HIPPO reveals climate surprises

A major pollution-mapping program that ends September 9 has turned up startling trends in climate-warming gases and soot. The data it collected over the past five years from a National Science Foundation aircraft show the tropics periodically belch huge plumes of nitrous oxide — a potent greenhouse gas — into the upper atmosphere. Arctic measurements show that the recent record summer retreats of ice cover have allowed seas there to exhale unexpected amounts of methane, another potent greenhouse gas.

Then there’s soot. Parts of the supposedly pristine Arctic skies host dense clouds of these black carbon particles. During some flights, “We were immersed in essentially clouds of black carbon that were dense enough that you could barely see the ground,” recalls Stephen Wofsy of Harvard University, a principal investigator in the program. “It was like landing in Los Angeles — except that you were 8 kilometers above the surface of the Arctic Ocean.”

Until a few years ago, scientists interested in mapping global emissions of climate-altering pollutants had to rely on Earth-based sensors or satellites’ eyes on the skies. Neither could identify at what altitude the pollutants tended to congregate. They also missed many highly localized or seasonal plumes of natural pollutants.

That all changed when a federal-university research partnership got access to NSF’s research plane: HIAPER (for High Performance Instrumented Airborne Platform for Environmental Research). Throughout a number of periodic runs, this aircraft repeatedly swooped up and down — from 150 meters above Earth’s surface to heights sometimes exceeding 13.7 kilometers (45,000 feet). All along the way, its instruments measured more than 50 greenhouse gases and black carbon.

The unparalleled altitude- and latitude- specific data collected as part of this program — named HIPPO (for HIAPER Pole-to-Pole Observations) — will soon be made available to researchers generally, notes Wofsy. He expects scientists will mine its data for many years, looking for additional climate trends.

Sky-truthing carbon dioxide levels
A primary goal of HIPPO was to investigate how well airborne pollutant concentrations match what computer models had predicted should exist. In some cases, as for soot, HIPPO data pointed to serious problems — oversimplifications — in those models. In other instances, such as for oxygen movement in and out of oceans, the new data generally validated computer predictions.

Currently, land plants and the oceans absorb roughly half of all carbon dioxide emitted, notes Britton Stephens, a scientist with the National Center on Atmospheric Research in Boulder, Colo. But details on which parts of which ecosystems do it, under what circumstances and how efficiently remains somewhat of an open book. Simply put: “We don’t understand their behavior at the current time well enough to predict their behavior into the future,” he says.

So airborne observations have been repeatedly compared to what computer models predict. And one example of where the models need fine tuning involves carbon dioxide, HIPPO indicates.

It revealed “large plumes of carbon dioxide over the Arctic,” Stephens reported Sept. 7 at a news briefing. These plumes didn’t come from the Arctic, he says, but bled into Arctic skies from industrial centers throughout the Northern Hemisphere.

“This was a bit of a surprise,” he says, because models had suggested that much of the carbon dioxide should have been sucked up by plants and seas close to where the gas was being emitted.

Another instance of where the models appear to fall short is on how well the mixing of near-surface parcels of air homogenize carbon dioxide concentrations.

Stephens pointed to data collected earlier this week by the HIAPER aircraft during a run from Kona, Hawaii, to Anchorage, Alaska. “This is the time of year when we see peak uptake [of the gas]” as a result of photosynthesis in land plants, he explains. And HIPPO indeed observed a large depletion of carbon dioxide near the surface, he notes — except “we were measuring it over the middle of the Pacific Ocean.”

Computer analyses had predicted a greater degree of mixing of clean and polluted air parcels, he says. Instead, there were sharp gradients in the gas among closely sampled regions.

Elsewhere, HIPPO offered welcome confirmation of a different model prediction: large plumes of oxygen coming out of the southern oceans during the austral summer. Stephens attributes these massive releases to the uptake of carbon dioxide by photosynthetic bacteria in the warming seas. Currently, it’s winter in the South. And HIPPO has just measured the opposite trend, Stephens says: a seasonal absorption of oxygen by oceans there.

Similar trends for these gases have been observed before throughout the Northern Hemisphere. But HIPPO shows that this normal pattern of winter absorption of oxygen and summer absorption of carbon dioxide “is somewhat decoupled” in the southern oceans. Indeed, Stephens concludes, at times “it was almost more significant that we measured an anti-correlation between oxygen and carbon dioxide than the actual numbers [of how much of either was present].”

Other revelations
More unexpected, Wofsy says, was the March 2010 finding of “a significant excess over the tropics of greenhouse gases — especially nitrous oxide — very high up in the atmosphere. That hadn’t been predicted by any models.”

So radical were the data that his team rushed them into print. Those data show a “bulge” in nitrous oxide emissions between the equator and 20° North latitude.

“It is clear that the enhanced nitrous oxide seen at altitude is a product of tropical emissions lofted to the middle and upper troposphere by convection,” the authors conclude in the Aug. 6, 2011, Geophysical Research Letters.  HIPPO data alone cannot confirm whether the release of this gas represents a “winking on and off” of emissions on time scales of days to weeks, the researchers said, or whether the releases occur more chronically but only occasionally shoot up to altitudes of between 2 kilometers and 14 kilometers.

The best explanation for these data, Wofsy and his coauthors write, is that rainfall or regional flooding spurs production of the gas (probably by soil microbes) — and when this coincides with sharp atmospheric updrafts, the pollutant is propelled high into the skies.

Something too new to fully understand (although a report on it is being prepared for publication), Wofsy says, is a finding of notable concentrations of methane in the Arctic’s atmosphere that trace back to the sea.

“Oceanographers have known for some time that there is production of methane in surface waters of the Arctic,” he says, but “it’s never been observed in the atmosphere.” Those oceanographic data, he says, suggest a source for this methane other than sediments or the melting of icy gas hydrates.

The phenomenon also appears very widespread. “We observed that the ocean surface releases methane to the atmosphere all over the whole of the Arctic Ocean,” Wofsy says.

Climate scientists have been concerned about whether the Arctic Ocean’s loss of summer ice cover might lead, through some feedback mechanisms, to boosting the release of methane. Concludes Wofsy: Thanks to HIPPO, “This hypothesized feedback has been observed for the first time.” And there are hints, he adds, that methane’s source may be something other than melting of gas hydrates.

One notable take-home message from HIPPO: Climate-altering pollution from the Northern Hemisphere — home to 95 percent of humanity — has been migrating everywhere, even into southern skies, says James Elkins of the National Oceanic and Atmospheric Administration, in Boulder. The data are “just very persuasive.”

Indeed, Wofsy adds, after reviewing HIPPO data, you’re left with an impression that pollution associated with human activities has exerted “an overwhelming influence” on Earth’s atmosphere. And that’s not, he adds, reassuring.