that the El Niño Southern Oscillation (ENSO) is not a new component of the global climate system, and that ENSO effects on South America were not new or negligible,” he said. “Sea surface temperatures of both the Atlantic and Pacific oceans in tropical and subtropical locations have a strong influence on the temperate forests of South America. Therefore, one can expect to find some ‘signal’ of these drivers in the collection of tree rings over a period of time.”
“Precipitation is related to the ocean-atmosphere interface and, in South America, predominantly dominated by the Atlantic Ocean,” he said. “The El Niño ENSO affects these patterns in four regions with large increases of precipitation (Ecuador, Argentina), drought (Northern Amazonia) and higher temperature (Ecuador, eastern Brazil). These changes affect tree growth in these and other regions of South America.”
D’Antoni developed a connection between rate of tree growth and sea surface temperatures using neural network software models. Using these models, he estimated past sea surface temperatures based only on tree ring widths. Wider tree rings indicate more tree growth. Precipitation and temperature control much of this growth.
Neural network models “learn” by observing patterns in today’s world and then make precise estimates. D’Antoni obtained tree-ring data produced by scientists who study the annual growth rings in trees. These researchers collected data from 25 sites, largely in the sub-Antarctic region of South America. With computer models, tree-ring width records and sea surface temperature data, D’Antoni established a pattern for the last few hundred years. When linked with sea surface temperatures, tree-ring growth patterns are proving to be exceptional starting points for researchers who are reconstructing past and predicting future climates.
D’Antoni and co-investigator Ante Mlinarevic of San Jose State University, San Jose, Calif., reconstructed past sea surface temperatures of the Atlantic and Pacific oceans for the period 1246 to 1995.
“The Atlantic SST appears more stable around 24.5 Celsius (76.1 degrees Fahrenheit); Pacific past SST varies in a much larger range around 21 degrees Celsius (69.8 degrees Fahrenheit) and reflects ENSO episodes in the past that are longer than the instrumentally recorded ones,” D’Antoni said.
One record kept by archeologist Jorge Marcos, Polytechnic School of the Littoral, Guayaquil, Ecuador, mentions “albarradas” (archeological and small historical dams) built by the people of Ecuador and copied by the European conquerors, D’Antoni said. Albarradas turned the damaging effect of runoff during ENSO episodes into a way of replenishing the groundwater table and aiding agriculture on the dry coast of Ecuador, according to D’Antoni.
The older albarradas are 2,270 years old, according to radiocarbon dating. These dams were in extensive use 1,000 years ago, and some albarradas are still being used.
D’Antoni stressed that records such as those of Marcos provide “circumstantial” evidence of the historical ENSOs. “Our reconstruction suggests that there were many ENSOs, some very intense ones, in the last 750 years,” D’Antoni said. “All of these experiences amount to a stronger support for prediction of future changes, which is one of NASA’s goals.”
While his findings eventually could lead to attempts by scientists to make long-range forecasts of levels of rainfall, humidity and other consequences of major climate changes, D’Antoni said he is still conducting basic science and is not ready to attempt climate change predictions. His immediate objective is simply to learn more about Earth’s climate, he said.###
NASA Ames Research Center, Moffett Field, Calif.
650/604-5026 or 650/604-9000
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