El Niño, the climate pattern that increases Pacific Ocean surface temperatures every three to seven years, has long been known to pummel the Sierra Nevada with snow, limit Peruvian anchovy fishermen’s harvest and bless the Hawaiian Islands with dry, beach-friendly weather. The question of whether the effects of El Niño have become more extreme in recent decades, as climate change has intensified, hasn’t accrued a consensus among scientists. But now, new research released last week, sponsored by the National Science Foundation and published in Science, strengthens the link between El Niño activity and climate change.
During an El Niño season–the next continues to be delayed but is expected to begin later this year–the force of trade winds in the western and central Pacific diminishes or even reverses, causing a spike in surface water temperatures. As the slackened winds allow–or the reversed winds slowly push–the warmer water east across the ocean, rainfall follows it.
El Niño and its cold-water counterpart La Niña, which occurs between El Niño episodes when the regular trade winds intensify their westward push, have global ramifications. Wildfires in Australia and famines in India have been associated with the climate pattern. The cycle of El Niño and La Niña also appears to have intensified in recent years. Searching for reasons why, scientists debated a link with climate change as long ago as 1997, when researchers at the National Center for Atmospheric Research published a study titled “El Niño and Climate Change.” They couldn’t identify a clear connection, but they believed there was an unidentified force at work–one that required further investigation. “t least part of what is happening… can not be accounted for solely by natural variability,” they wrote.
A year later, experts at the Nevada-based Western Regional Climate Center, which disseminates climate data and conducts research, also contemplated whether global warming was goosing El Niño. They were more overtly suspicious of a linkage, but again, lacked specific evidence. In a post on the center’s website, they noted:
It is plausible that a warmer earth would produce more and stronger El Niños. There is some evidence that the earth has warmed over the past two decades, and there is no doubt that El Niño has been much more frequent in that time. If the evidence of a warming earth is taken at face value (not universally accepted), there still remains a wide spectrum of opinions on whether we are seeing a manifestation of human modification of global climate, or whether the natural climate system would be exhibiting this behavior anyway.
In the new study, conducted by the Georgia Institute of Technology and the Scripps Institute of Oceanography, scientists traveled to the central tropical Pacific, where the variations in El Niño-driven temperature and precipitation patterns are most acute. Studying the region’s coral gave them a window into the historical effects of El Niño.
They extracted core samples from large coral rocks that had been pushed by storm activity onto Christmas (Kiritimati) and Fanning Islands, tiny spits of land within Kiribati’s Northern Line Islands. Using radioactive dating, they ascertained the ages of 17 samples, each of which spanned 20 to 80 years in time, allowing them to create a patchwork timeline covering 7,000 years.
Then they looked at the ratio of oxygen isotopes within the coral skeletons as a way of measuring variations in weather patterns. Since temperature and rainfall affect isotope ratios, they were able to glean the environmental conditions present during each phase of the corals’ lifespans. Dips and surges in rain and sea surface temperatures left an imprint in the coral samples, and in their analysis, scientists found significantly more intense and variable El Niño activity in the 20th century than most other periods represented.
“The level of variability we see in the 20th century is not unprecedented,” said the study’s lead author, Georgia Institute of Technology’s Kim Cobb in a statement, noting a similarly severe period in the 17th century. “But the 20th century does stand out, statistically, as being higher than the fossil coral baseline.”
The researchers reluctantly went a step further to connect the increase in El Niño activity to climate change: “We kind of answered the question, is El Niño changing with respect to recent natural variability?” said Cobb. “The answer is yes, tentatively so.” Yet despite the bounty of new data, researchers say they would need to go back even further in time to make a more definitive linkage between climate change and El Niño activity.
They were less ambiguous about the impact of the study on future climate change research. The new data will help other scientists investigate past climate change events in both paleoclimate records and model simulations, Cobb said. “Prior to this publication, we had a smattering of coral records from this period of interest,” she explained. “We now have tripled the amount of fossil coral data available to investigate these important questions.”