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Flights, satellites, and carbon

Building a 3-D picture of Earth’s greenhouse gases

by Bob Henson

HIAPER approaching Deadhorse, Alaska

This photo taken from a camera mounted on the wing of HIAPER shows the aircraft approaching Deadhorse, Alaska.

In the beginning, there was the Keeling curve. This world-famous graph of global atmospheric carbon dioxide, showing an inexorable year-by-year rise since 1958, emerged from painstaking measurements begun by Charles Keeling (Scripps Institution of Oceanography).

It was the long atmospheric lifetime of carbon dioxide that allowed Keeling and colleagues to use observations from a single point—the top of Mauna Loa, Hawaii—to chart CO2’s average increase across the globe with confidence. Since the 1970s, NOAA and other partners have built a network of more than 60 ground-level stations worldwide that measure CO2 continuously or weekly. These readings, plus aircraft and satellite data, have solidified our understanding of CO2’s global presence and shed light on some of its regional and seasonal workings.

But there remains a little-sampled world of CO2 variations in time and space, with peaks and dips that are news to most laypeople and poorly understood even by experts. Now, a Japanese satellite and an ambitious three-year field project led by scientists from Harvard University, Scripps, NCAR, NOAA, and elsewhere are beginning to connect the dots. Despite the loss of a different satellite in February, these projects will help fill critical gaps in our knowledge of CO2 as it travels around the globe and into and out of flora, fauna, soil, and oceans.

There’s more than scientific interest in the quest to map CO2’s variations in space and time. Leaders around the nation and the world are watching the research closely as they ponder new agreements to restrict CO2 emissions. Some countries or regions stand to gain credit for preserving forests that absorb carbon. But it’s not yet certain which forests are doing the most, and the answer could have major fiscal implications.

“Huge sums of money could exchange hands based on where the carbon appears to be going,” says NCAR’s Britton Stephens, who shook up the world of carbon assessment in 2007 with a short paper in Science.

Britton Stephens

Britton Stephens. (Photo by Carlye Calvin.)

Mixing it up

Working with 21 coauthors on the Science paper, Stephens pulled together a set of scattered airborne and surface observations and used them to infer seasonal variations in CO2 by altitude. The result surprised many. Scientists had long thought that northern midlatitudes were absorbing far more carbon than the deforestation-ravaged tropics, though they couldn’t tell exactly where the northern carbon was going (thus, the so-called “missing carbon sink”).

Stephens and colleagues examined what was happening to CO2 through the depth of the atmosphere and found that the ebb and flow of seasonal changes in vertical mixing hadn’t been factored correctly into previous models. Based on this, they the came up with a new paradigm: northern forests were likely absorbing far less than believed, and intact tropical forests might be absorbing much more than thought.

The new hypothesis cried out for an explicit test, and a very useful tool came along just at the right time. The NSF/NCAR Gulfstream V jet arrived at Rocky Mountain Metropolitan Airport in 2005. Soon afterward, Stephens gave Ralph Keeling (Scripps) and Steven Wofsy (Harvard) a tour of the G-V, which is also known as the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER).

Wofsy, Keeling, and Stephens ended up joining forces to design a field project that would take advantage of the G-V’s strengths in range, altitude, and interior capacity. The idea was to measure CO2 across the globe over different seasons, altitudes, and latitudes.

The first phase of this project, dubbed HIPPO (HIAPER Pole-to-Pole Observations), unfolded on 8–30 January, with Wofsy as principal investigator. Four subsequent HIPPO phases will play out through mid-2010, with flights about every seven months. Participants hope this will provide just enough data points to capture the broad sweep of the seasonal cycle.

Steven Wofsy

Steven Wofsy with HIAPER. (Photo by Carlye Calvin.)

HIPPO took the G-V on a grand circle around the Pacific (see map) that gave the lead scientists an observational and visceral sense of CO2’s global reach. Harvard’s Wofsy summed up the feel of the HIPPO flights at a press conference, noting: “It’s quite an experience to fly in an airplane at one point in time above the ice sheet—above the floating ice in the Arctic Ocean, with moonlight illuminating it—and then a short while later to be in American Samoa, a lush tropical paradise, and a short while later in New Zealand, and a short while after that in the Southern Ocean. It really gave us a tremendous aesthetic impression of the whole atmosphere and a connectedness of the globe.”

The G-V’s long range and high-flying ability is what made a far-reaching project like HIPPO possible. However, its status as a modified corporate jet also helped. Because Gulfstream V aircraft are used commercially around the world, repairs and spare parts are relatively easy to come by. The day before HIPPO was to begin, the G-V’s main entrance door malfunctioned, but parts arrived quickly and only one day was lost.

“The mission was a great success from an operational standpoint,” says NCAR’s Pavel Romashkin. He gives kudos to Gulfstream and Raytheon, as well as NSF’s Office of Polar Programs and stopover hosts Mark Cunningham (site manager at NOAA’s Samoa Station) and Phil Ambler (Raytheon manager of operations) for smoothing HIPPO’s journey.

Where is all that CO2 going?

Measurements conducted by Scripps and NOAA since 1957 atop Mauna Loa, and by teams at other spots around the globe, confirm that the globally mixed concentration of carbon dioxide in the atmosphere has increased by 1% to 3% per year. Some of the variation is due to economic booms and downturns, such as recessions in the early 1980s and early 1990s, that influence how much CO2 is emitted. However, the main factor appears to be how much CO2 is added by plant burning and decay and pulled out by plant growth in a given year—and in some years, the net total is surprisingly close to zero.

An oft-quoted statistic is that just over half of all human-produced emissions are absorbed roughly equally by oceans and plants, with the rest staying in the atmosphere. However, the statistic hides a range that’s surprisingly wide (see graph). Since 1960, some years have seen as little as 30% of fossil-fuel emissions remaining in the atmosphere, while in other years more than 80% has remained airborne.

Scientists believe much of this variability is due to climate wrenches—from El Niño to volcanic eruptions such as 1991’s Mount Pinatubo—that influence global rainfall and drought. These dictate how much vegetation grows or burns in a given year, especially in the tropics, and thus how much CO2 is taken up. Compared to the major variability in plant uptake, ocean uptake is relatively steady from year to year.

 Graphs showing how global atmospheric concentrations of CO2 have evolved since the late 1950s

The above images—Figures 7.4 (a) and (b) from Chapter 7 of the IPCC’s 2007 Working Group 1 report—show how global atmospheric concentrations of CO2 have evolved since the late 1950s. (a) The increase that would have occurred if 100% of fossil-fuel emissions remained in the atmosphere (top black line) is compared to the actual yearly increases measured by Scripps (narrow bars) and the five-year average increases from Scripps (black) and NOAA (red). (b) The fraction of fossil-fuel emissions remaining in the atmosphere is shown as measured by Scripps each year (narrow bars) and averaged across five-year periods (solid black line). Expressed as a percentage, the fraction varies from around 30% in the early 1990s to around 80% in several other years. (Figures courtesy IPCC.)

Measuring carbon second by second

Along with measuring CO2, HIPPO assessed a wide array of greenhouse gases and other constituents at 1- to 10-second intervals. A mass spectrometer developed by James Elkins and operated by James Moore (both of NOAA) analyzed other constituents once each minute. Much of the equipment in HIPPO was designed expressly for the project. As much as possible, the instruments gathered data in real time rather than gathering air in flasks for later analysis.

HIAPER flight path

The HIPPO project took the NSF/NCAR Gulfstream V (HIAPER) on a three-week circuit around the Pacific that began in Boulder and encompassed Alaska, the Arctic Ocean, Hawaii, American Samoa, New Zealand, Antarctica, Tahiti, Easter Island, and Costa Rica (each shown in numerical order). (Illustration by Steve Deyo.)

“Essentially, we have a flying laboratory that we’re taking around the world, sucking in air and doing the measurements as we go,” Stephens says.

HIPPO scientists will be able to construct vertical as well as latitudinal cross-sections, since the G-V dipped as low as 300 meters (1,000 feet) and soared as high as 14,000 m (47,000 ft) along its route. The project is also drawing on dozens of ground-based CO2 stations, many sponsored by NOAA and the U.S. Department of Energy, in order to help verify and extend the airborne measurements.

“I have never been part of such an exciting field experiment,” said Keeling as HIPPO’s first phase unfolded. “This is the first time anyone has systematically tried to map the distribution of carbon dioxide and related gases from the Arctic to the Antarctic and from the surface to the upper atmosphere. Oceanographers have been doing similar mapping of the ocean for decades. But for the atmosphere, the approach is revolutionary. Each day we get a snapshot of another piece of the world. We are assembling a global picture, flight by flight.”

According to Wofsy, an early look at the first phase of HIPPO results hints that the atmosphere is more uniform in the vertical direction, and more compartmentalized in the horizontal, than in many existing models. However, he says, “this experiment will require some time to mature. We’re going to take four more shots at it, and only after we have all of those can we make a definitive statement.” As was the case for those who discovered King Tut’s tomb, he adds, “we have seen how lovely the jewels are, but we have just gotten our hands on them and we don’t yet know what they mean.”

The first paragraph of this article has been revised to include the correct starting year (1958) of carbon dioxide observations at Mauna Loa, Hawaii.

Despite lost satellite, hopes remain high for sensing CO2 from space

An artist’s conception of Japan’s Ibuki (GOSAT) satellite. (Courtesy Japan Aerospace Exploration Agency.)

An artist’s conception of Japan’s Ibuki (GOSAT) satellite. (Courtesy Japan Aerospace Exploration Agency.)

With society clamoring for more detail on carbon dioxide, space agencies have been pushing to launch CO2 sensors. Until this year, there were two main CO2-sensing instruments in space, both launched in 2002: the Atmospheric Infrared Sounder (AIRS), from NASA, and SCIAMACHY, from the European Space Agency (ESA).

Two new tools were slated to join this group early this year. Japan’s Ibuki (meaning “breath” in Japanese), also known as GOSAT, was deployed on 23 January. However, NASA’s Orbiting Carbon Observatory (OCO) crashed into the ocean near Antarctica on 24 February after a protective shield failed to separate from the satellite after takeoff.

Both GOSAT and OCO had CO2 detection as their primary missions. Whereas AIRS focused on the mid-troposphere, and SCIAMACHY uses a relatively low-resolution spectrometer with limited sensitivity, GOSAT uses higher-resolution spectroscopy to measure CO2 through the entire atmosphere. Flying above swaths of land and ocean, GOSAT employs reflected sunlight to increase its sensitivity in the atmosphere’s lowest kilometer, where CO2 varies the most.

OCO’s trio of spectrometers was to have deduced the presence of CO2 at resolutions of 1 to 2 parts per million, making the instrument three times as precise as any previous satellite-based trace gas sensor. Among the many researchers crestfallen at the loss of OCO were scientists and students at Colorado State University and NOAA/CSU’s Cooperative Institute for Research in the Atmosphere. Graeme Stephens and Denis O’Brien were in charge of primary data processing for CO2 products, while a group led by Scott Denning planned to use the CO2 retrievals to map sources and sinks through a suite of carbon cycle models.

“We’ve been waiting nine years for these data, which would have revolutionized the study of the global carbon cycle. So obviously it’s a major disappointment to lose the mission,” says Denning. “We hope that NASA will fly a replacement mission, and there’s also the possibility that an even better instrument will be flown in the coming decade [see ASCENDS discussion below]. But of course, developing and building these tools is difficult, slow, and expensive.”

In the meantime, Denning and others will call on data from GOSAT and other sources to continue their research. NOAA’s Pieter Tans and colleagues are hoping to incorporate data from GOSAT into their CarbonTracker system starting later this year.

Further into the future, there’s another promising data source. If funded by NASA, ASCENDS (Active Sensing of CO2 Emissions over Nights, Days, and Seasons) will include a laser that allows it to detect CO2 at night and through areas of scattered clouds. ASCENDS would also be able to sample sun-limited locales, such as northern Europe and polar regions in wintertime. Edward Browell (NASA Langley Research Center) says the technology can be in place to launch ASCENDS by as early as 2015. ESA is also considering two new carbon observatories that would launch in the mid-2010s.

On the Web

HIPPO (NCAR Earth Observing Laboratory)

Ibuki (Japanese Space Agency)


CarbonTracker (NOAA)




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