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Why has global warming paused? Pacific Ocean's 'engine room' running cool. (+video)

Despite years of record heat, the rate of global warming has been almost zero in recent years, puzzling scientists. The cycles of the tropical Pacific could hold the answer.

The 2013 Arctic ice melt
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Global warming has been put on a 15-year (and counting) hold by a prolonged period of cold ocean temperatures in the eastern tropical Pacific – part of a natural climate pattern that should allow the rate of warming to pick up when the pattern shifts, according to a new study.

During the past 15 years, warming has continued. Indeed, the decade from 2000 to 2009 was the warmest on record globally, with 12 of the 14 warmest years on record falling between 2001 and 2012. But the warming has occurred so slowly that, statistically, the rate of warming per decade could just as easily have been zero, researchers say. This real-world pace was far slower than the pace found in computer simulations of climate change for the same period

The hiatus triggered finger-wagging from some of the more strident climate-change skeptics, as well as chin-scratching among many climate scientists. With carbon-dioxide emissions rising relentlessly to levels not seen in hundreds of thousands of years, how could atmospheric temperatures fail to respond in a stronger way, many asked.

To tackle that question, two researchers at the Scripps Institution of Oceanography in La Jolla, Calif., turned to a sophisticated climate model, but used it in what they say is a novel way.

Typically, researchers feed an array of data into the model and then turn it loose to see how the climate system evolves with time. But for this study, researchers Yu Kosaka and Shang-Ping Xie did not let the model compute sea-surface temperatures in the eastern tropical Pacific – a patch of ocean representing less than 10 percent of the Earth's surface. Instead, they specified those data.

The reason is that the distribution of sea-surface temperatures along the tropical Pacific can have far-ranging effects on climate, Dr. Xie says.

"That's the engine room of the global climate system," he says. "If you mess up the engine room, you'll get a huge response."

The region undergoes both short- and long-term cycles in sea surface temperature. The short term cycles are known as El Niño and La Niña, and they exchange dominance a few times times a decade. They fall within longer-term swings known as the Topical Pacific Decadal Oscillation, during which one phase can hold sway for several decades. The cool phase currently is in place, prompting the researchers to test its influence on the hiatus.

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