So if global warming is still the future, what's causing the temporary pause, and why should anyone worry that more warming is coming?
The IPCC report attributes this hiatus to short-term factors that result in temporary cooling periods, including volcanoes, solar cycles, absorbent oceans, non-greenhouse-gas pollutants, and a string of other temporary-yet-powerful natural forces.
Blame the volcanoes
While greenhouse gases are trapping heat, volcanoes are doing their best to block it out.
Volcanic eruptions send large quantities of sulfur dioxide into the stratosphere, a slice of Earth's atmosphere that begins about eight miles above the earth's surface. These emissions — known as volcanic aerosols — block the sun's light and heat from reaching the lower atmosphere and heating the planet.
The aerosols do not stay in the stratosphere permanently, but they can linger for years. And depending on the frequency and size of volcanic eruptions over a given period, the aerosols' concentration in the atmosphere waxes and wanes. In eras of increasing concentrations, the aerosols form a more effective heat shield around the planet and temporarily work against global warming.
That's exactly what's happening right now, according to a study released in March. Since 2000, volcanic aerosols increased their heat-blocking ability by between 4 percent and 7 percent, according to the study from a team of researchers at NASA, the National Oceanic and Atmospheric Administration, the University of Colorado, and elsewhere.
But while volcanoes are contributing to a temporary slowdown in global warming, they're not a permanent solution, said Colorado professor Brian Toon. "Overall these eruptions are not going to counter the greenhouse effect," Toon said in a statement accompanying the study's release. "Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse-gas emissions from human activity just continue to go up."
The sun is dimmer ... for now
During the 20th century, scientists believed that the sun emitted energy steadily enough that it couldn't significantly affect the Earth's climate. In 2001, a Science study found that solar highs and lows coincided with terrestrial climate cycles. It was a long-term trend — the climate of the northern Atlantic Ocean has warmed and cooled nine times in the last 12,000 years. The sun undergoes small-scale changes in strength too, and throughout most of the 1900s, a considerable increase made scientists wonder if short-term hotter output made for a hotter Earth. But then, by the beginning of this century, the sun's strength declined, and warming of the Earth's surface temperature stalled.
"If the sun had been warming the Earth, that should have come to an end, and we should have seen temperatures start to go the other way," space environment physics professor Michael Lockwood told National Geographic in 2007. But that didn't happen; despite the ebb and flow of solar output, Earth steadily continued to heat up.