Flying HIPPO Samples Earth's Atmosphere

The conclusion of a three-year mission promises to provide the 'first global portrait of greenhouse gases.'

When it comes to measuring greenhouse gas and particle emissions in the atmosphere, researchers have largely been confined to sampling from ground stations. While the information collected has added immensely to scientific understanding of global warming pollution, relying on those ground stations alone has been far from ideal. For one thing, they may be several thousand miles apart, leaving gaps in the data that have to be filled by computer models that estimate how those gases are distributed vertically through the atmosphere.

"Tracking carbon dioxide and other gases with only surface measurements has been like snorkeling with a really foggy mask," said Britton Stephens, a scientist with the National Center for Atmospheric Research (NCAR) in a press release on Wednesday. That release was issued to announce the conclusion this week of a three-year project designed to fill in the gaps and allow researchers to create "the first global portrait of greenhouse gases."

Key to the mission has been a Gulfstream V aircraft, owned by the National Science Foundation (NSF) and operated by NCAR, and equipped with a suite of specially-deigned instruments to sample a wide range of constituents in the atmosphere. The plane is known as the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER), and the three-year project, which has involved five flight legs from Arctic to Antarctic, is called HIPPO, for HIAPER Pole-to-Pole Observations.

"With HIPPO, we now have views of whole slices of the atmosphere," said Steven Wofsy, of Harvard University's School of Engineering and Applied Sciences in the same press release. "We've been quite surprised by the abundance of certain atmospheric components and the locations where they are most common."

During a conference call with reporters on Wednesday, Stephens noted that, "Of all the carbon dioxide emitted by human activities, approximately one-half is taken up by land plants and the oceans." When attempting to model future climate scenarios, he continued, "the first source of uncertainty is what people will do. The second is what plants and oceans will do. Some models predict that, by 2050, they will continue to take up half. Some predict by then it will be almost none. The great thing about HIPPO is that we now have a clear picture of how CO2 mixes in the atmosphere. We've been able to see exactly how much CO2 goes in and out of land plants."

Wofsy added that the HIPPO flights were unlike most such sampling missions, which may maintain a steady altitude while researchers stare at their on-board instruments.

"The airplane is constantly visiting the surface," he explained during the call. "Every two degrees of latitude, it would descend to 500 feet or even lower, and then back up to 30,000 feet or higher. It was a very exciting mission to be on. You're actually visiting these places."

At times during the flights, Wofsy continued, "you could really visualize the rapid change in the Arctic," such as on one occasion when the plane became "immersed in dense clouds of black carbon, like smog in Los Angeles. The recession of ice that has occurred over the last decade and a half was also very obvious to us."

Having gathered the data, researchers now face what may be a decade-long analysis; in the meantime, the database will be made publicly accessible, while some preliminary observations are already clear. While the Intergovernmental Panel on Climate Change (IPCC)has long predicted high levels of greenhouse gas emissions from the tropics (much in the form of nitrous oxide from tropical soils), HIPPO surveys suggest that those estimates are too low. Conversely, they indicate that climate models tend to over-predict how much black carbon is in the upper atmosphere.

Of particular interest, said Wofsy, were observations of methane, a potent greenhouse gas, being released from the Arctic Ocean.

"People have been concerned that Arctic warming would lead to feedbacks of methane release," he said. "We found that the ocean surface is releasing methane over the whole of the Arctic Ocean, at least in the Atlantic Ocean region we observed. It is not clear that this is being released from previously-identified reservoirs, but just by removing the cover of sea ice, we are enabling that release to happen. We can't say how globally significant it is. It appears to be of a sufficient size to be globally significant, but we can't quantify it yet."

Photograph: The NSF/NCAR Gulfstream V in Anchorage, Alaska. (UCAR, photo by Carlye Calvin) UCAR/NCAR video shows the flight plan for the fifth of HIPPO's five transects.)