Tree rings reveal El Niño tied to recent global warming

Every seven years – or sometimes every two years, or maybe four years – El Niño turns a bucket of rainfall over western South America, sending floodwaters cascading over coastal Ecuador and typhoons spiraling into Peru. Other parts of countries get no rain at all, withering in a crippling dry heat. Fish die, and so do the birds that live off them. The economy of those coastal countries buckles. Nations wait for respite.

For all the damage it does, El Niño, a rise in water temperatures off western South America's coast, is difficult to track. Currently, National Oceanic and Atmospheric Administration forecasts only go out six months. On Monday, the Proceedings of the National Academy of Sciences published a report by lead researcher Josef Ludescher of Germany's Giessen University that suggests a method of forecasting that could extend that outlook out to one-year. [Editor's note: Updated July 2, 2013]

But new research suggests that the phenomenon was uncharacteristically active in the late 20th century, relative to the previous seven centuries. That in turn suggests that El Niño is more responsive to climate change than previously believed and that it might be possible to better predict future conditions.

El Niño is not a well-understood phenomenon. So, pinning its behavior on manmade global warming is challenging, since El Niño waxes and wanes like a fair-weather friend over relatively short lengths of time. Those short busts and booms could be pegged to short-term, natural climate changes, as opposed to a long-term, warming of the Earth.

But this latest research, published in Nature Climate Change, looks with longer hindsight on the phenomenon, reflecting on some 700 years of its behavior. That long look back revealed some clear patterns that suggest the phenomenon is waltzing in step with global warming.

Scientists compiled some 2,222 tree-ring chronologies of the past seven centuries from both the mid-latitudes in both hemispheres and the tropics. Tree rings can provide an accurate record of historical climate pattern, packing in their width and color information about the precipitation, wind, and temperature conditions at the time at which the tree was growing.

Scientists found that the tree ring patterns in the 20^th -century suggested that El Niño had been more active then than during the last seven centuries, meaning  that long-term El Niño patterns have dovetailed with the global warming that characterized that century.

“This suggests that many models underestimate the sensitivity to radiative perturbations in greenhouse gases,” said Shang-Ping Xie, co-author and meteorology professor at the International Pacific Research Center. “Our results now provide a guide to improve the accuracy of climate models and their projections of future ENSO activity.”

“If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts,” she said.