"We had this puzzle — the concentration of carbon dioxide was over 400 ppm, last year we had record summer heat waves in the U.S., record retreat of Arctic sea ice. All of these things are consistent with the general warming of the climate," Xie said. "Yet, if you plot the global temperature, you see a flattening average over the last 15 years. On the one hand, scientists are saying carbon dioxide is causing the general rise of global temperatures, but on the other hand, in recent years there is no warming, so something very strange is going on."
Xie and his colleagues set out to solve this mystery using climate models to reproduce the long- and short-term trends based on global climate records from the past 130 years. The researchers found that sea-surface temperatures in the tropical Pacific Ocean, in spite of anthropogenic or manmade effects of global warming, were key ingredients in creating the flattening global temperatures seen in the past 15 years.
"In our model, we were able to show two forces: anthropogenic forces to raise global average temperature, and equatorial Pacific cooling, which tries to pull the temperature curve down, almost like in equilibrium," Xie said.
The effect is similar to the El Niño and La Niña cycles, which are parts of a natural oscillation in the ocean-atmosphere system that occur every three to four years, and can impact global weather and climate conditions, Xie explained. El Niño is characterized by warmer-than-average temperatures in the waters of the equatorial Pacific Ocean, while La Niña typically features colder-than-average waters.