Nature, 17 July 2003
Beach combing suggests El Niño is independent from overall global climate
When local officials turned the yacht away from the white sandy beaches of Kiritimati Island in the tropical Pacific Ocean, the crew's hearts sank. But Kim Cobb and her colleagues were not hankering after long drinks in the sun. They had hoped to delve into past climate by drilling into rare deposits of fossil coral.
Happily, the nearby safe haven of Palmyra Island turned out to be an unexpected goldmine. Its corals revealed that, out of five time windows between AD 928 and 1998, the largest short-term climate variability occurred between 1635 and 1670 - rather than during the global-warming-ridden twentieth century.
This is interesting because Palmyra lies at the centre of Earth's most powerful climate rollercoaster, El Niño. This cycle, which repeats every three to seven years, transfers huge amounts of heat across the equatorial Pacific Ocean.
Cobb, now at the California Institute of Technology in Pasadena, and her colleagues analysed corals that were probably ripped off their reefs and swept onto the island during large storms during the past millennium. The fossils reveal the temperature and salinity at the time when the corals were growing.
From deposits of different vintages along Palmyra's beaches, the team reconstructed the month-to-month variability of past climate in the region. The new picture of the past places the present in perspective. The strong El Niño cycles that we have experienced in recent decades are well within the natural range, despite the average climate having already shown clear signs of human influence. What's more, the corals hint that El Niño is independent of the mean climate, at least within the range of past temperatures. So the cycle may not be as sensitive to global warming as has been feared.
The equatorial Pacific is like a giant bathtub - a layer of warm water sits atop colder bottom waters. Easterly trade winds constantly sweep its sun-heated surface to the Indonesian side, exposing cold water at the Chilean and Peruvian coasts. When El Niño strikes, these trade winds slacken, allowing warm water to gather off Chile and Peru. The resulting heat flow has profound effects on climate and ecology on both sides of the ocean.
The pendulum's return swing is called La Niña. This brings cool water to the South American coast, while a new warm pool gathers off Southeast Asia, ready for the next cycle.
The worldwide effects of this oscillation can be dramatic - perhaps unsurprisingly, given that the Pacific Ocean spans two-fifths of the Equator and stores an enormous amount of heat. The 1997-98 El Niño, for example, caused severe droughts that set vast areas of Indonesian forest ablaze. South America, meanwhile, suffered storms and flooding. During a typical warm phase, Pacific skipjack tuna go east and Argentinian maize yields go up; rainfall in Southern Africa slumps, and the northern United States enjoys warm, dry winters.
Computer models - our only chance of a dry run at climate change - have not been forthcoming on
the subject of El Niño's future. Different models respond so differently to global warming that they yield evidence to suit every political agenda.
Hence the excitement about Cobb's new climate record. It shows El Niño playing to its own tune, apparently without much connection to mean global climate or greenhouse-gas emissions - so far. No wonder computer models are divided. But this independence will not save us from torrential rains or blazing droughts caused by climate change - there is now a scientific consensus that human interference is probably exacerbating extreme weather events.
Strictly speaking, a question mark remains: mean climate needs to be assessed from independent data. Cobb's group used their own climate reconstruction, supplemented by others based mainly on tree rings from regions outside the tropics. Tropical mean climate over the past millennium is still a great unknown. The team hopes to probe more corals to resolve this problem - perhaps even those on Kiritimati Island.
Cobb, K. M., Charles, C. D., Cheng, H. & Edwards, R. L. El Niño/Southern Oscillation and tropical Pacific climate during the last millennium. Nature, 424, 271 - 276, (2003)
17 July 2003