1000 Years of El Niño on the Nile
Flood gauges used for millennia along the Nile River in Egypt are helping scientists trace the history of El Niño’s global influence. The records show that El Niño has reared its head more often in the 1980s and 1990s than in any other pair of decades in the past 10 centuries, according to a report in the 15 February Geophysical Research Letters.
Researchers look for clues preserved in corals, mollusk shells, and ice cores to unravel the past comings and goings of El Niño, a warming of the tropical Pacific Ocean that influences world weather patterns. Most recently, banded sediments in a lake bed in Ecuador suggested that El Niño rarely struck between 5000 and 12,000 years ago (Science, 22 January, p. 467). Most of these data lack the year-to-year details that climatologists crave. However, simple river gauges called "nilometers" have measured the seasonal height of the Nile River since the pharaohs reigned. Accurate records of these flows date back to 622 A.D.
Drawing upon work by the late oceanographer William Quinn of Oregon State University in Corvallis, hydrologists Elfatih Eltahir and Guiling Wang of the Massachusetts Institute of Technology in Cambridge analyzed the El Niño-Nile connection through 1997. Correlating Nile river heights with records of Pacific Ocean temperatures dating to 1872, the duo found that about 30% of the Nile's annual fluctuations in water level could be linked to El Niño. The ocean warming tends to cause droughts in parts of Africa with tributaries that feed the Nile, lowering its level. La Nina coolings, on the other hand, seem to trigger rainfall there. "There is a lot of variability, but the Nile flood is a good sensor for El Niño," Eltahir says. Analysis of the full set of water level patterns shows that El Niño has flared up more frequently, and with longer durations, since the mid-1970s than in any 2-decade interval since the end of the last millennium. If that trend continues, Eltahir argues, it would be a "clear sign" of a major change in global climate--but not necessarily due to human influences, he notes.
"This is very useful baseline data," says archaeologist Daniel Sandweiss of the University of Maine in Orono. "Few records allow us to look at El Niño frequencies with any detail prior to written records from the mid-1500s." But he agrees with Eltahir that using El Niño as an indicator of human impacts on global climate is risky, because the active El Niño period 1000 years ago obviously wasn't triggered by industrial emissions of greenhouse gases.
--Robert Irion