Why Global Warming Slowed in the 2000’s: Another Possible Explanation

The world is getting progressively warmer, and the vast majority of evidence points to greenhouse gases spewed into the atmosphere by humans — carbon dioxide (CO2), especially — as the main culprit. But while the buildup of greenhouse gases has been steadily increasing, the warming goes in fits and starts. From one year to the next it might get a little warmer or a lot warmer, or even cooler.

That’s because greenhouse gases aren’t the whole story. Natural variations in sunlight and ocean currents; concentrations of particles in the air, manmade and otherwise; and even plain old weather variations can speed the warming up or slow it down, even as the underlying temperature trend continues upward. And while none of those factors is likely to change that trend over the long haul, scientists really want to understand how they affect projections of where our climate is heading.

The latest attempt to do so just appeared in Science Express, the online counterpart of the journal Science, where a team of climate scientists is reporting on their investigations of airborne particles, or aerosols, in the stratosphere. It’s well known, says co-author John Daniel, of the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo., that these particles have a cooling effect, since they reflect sunlight that would otherwise warm the planet.

Mt. Pinatubo's erruption in the Philippines, in 1991. Credit: USGS.

It’s also well known that major volcanic eruptions, like Mt. Pinatubo’s in the Philippines in 1991, can pump lots of aerosols into the stratosphere — and indeed, Pinatubo alone temporarily cooled the planet for about two years. The explosion of Mt. Tambora in 1815 had even more catastrophic effects, which you can imagine given that 1816 came to be known as “the year without a summer.” But what lots of people thought, says Daniel, “is that since there haven’t been any eruptions on that scale recently, aerosols have become relatively unimportant for climate.”

That, says the study, is not true: even without major eruptions, aerosols in the stratosphere increased by about 7 percent per year from 2000 to 2010. Plug that figure into climate models, and they predict a reduction in the warming you’d otherwise expect from the rise in greenhouse gases by up to 20 percent.

In the real world, as it happens, the rise in temperature slowed during that same decade. “That,” says Daniel, “was the motivation for doing this research. It could have just been natural climate variability, but we wondered if it could be something else.” Some climate scientists attribute the slowdown to heat being temporarily stored in the deep oceans, but stratospheric aerosols could clearly be part of the answer as well.

Whether these aerosols are natural or manmade, however, is something the scientists didn’t address. Just last week, a paper in Proceedings of the National Academy of Sciences (PNAS) suggested the cause was a construction boom of coal-fired power plants in China over the same decade. The new study doesn’t necessarily contradict that. “Human emissions could play a role,” says Daniel, although the PNAS study was talking about aerosols in the lower atmosphere, not the stratosphere. “But even in the absences of colossal volcanic eruptions,” he says, “smaller eruptions could still add up.”

The other difference between the two studies is that the one from last week looked at the relatively slow temperature rise over the most recent decade and tried to tease out what might have changed since the previous decades, when the warming was faster. The new one took actual observations of aerosols and tried to predict what the temperature rise should be. That sort of approach tends to produce more credible results, since an incorrect prediction would stick out like a sore thumb.

Where the two studies emphatically agree is that if the level of aerosols goes down — due to a lull in eruptions, or a reduction in coal-plant pollution, or both — the pace of warming would likely pick up. That would mean that current projections for up to a 4.5°C increase in global average surface temperatures by the end of the century might turn out to be an underestimate. And if aerosol levels increase, the temperature in 2100 could be lower than everyone expects.