"If the planet is warming, why have temperatures been steady for a decade?"
That question is now the go-to counterpoint for global-warming skeptics, and it has long been a sticking point for scientists as they try to explain their climate conclusions to an increasingly polarized public.
The debate was reborn anew last week when a leaked draft of the United Nations' Intergovernmental Panel on Climate Change upcoming report conceded that warming has largely paused over the past decade, prompting outcry from skeptics and leading conservative news outlets (including Fox News) to play up the pause in their reporting.
Climate scientists largely agree that warming has paused over the past decade (especially in measurements of surface temperature), but they say that break is temporary, and the near-consensus on human-caused global warming remains unbroken.
"There has been a slowdown or hiatus in the rate of change of global temperature in the 21st century, and that's real," says David Gutzler, an earth- and planetary-sciences professor at the University of New Mexico who contributed to the IPCC report. "Most of us think that this is probably a temporary hiatus as opposed to a cessation of global warming."
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.
While both millennial and decadal fluctuations in solar strength can contribute to changes in Earth's climate, scientists say the effects of increasing greenhouse gases in the atmosphere far outweigh our sun's effects.
Greenhouse gases heat the planet, other pollutants cool it
The production and use of fossil fuels and greenhouse gases often produces soot and ash, pollutants that rise into the air. There, they can change the physical properties of clouds by making them more reflective. Like volcanic aerosols, these cloud-dwelling pollutants block the sun's heat.
The clouds reflect more of the sun's energy back into space, keeping the surface temperature of the Earth from rising. In an interesting twist, programs that aim to reduce air pollution in heavy industrial areas could end up freeing some room for the sun's rays to penetrate the atmosphere.
The oceans have been stockpiling extra heat
As the Earth accumulates heat, that additional energy has to go somewhere — deep inside the high seas. The Earth's oceans have become warmer since 1955, according to the Environmental Protection Agency.
The ocean-atmosphere system works like this: As oceans store heat energy, the rate at which the atmosphere warms slows. Oceans don't always take in heat the same way — changes in currents and temperatures that occur naturally can change the rate of uptake every few decades, said Gutzler, altering the surface temperature of the Earth. In the last decade, the oceans' absorption of heat has contributed to cooling of that temperature rather than warming. But again, this decadal fluctuation can cloud the bigger picture.
It's all really, really complicated
Climate models incorporate vast numbers of dynamic factors, everything from melting permafrost in Russia to coal development in China to deforestation in the Amazon. Scientists are still scrambling to provide a more nuanced understanding of what global warming will look like. Indeed, many of the forces that checked warming over the past decade may accelerate it in years to come.
Within that butterfly-effect-like chaos, Gutzler said it's possible that the predictive climate models scientists use are partially wrong — not about the fact that the planet is warming, but about how, when, and where that warming will occur.
"Just like weather, once you get a month or two out, individual events are unpredictable, like decadal events are unpredictable," he said.