BioEd Online

Climate change caused by black carbon, also known as soot, emitted during a decade of commercial space flight would be comparable to that from current global aviation, researchers estimate.

The findings, reported in a paper in press in Geophysical Research Letters¹, suggest that emissions from 1,000 private rocket launches a year would persist high in the stratosphere, potentially altering global atmospheric circulation and distributions of ozone. The simulations show that the changes to Earth's climate could increase polar surface temperatures by 1 °C, and reduce polar sea ice by 5–15%.

"There are fundamental limits to how much material human beings can put into orbit without having a significant impact," says Martin Ross, an atmospheric scientist at the Aerospace Corporation in Los Angeles, California and an author of the study.

Private space flight is a rapidly maturing industry. Spaceport America, a launch site in Las Cruces, New Mexico, opened its first runway on 22 October. During the next three years, companies such as Virgin Galactic, headquartered at Spaceport America, expect to make up to two launches per day for space tourists. Meanwhile, the NASA Authorization Act passed by US Congress in September provides US$1.6 billion in private space-flight investments to develop vehicles to take astronauts and cargo into orbit.

Commercial rockets burn a mixture of kerosene and liquid oxygen. But several private space-flight companies, such as Virgin Galactic, may soon use a more economical 'hybrid' rocket engine that ignites synthetic hydrocarbon with nitrous oxide, says Ross. These hybrid engines emit more black carbon than a kerosene and oxygen engine, he adds.

"Rain and weather wash out these particles from the atmosphere near Earth's surface, but in the stratosphere there isn't any rain and they can remain for 3 to 10 years," says Michael Mills, an atmospheric chemist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and another author of the paper.

Soot surprise

The researchers ran global atmospheric models of an injection of about 600 tonnes of black carbon per year at a single location: Las Cruces. The results showed a soot layer in the stratosphere that stays within 10° latitude of the launch site, says Ross. Furthermore, around 80% of the black carbon remained in the Northern Hemisphere, spreading out to between 25° and 45° northern latitude.

The black carbon layer caused the temperature to decrease about 0.4 °C in the tropics and subtropics, whereas the temperature at the poles increased by between 0.2 and 1 °C, he says, emphasizing that the exact details would have to be refined with further models. The black carbon also caused ozone reductions of up to 1.7% in the tropics and subtropics, and increases of 5–6% in the polar regions.

The results are surprising, says Simone Tilmes, an atmospheric chemist at NCAR who was not involved in the study. "What's interesting is that if you force the whole climate system in one point or one hemisphere you can make big changes," she says. Further, more detailed studies examining the circulation of particulates will to help to reduce some of the uncertainties in the model, she adds.

Ross and his team hope to organize scientists, engineers and members of the private space-flight industry to discuss the kinds of measurements that need to be made to produce more definitive results.

"The goal here is to support the commercial space industry so that it can develop normally," says Ross.

He compares the problem to another one facing the industry: space debris — waste that remains in orbit and can present a potential collision risk to astronauts. "We have to come together to take care of the space commons," he says.