Observing Earth

Studying complex environmental systems, such as atmospheric chemistry and climate, requires a global, long-term view—the view from a satellite.

“You can have access to regions that are very, very difficult to go study and make sustained measurements in, such as the polar regions, rainforests of Central America, and other areas that are under-instrumented and under-studied,” says Michael Gunsonof the National Aeronautics & Space Administration.

Civilian Earth-observing satellites date to 1960, when NASA launched its first Television Infrared Observation Satellite to map cloud cover. The satellite worked for just 78 days, but it proved the potential of satellites to monitor environmental conditions from space.

They all make an essential contribution to environmental science. Aside from providing data from hard-to-reach places, they enable wide-angle views. For CO_2, for example, there are only about 100 monitoring stations around the globe. “That is just absolutely inadequate,” says Inez Fung, a professor of atmospheric science at the University of California, Berkeley. “There’s a lot of room for imagination for what goes on between the stations.”

Over the next few years, NASA plans to launch several new satellites that will help researchers study chemistry both low and high in the atmosphere, as well as Earth’s water cycle through soil moisture and ice-sheet measurements. Other countries are doing the same. But as critical as such satellites are for studying complex environmental systems, researchers are concerned about the future: There is no good path either in the U.S. or internationally to set and execute long-term priorities for space-based observations.

The current flagship of NASA’s Earth science satellite missions is Terra. Terra launched in 1999 and carries five instruments: ASTER, which collects visible and infrared light to map Earth’s land surface temperature, reflectance, and elevation; CERES, which monitors Earth’s radiation budget and cloud properties; MISR, which uses reflected sunlight to illuminate atmospheric aerosol and cloud types and properties, as well as vegetation distribution; MODIS, which tracks aerosols and clouds with a wider view, along with snow and ice cover and photosynthetic activity; and MOPITT, which maps carbon monoxide and methane using infrared bands.

Other satellites launched in the early 2000s were also big platforms. Aqua, which is generally focused on Earth’s hydrologic cycle, has its own CERES and MODIS plus additional instruments to better characterize atmospheric water vapor, clouds, precipitation, sea surface temperature and winds, sea ice, snow, and soil moisture. Aura, which looks at atmospheric chemistry and dynamics, carries instruments that collectively can track O₃, H₂O, OH, HO₂, CO, CH₄, CH₂O, C₂H₂O₂, CH₃CN, N₂O, HNO₃, SO₂, ClO, HCl, BrO, and OClO. One instrument, called HIRDLS, also measured chlorofluorocarbons, but it ceased operating in 2008. Although most chemical measurements reveal only the total amount of a particular chemical from the bottom to the top of the atmosphere, the particular spectral bands of ozone allow researchers to differentiate how much is next to the Earth in the troposphere, where ozone is a health-harming pollutant, versus higher up in the stratosphere, where it serves to shield Earth from ultraviolet radiation.

More recent satellite missions have tended to be more highly focused on one aspect of Earth’s environmental system. OCO-2 is one example. OCO-2 carries spectrometers that look at the sunlight reflected off of Earth and measure two CO_2 absorption bands and one O₂ absorption band, all in the near-IR. The output is the mole fraction of CO_2 in dry air, with better precision and resolution than existing satellites provide. CO_2 doesn’t vary by more than a percent from the bottom to the top of the atmosphere, “so you need to measure it extremely well,” says Paul O. Wennberg, a professor of atmospheric chemistry at California Institute of Technology and part of OCO-2’s science team. OCO-2 is just starting to send in its first data now, and although “things look great,” Wennberg says, he’s cautious about pinning a limit of detection on the measurements just yet. He hopes to get close to 0.5 ppm, compared with an overall CO_2 concentration of about 400 ppm.

And OCO-2’s spatial resolution is 3 km². “It’s really the sharpest eye in the sky for CO_2,” UC Berkeley’s Fung says. Overall, OCO-2 will provide a much better view of where CO_2 comes from and where it goes than scientists and policymakers currently have. “It’s very difficult to do any carbon management strategy when you don’t have good information,” Fung says.

Beyond OCO-2, atmospheric chemists are also looking forward to the Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite, which is being developed now for launch in 2018 or 2019. In part, TEMPO will provide continuity from 10-year-old Aura by monitoring key pollution and air quality compounds: O₃, NO₂, SO₂, CH₂O, and C₂H₂O₂, along with water vapor, aerosols, and cloud properties. Other molecules, such as halogen oxides that come out of seawater, may also be possible, says Kelly Chance, a senior physicist at the Smithsonian Astrophysical Observatory and TEMPO’s principal investigator.

Aura, OCO-2, and many other satellites follow a so-called sun-synchronous orbit, which means that they orbit Earth in line with the sun, getting a global data set each day. TEMPO, in contrast, will sit in a so-called geostationary orbit and collect data only over North America, repeating its measurements every hour. Whereas Aura’s pollution-monitoring instrument has a spatial resolution of 13 km², TEMPO’s will be about 9.8 km². TEMPO’s view will reach far enough north to monitor emissions from the Alberta oil sands.

Scientists are hopeful that TEMPO will push forward their understanding of atmospheric chemistry. “Most of the previous work using satellite data has been focused on just getting the emissions right,” says Ronald C. Cohen, a UC Berkeley chemistry professor and director of the Berkeley Atmospheric Science Center. “The next frontier is to see how the chemistry is different in different locations and how plumes evolve.”

“What we’re seeing now in the air pollution world is a transition from local to regional to global,” adds Ross J. Salawitch, a professor of atmospheric and oceanic science at the University of Maryland. “It used to be that pollution was so bad that we had to control local sources. In a lot of U.S. states that’s been done, so now we need to pinpoint what’s coming from further away. Satellite data are vital for this.” Although TEMPO’s sole focus will be North America, similar satellites are being developed by Europe and South Korea to monitor their parts of the world. Computer models may help connect the data sets, even as better data will also help to improve the models.

For all the immediate importance of measuring pollutants for air quality and health, the bigger picture for Earth is climate. And for climate, satellites must measure water. “Water is the big feedback,” says NASA’s Gunson, who is program manager for global change and energy at the Jet Propulsion Laboratory (JPL) and also project scientist for OCO-2. “The hydrologic cycle is where we will see some of the most important consequences of climate change,” whether the effects are in sea-level rise or precipitation and the availability of freshwater. The hydrologic cycle is also the source of a lot of uncertainty about the consequences of climate change.

Terra, Aqua, and other satellites are perfectly positioned to help assess the amount of water vapor in the air; how efficiently water turns into precipitation through clouds; distribution of rain, snow, and ice; soil moisture; connections between ocean salinity, density, and weather patterns; and links between sea level and ocean heat distribution.

But to make those assessments and understand the broader interplay among atmospheric chemistry, hydrology, and climate—and what global warming might mean on a local scale—requires long-term measurements. Climate is such a complicated problem that one or two years of observations are insufficient. It’s only after a decade or more that researchers can start to tease out trends that yield clues to the underlying physical processes. Those processes can then be used to improve climate models to understand things such how climate change will affect air quality or resources, says Anne R. Douglass, who is the Aura project scientist and helps develop the NASA Goddard Earth Observing System Chemistry Climate Model.

But Terra is 15 years old, Aqua is 12, and more satellites launching now have design lives of just three years. They’ll probably last longer, but that’s small comfort when it can take a decade to plan, build, and launch a new one. Scientists fret that the U.S. has struggled to set up a mechanism through which long-term measurement priorities are set and executed.

“NASA is an agency that prides itself in doing the first and the best thing, while NOAA has traditionally been more of a long-term operational agency,” Caltech’s Wennberg says. A “valley of death” between the agencies on the transition from research instruments to sustained monitoring got even worse when an ill-fated attempt to merge the U.S. civil and military meteorological satellite programs was canceled in 2010 because of cost overruns, and NOAA retrenched to focus more closely on weather at the expense of other monitoring.

Recession-era budget cuts didn’t help matters, either. NOAA’s existing satellite program already consumes about 40% of its budget, notes Antonio J. Busalacchi Jr., director of the Earth System Science Interdisciplinary Center at the University of Maryland. Increasing the budget for satellites might mean decreasing the funds available for, say, the fisheries service or nonsatellite oceanic and atmospheric research. Even NOAA’s weather satellites, unequivocally considered essential for weather forecasting, may see a gap in upcoming years if existing instruments fail before new ones launch.

The situation has left NASA building “wonderful new sensors to measure things we really want to understand,” Wennberg says, with little hope that they will find their way onto long-term missions.

Researchers do see some indications that things might be turning around. The White House Office of Science & Technology Policy produced the first National Strategy for Civil Earth Observations in 2013, and the National Science & Technology Council released a National Plan for Civil Earth Observations in July. The documents are more frameworks for how to assess projects rather than a true path forward, but scientists see it as progress.

A more general satellite trend toward miniaturization and packaging instruments into 10-cm “CubeSats” rather than the buslike Aura also bodes well for sustainable programs. “My view is that the day is not far away before we can do really meaningful science” on relatively tiny platforms, says Graeme Stephens, director for the Center for Climate Sciences at NASA JPL. The smaller sizes require fewer resources to build and also cost far less to launch. Commercial spaceflights may also provide lower-cost launch options.

Ultimately, a long-term outlook is important not only to understand the complex systems of atmospheric chemistry and climate, but also for how to inform decisions about resource management or food security. Twenty years ago, climate change was “a research problem that had a certain degree of abstract nature to it because we weren’t living in such rapidly changing times,” Gunson says. “Now we have these crises, like droughts that start to affect water availability. The question is, How do we provide information to address those problems and identify adaptation strategies?”