A new study reports that global warming may have hit a speed bump and could go into hiding for decades.
Earth's climate continues to confound scientists. Following a 30-year trend of warming, global temperatures have flatlined since 2001 despite rising greenhouse gas concentrations, and a heat surplus that should have cranked up the planetary thermostat.
"This is nothing like anything we've seen since 1950," Kyle Swanson of the University of Wisconsin-Milwaukee told Disocvery News.
"Cooling events since then had firm causes, like eruptions or large-magnitude La Ninas. This current cooling doesn't have one," Swanson added.
Instead, Swanson and colleague Anastasios Tsonis think a series of climate processes have aligned, conspiring to chill the climate. In 1997 and 1998, the tropical Pacific Ocean warmed rapidly in what Swanson called a "super El Nino event."
It sent a shock wave through the oceans and atmosphere, jarring their circulation patterns into unison.
When added up with the other four years since 2001, Swanson said the overall trend is flat, even though temperatures should have gone up by 0.2 degrees Centigrade (0.36 degrees Fahrenheit) during that time.
The discrepancy gets to the heart of one of the toughest problems in climate science, identifying the difference between natural variability from human-induced change.
But just what's causing the cooling is a mystery.
Sinking water currents in the north Atlantic Ocean could be sucking heat down into the depths. Or an overabundance of tropical clouds may be reflecting more of the sun's energy than usual back out into space.
"It is possible that a fraction of the most recent rapid warming since the 1970's was due to a free variation in climate," Isaac Held of the National Oceanic and Atmospheric Administration in Princeton, New Jersey told Discovery News. "Suggesting that the warming might possibly slow down or even stagnate for a few years before rapid warming commences again," he added.
"When the climate kicks back out of this state, we'll have explosive warming. Thirty years of greenhouse gas radiative forcing will still be there and then bang, the warming will return and be very aggressive," Swanson said.