Off the coast of Namibia, for several months a year, a layer of smoke drifts over a persistent deck of low clouds. It's the perfect place to investigate the thorniest problem in all of climate science: how haze and clouds interact to influence global warming, either boosting or moderating it. Now, after weeks of delay, an airborne research campaign is getting started in this diaphanous natural laboratory.
The smoke, from fires deep in Africa, is nearly invisible to satellites in space, and because the southeast Atlantic Ocean has few islands, the layers are hard to study from below. But flying directly into them from Namibia has posed its own hurdles—bureaucratic ones, which NASA has now cleared. On 29 August, research planes will begin nearly a month of flights into the heart of the smoke and clouds, taking off every other day from Walvis Bay, Namibia, with plans to return in 2017 and 2018.
Complementary efforts from France and the United Kingdom would have expanded the sampling area but were postponed when the teams couldn't get diplomatic clearances from Namibia. NASA itself almost canceled this season's flights before getting permission last week. Mission leaders were relieved and eager to begin their studies of cloud and haze effects, which "constitute the largest uncertainties in our models of future climate—that's no exaggeration," says Jens Redemann, an atmospheric scientist at NASA's Ames Research Center in Mountain View, California, and the principal investigator for ObseRvations of Aerosols above CLouds and their IntEractionS (ORACLES).
Like Chile and California, Namibia has a near-constant coastal fog: a bank of low-lying, relatively thin stratocumulus clouds sitting just offshore. It also has a supply of smoke from the natural and human-caused fires that whip across the savannas of southern Africa. In August, the prevailing winds shift and, for about 3 months, blow the smoke out to sea and above the cloud deck. Near shore, the clouds and smoke stay separated in tidy layers, but farther west the smoke settles and mixes with the clouds. This gives scientists an ideal laboratory to observe the varied and complex interactions of the clouds and smoke and their climate effects (see graphic above).
Atmospheric scientists already have a decent handle on how clouds and smoke work on their own. For instance, bright, white clouds reflect light to space, cooling the planet. Aerosols can also have a cooling effect, if they are bright, like the sulfate particles emitted by volcanoes. Sooty black carbon, on the other hand, tends to absorb light and heat the planet. Although scientists don't know exactly how the African smoke evolves chemically as it drifts over the ocean, they think that because it originates from burning vegetation, it contains a lot of brown carbon. That would put its brightness somewhere between that of soot and sulfate—able to both trap and reflect heat.
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Things get more complicated when clouds and smoke overlap. When the African smoke blows over the top of the white cloud deck, it absorbs some of the heat that the clouds would have reflected, which turns up the thermostat. But at breaks in the cloud deck, smoke has the opposite effect: It is brighter than the dark ocean surface, reflecting solar radiation and reducing warming.
Smoke particles also influence clouds. By absorbing heat, aerosols can evaporate nearby cloud droplets—making the cloud less reflective and compounding the heating effect. On the other hand, by warming the atmosphere, aerosols can stabilize the air and protect clouds from drying out and thinning. Still other effects—which primarily promote cooling—occur when the aerosols begin mixing into a cloud. The aerosols create additional seeds around which water vapor can condense, boosting the number of cloud droplets and making the cloud more reflective. Because the added droplets are smaller, they are less likely to be rained out, which enhances cloud longevity.
The smoke is virtually invisible against a bright cloud, which handicaps satellites looking down from above. So the $30 million ORACLES mission will fly straight into the mix. The mission's workhorse is NASA's P-3, a low-flying turboprop originally developed by the U.S. Navy to hunt for submarines. Flying at altitudes between 4 and 6 kilometers, it will carry instruments that look up and down and sample the air directly. Flight paths will vary, with lots of turns and sudden changes in altitude. The pilots love it, Redemann says, but the scientists—not so much. "It's not rare that multiple people vomit."
Some flights will try to stay on a particular line of latitude or longitude, no matter what the clouds and smoke are doing that day, because climate modelers need data collected along a transect. Others will be flights of opportunity, following the weather. One such plan is a flight to Ascension Island, 1600 kilometers off the African coast, that will seek to follow the smoke layer westward to see how it changes chemically with time.
ORACLES will also deploy a camera-carrying ER-2—a variant of the venerable U-2 spy plane—that will cruise at an altitude of 20 kilometers, providing contextual images. "It's our personal satellite," Redemann says. Meanwhile, a battery of sensors and radars is watching from the ground, on Ascension Island. Part of a separate experiment run by the U.S. Department of Energy called Layered Atlantic Smoke Interactions with Clouds (LASIC), the ground station recently captured samples of the downwind smoke to see how its properties changed during its drift westward. "It was both exciting and a relief," says Paquita Zuidema, an atmospheric scientist at the University of Miami in Coral Gables, Florida, and LASIC lead scientist. "You'd hate to think that the smoke is just blowing by you overhead, without being able to take direct measurements."
What the ORACLES team learns should sharpen forecasts of regional climate change. Climate models suggest that, in a hotter future, weather patterns will shift and southern Africa will become dryer. But that prediction might not hold if African nations tamp down fires in order to limit smoke, a health hazard. Eliminating aerosols could reduce their tendency to evaporate rain clouds, and the region could get wetter, says Rob Wood, ORACLES deputy principal investigator and an atmospheric scientist at the University of Washington, Seattle. "There's a potential for [the effect] to mitigate the greenhouse gas changes in precipitation," he says.
The team also hopes to shrink broader uncertainties about clouds and aerosols. In a recent study, for instance, well-respected climate models were shown to have completely opposing estimates for the overall effect of the clouds and smoke in the southeast Atlantic: Some found net warming, whereas others found cooling.
When ORACLES scientists return to Namibia next August, they hope to be joined by French and U.K. researchers, who postponed their missions for at least a year when they didn't get diplomatic clearances. Researchers with the U.K. mission, called CLARIFY (Cloud-Aerosol-Radiation Interactions and Forcing for Year 2016), initially looked at relocating its research aircraft to Ascension Island, but they couldn't make the switch work logistically. "Everybody is quite frustrated," says Steven Abel, a CLARIFY principal investigator with the U.K. Met Office in Exeter.
NASA, too, ran into obstacles. Namibia gained its independence in 1990 from South Africa, after a long struggle led by a Socialist Party supported by Cuba and the Soviet Union—enemies of the United States at the time. Some suspected that Namibia's defense ministry harbored lingering suspicions of the United States that slowed approval of the project. "I have to assume it's discomfort with having an ex-high-altitude spy plane and an ex-submarine hunter placed in their country," Zuidema says.
Bob Swap, an ORACLES program scientist in Washington, D.C., spent the month of July in Namibia shuttling between ministries and trying to rally support. "You camp out and you're on their time schedule not yours, and you're showing respect," he says. Eventually, ORACLES got on the agenda of a presidential cabinet meeting on 16 August. The bureaucratic skies cleared; visas for the mission scientists and flight permits for the aircraft were proffered days later.