Ancient trees reveal that the El Niño and La Niña events that wreak havoc on climate worldwide have been even more extreme than anyone knew, a revelation that suggests wilder swings in the future as the world gets warmer.
El Niño and La Niña are the warm and cold phases, respectively, of the pattern known as the El Niño Southern Oscillation (ENSO) in the eastern half of the tropical Pacific.
Forecasting how this pattern will behave a few months in advance is now routine. However, the task of predicting its long-term behavior is hampered by the relatively short history scientists have of monitoring it.
Now an international team of scientists has peered back into the history of this climate pattern by analyzing trees up to 1,100 years old. [ Earth in the Balance: 7 Crucial Tipping Points ]
If you cut into the trunk of a tree, you can see that it is divided into rings, each one of which represents a tree's growth in a given year. These growth rings can serve as records of the environmental conditions a tree experienced in that year.
The scientists analyzed the North American Drought Atlas, which has data from centuries-old trees to deduce the history of drought in North America, particularly the southwestern United States. They used this data to reconstruct the intensity of El Niño and La Niña events over the past 1,100 years.
During El Niño, the unusually warm surface temperatures in the eastern Pacific lead to changes in atmospheric circulation, causing unusually wetter winters in the southwestern United States and thus wider tree rings (representing more growth of the tree). Unusually cold eastern Pacific temperatures during La Niña lead to drought, less growth and narrower rings.
The team's findings agreed well with sea surface temperature records that scientists had already collected in the tropical Pacific over the course of 150 years. They also closely matched data on ENSO captured in both living corals and ones that lived centuries ago around the Palmyra Atoll in the central Pacific. This suggests the tree ring data reflect how ENSO has behaved for the past millennium.
"The towering trees on the mountain slopes of the U.S. Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño," said study team member Jinbao Li of the University of Hawai'i at Mānoa. "The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1,100 years."
The tree rings reveal that over the centuries, the intensity of this climate pattern has been highly variable, with decades of strong El Niño and La Niña events and decades of little activity. The least variability happened during the Medieval Climate Anomaly in the 11th century, whereas the highest variability occured between the 18th and 20th centuries.
Many of the El Niño and La Niña events of the last millennium were more intense than the ones scientists have direct data on. Overall, the world has seen a trend of increasing swings to extremes over the past millennium, researchers said.
These changes in the intensity of ENSO appear to be linked to the tropical Pacific climate. Samples taken from lake sediments in the Galapagos Islands, the northern Yucatan in Mexico and the Pacific Northwest suggest that the eastern-central tropical Pacific climate swings between warm and cool phases, each lasting from 50 to 90 years. During warm phases, El Niño and La Niña events were more intense than usual, and during cool ones, they deviated little from the long-term average.
Given the current phase of global warming, which is causing temperatures in the tropical Pacific to rise, the world might see "enhanced ENSO variability — more severe El Niños and La Niñas, and more extreme climate conditions around the globe," Li told OurAmazingPlanet.
However, forecasts of how ENSO might behave in the future are complicated by a host of interactions between the ocean and atmosphere, and better climate models are needed before scientists can arrive at such predictions, he added.
The scientists detailed their findings in the May 6 issue of the journal Nature Climate Change.