Why, despite steadily accumulating greenhouse gases, did the rise of the planet’s temperature stall for the past decade?
MAUNA LOA OBSERVATORY, Hawaii — At nightfall, 11,000 feet up, under the summit of a looming volcano, the black lava moonscape cools as the sun’s tropical heat escapes upward. Settling, subsiding, some of the world’s purest air — a sample of the entire central Pacific atmosphere — descends on the dusk, cloaking Mauna Loa in stillness.
That’s when John Barnes flips on his emerald-tinged laser and shoots it into the sky.
While it can reach up 60 miles, the primary target of Barnes’ laser is the stratosphere, the cold, cloudless layer that sits atop the planet’s bustling weather — home of commercial jets and the ozone hole. The laser’s concentrated 20 watts, a green beam visible miles away from the volcano, reflect off any detritus in its path, these wisps of evidence collected by the observatory’s three large mirrors.
Barnes has kept a lonely watch for 20 years. Driving the winding, pothole-strewn road to this government-run lab, he has spent evening after evening waiting for the big one. His specialty is measuring stratospheric aerosols, reflective particles caused by volcanoes that are known to temporarily cool the planet. Only the most violent volcanic eruptions are able to loft emissions above the clouds, scientists thought, and so Barnes, after building the laser, waited for his time.
For nearly 20 years, John Barnes has fired his green laser into the skies above Hawaii’s Mauna Loa volcano, monitoring particles suspended high above the weather. He waited. And waited.
To this day, there hasn’t been a major volcanic eruption since 1991, when Mount Pinatubo scorched the Philippines, causing the Earth to cool by about a half degree for several years. But Barnes diligently monitored this radio silence, identifying the background level of particles in the stratosphere. And then, sitting in his prefab lab four years ago, not far from where Charles Keeling first made his historic measure of rising atmospheric carbon dioxide levels, Barnes saw something odd in his aerosol records.
“I was just updating my graph, and I noticed that, ‘Hey, this is increasing,'” Barnes said during a recent interview. It was unexpected. Where were these particles coming from, without a Pinatubo-style eruption? “No one had seen that before,” he said.
Barnes had uncovered a piece of a puzzle that has provoked, frustrated and focused climate scientists over the past half decade. It is a mystery that has given cover to forces arrayed against the reality of human-driven global warming. And it is a question that can be easily stated: Why, despite steadily accumulating greenhouse gases, did the rise of the planet’s temperature stall for the past decade?
“If you look at the last decade of global temperature, it’s not increasing,” Barnes said. “There’s a lot of scatter to it. But the [climate] models go up. And that has to be explained. Why didn’t we warm up?”
The question itself, while simple sounding, is loaded. By any measure, the decade from 2000 to 2010 was the warmest in modern history. However, 1998 remains the single warmest year on record, though by some accounts last year tied its heat. Temperatures following 1998 stayed relatively flat for 10 years, with the heat in 2008 about equaling temperatures at the decade’s start. The warming, as scientists say, went on “hiatus.”
The hiatus was not unexpected. Variability in the climate can suppress rising temperatures temporarily, though before this decade scientists were uncertain how long such pauses could last. In any case, one decade is not long enough to say anything about human effects on climate; as one forthcoming paper lays out, 17 years is required.
For some scientists, chalking the hiatus up to the planet’s natural variability was enough. Temperatures would soon rise again, driven up inexorably by the ever-thickening blanket thrown on the atmosphere by greenhouse gases. People would forget about it.
But for others, this simple answer was a failure. If scientists were going to attribute the stall to natural variability, they faced a burden to explain, in a precise way, how this variation worked. Without evidence, their statements were no better than the unsubstantiated theories circulated by climate skeptics on the Internet.
“It has always bothered me,” said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research. “Natural variability is not a cause. One has to say what aspect of natural variability.”
Trenberth’s search has focused on what he calls “missing energy,” which can be thought of as missing heat. The heat arriving and leaving the planet can be measured, if crudely, creating a “budget” of the Earth’s energy. While this budget is typically imbalanced — the cause of global warming — scientists could account for where the heat was going: into warming oceans or air, or melting ice. In effect, the stall in temperatures meant that climatologists no longer knew where the heat was going. It was missing.
The hunt for this missing energy, and the search for the mechanisms, both natural and artificial, that caused the temperature hiatus are, in many ways, a window into climate science itself. Beneath the sheen of consensus stating that human emissions are forcing warmer temperatures — a notion no scientist interviewed for this story doubts — there are deep uncertainties of how quickly this rise will occur, and how much air pollution has so far prevented this warming. Many question whether energy is missing at all.
For answers, researchers across the United States are wrestling with a surge of data from recent science missions. They are flying high, sampling the thin clouds crowning the atmosphere. Their probes are diving into deep waters, giving unprecedented, sustained measures of the oceans’ dark places. And their satellites are parsing the planet’s energy, sampling how much of the sun’s heat has arrived, and how much has stayed.
“What’s really been exciting to me about this last 10-year period is that it has made people think about decadal variability much more carefully than they probably have before,” said Susan Solomon, an atmospheric chemist and former lead author of the United Nations’ climate change report, during a recent visit to MIT. “And that’s all good. There is no silver bullet. In this case, it’s four pieces or five pieces of silver buckshot.”
This buckshot has included some familiar suspects, like the Pacific’s oscillation between La Niña and El Niño, along with a host of smaller influences, such as midsize volcanic eruptions once thought unable to cool the climate. The sun’s cycles are proving more important than expected. And there are suspicions that the vast uptick in Chinese coal pollution has played a role in reflecting sunlight back into space, much as U.S. and European pollution did decades ago.
These revelations are prompting the science’s biggest names to change their views.