Tuesday, May 24, 2011

Warmer climate = stronger El Niño? Tree rings say maybe.

How can we learn about ancient climates? Tree rings have been a valuable clue, along with ocean sediments, chemical analysis of slow-growing corals and other record keepers of the natural world.

University of Hawai`i scientists have now found that they can track El Niño events in the rings of trees of the North American Southwest. And they've found that El Niño cycles tend to be stronger during warmer climates.

They are careful not to outright predict that global warming will mean stronger El Niño and La Niña events, but they said the data they found can be valuable in fine-tuning climate models.

El Niño, and its counterpart La Niña, involve large-scale movements of heat within the Pacific, which have related impacts on global climate. The warm phase, El Niño, features a tongue of warm water that moves eastward into the equatorial Pacific, while cooler water represents La Niña and the climate “cool” phase. Associated with them are major changes in rainfall patterns, storms and other weather features. For Hawai`i, El Niño can mean winter drought an increased hurricane activity.

One question: what will happen to El Niño patterns as the global climate warms. And one problem with that question is how to gather the information needed to understand the issue. El Niño has been around for centuries, but scientists' instruments have only been keeping track of climate for decades.

Now, researchers have found that they can track El Niño events in the tree-ring records of the U.S. Southwest over a period of 1100 years.

Meteorologists Jinbao Li and Shang-Ping Xie of the International Pacific Research Center, published their findings in the May 6 issue of Nature Climate Change.

El Niño events, with their wetter winters in the Southwest, create wider tree rings. Cold La Niña conditions promote drought there, and correspondingly narrower tree rings.

To confirm the findings, the team was able to compare the tree ring data with modern instrument data, as well as isotope concentrations in both living and ancient corals from Palmyra Atoll.

“Our work revealed that 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, which shows its signature in their yearly growth rings. 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,” Li said.

Their findings: “During warm phases, El Niño and La Niña events were more intense than usual. During cool phases, they deviated little from the long-term average as, for instance, during the Medieval Climate Anomaly when the eastern tropical Pacific was cool,” said a press release

“Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” Xie said. “Current models diverge in their projections of its future behavior, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming.”

This research was funded by the National Science Foundation, National Oceanic and Atmospheric Administration, Japan Agency for Marine-Earth Science and Technology, National Basic Research Program of China, and the National Natural Science Foundation of China.

© Jan TenBruggencate 2010

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