"Abrupt changes in water temperatures occurring over intervals of up to 25 years suggest that global warming may result as much from natural cyclical climate variations as from human activity," said Benjamin Giese, oceanography professor in the College of Geosciences.
"Climate models constructed here at Texas A&M University were used to analyze ocean surface temperature records in the tropical Pacific since 1950. The results suggest that as much as one-half of all global surface warming since the 1970's may be part of natural variation as distinct from the result of greenhouse gases,"
Giese and graduate student Amy J. Bratcher published the results of their analysis in the Oct. 8 issue of Geophysical Research Letters.
Surface air temperature records maintained over the past 120 years serve as the main evidence for hypotheses linking global warming to increased greenhouse gases generated by manmade (anthropogenic) causes. These records show the average global air temperature has risen by about one-half degree Centigrade over the last 50 years. But while the general air temperature trend seems to be undisputedly upward, this upward trend varies considerably.
"How much of this variability is attributable to natural variations and how much is due to anthropogenic contributions to atmospheric greenhouse gases has not yet been resolved," Giese said. "Recent studies indicate that it is difficult to separate intrinsic natural variance from anthropogenic forcing in the climate system."
Giese believes their analysis of tropical Pacific Ocean data indicates long-term upward changes in ocean temperatures precede global surface air temperature changes by about four years. These ocean temperature fluctuations are in turn preceded by an increase in subsurface water temperatures by about seven years.
"Thus, the results suggest that much of the decade to decade variations in global air temperature may be attributed to tropical Pacific decadal variability," Giese observed. "The results also suggest that subsurface temperature anomalies in the southern tropical Pacific can be used as a predictor of decadal variations of global surface air temperature."
For example, in 1976 an abrupt change in the temperature of the tropical Pacific Ocean preceded a rise of two-tenths of a degree in global air temperatures.
"This phenomenon looks like El Nino, but with a much longer time scale - El Nino occurs over a period of from nine to 12 months, but this fluctuation lasts for about 25 years," he continued. "In 1976, the ocean temperature change in question occurred very quickly, moving from cooler than normal to warmer than normal in about a year."
Bratcher and Giese report that now conditions in the tropical Pacific are similar to those prior to the 1976 climate shift, except with the opposite sign. If conditions develop in a similar way, then the tropical Pacific could cool back to pre-1976 conditions.
"The subsurface tropical Pacific has shown a distinct cooling trend over the last eight years, so the possibility exists that the warming trend in global surface air temperature observed since the late 1970's may soon weaken," Giese observed.
"This natural variation would help to counter the greenhouse gas warming effect. In fact, careful study reveals that global warming and cooling has occurred in the past in cyclical patterns."
Giese's work involves constructing computer models that incorporate years of weather data to reveal recurring patterns of oscillation and help identify mechanisms that may affect climate. He focuses on climate oscillations that are not directly forced by such things as changing amounts of sunlight, but instead are mechanisms of internal climatic variation for which scientists have as yet isolated no particular cause.
"Our model results terminated at the end of 2001," he said. "Now we're waiting to see what their long-term effects may be on global temperatures.
"Our results don't preclude the possibility that anthropogenic sources of greenhouse gases have contributed to global warming. We're just suggesting that the human forced portion of global warming may be less than previously described."