21 June 2022
written by Karly Beaumont and Theo Stein, NOAA Communications
Projected growth in rocket launches for space tourism, moon landings, and perhaps travel to Mars has many dreaming of a new era of space exploration. But a NOAA study suggests that a significant boost in spaceflight activity may damage the protective ozone layer on the one planet where we live.
Kerosene-burning rocket engines widely used by the global launch industry emit exhaust containing black carbon, or soot, directly into the stratosphere, where a layer of ozone protects all living things on the Earth from the harmful impacts of ultraviolet radiation, which include skin cancer and weakened immune systems in humans, as well as disruptions to agriculture and ecosystems.
According to new NOAA research published in the Journal of Geophysical Research Atmospheres, a 10-fold increase in hydrocarbon fueled launches, which is plausible within the next two decades based on recent trends in space traffic growth, would damage the ozone layer, and change atmospheric circulation patterns.
"We need to learn more about the potential impact of hydrocarbon-burning engines on the stratosphere and on the climate at the surface of the Earth," said lead author Christopher Maloney, a CIRES research scientist working in NOAA's Chemical Sciences Laboratory. "The relative impacts of different rocket types on climate and ozone should be better understood with further research."
Launch rates have more than tripled in recent decades, Maloney said, and accelerated growth is anticipated in the coming decades. Rockets play a unique role in the middle atmosphere because they are the only source of human-produced aerosol pollution at those altitudes.
The research team used a climate model to simulate the impact of approximately 10,000 metric tons of soot pollution injected into the stratosphere over the northern hemisphere every year for 50 years. Currently, about 1000 tons of rocket soot exhaust are emitted annually. The researchers caution that the exact amounts of soot emitted by the different hydrocarbon fueled engines used around the globe are poorly understood.
They found that this level of activity would increase annual temperatures in the stratosphere by 0.5 - 2° Celsius ( or approximately 1 - 4° Farenheit), which would change global circulation patterns by slowing the subtropical jet streams as much as 3.5%, and weakening the stratospheric overturning circulation.
Stratospheric ozone is strongly influenced by temperature and atmospheric circulation, noted co-author Robert Portmann, a research physicist with the Chemical Sciences Laboratory, so it was unsurprising to the research team that the model found changes in stratospheric temperatures and winds also caused changes in the abundance of ozone. The scientists found that ozone loss occurred poleward of 30 degrees North, or roughly the latitude of Houston, in nearly all months of the year. The maximum loss of 4% occurred at the North Pole in June. All other locations north of 30° N experienced at least some ozone loss throughout the year. This spatial pattern of ozone loss directly coincides with the modeled distribution of black carbon and the warming associated with it, Maloney said.
"The bottom line is projected increases in rocket launches could expose people in Northern Hemisphere to increased harmful UV radiation," Maloney said.
The research team also simulated two larger emission scenarios of 30,000 and 100,000 tons of soot pollution per year to better understand the impacts of an extremely large increase in future space travel using hydrocarbon-fueled engines, and more clearly investigate the feedbacks that determine the atmosphere's response. Results showed that the stratosphere is sensitive to relatively modest black carbon injections. The larger emission simulations showed a similar, yet more severe climate response than the 10,000 metric ton case.
The study built on previous research conducted by members of the author team, who have collaborated on this research area before. A 2010 study led by co-author Martin Ross, a scientist with The Aerospace Corporation, first explored the climate impact of a hypothetical increase in soot-producing rocket launches. A second study performed at NOAA in 2017, on which Ross was a co-author, examined the climate response to water vapor emissions from a proposed reusable space launch system utilizing cleaner hydrogen-fueled rockets.
"Our work emphasizes the importance of ozone depletion caused by soot particles emitted by liquid-fueled rockets," Ross said. "These simulations change the long-held belief that spaceflight's only threat to the ozone layer was from solid-fueled rockets. We've shown that particles are where the action is for spaceflight's impacts."
While the new research describes the influence that soot in rocket exhaust has on the climate and composition of the stratosphere, the scientists said it represents an initial step in understanding the spectrum of impacts on the stratosphere from increased space flight.
Combustion emissions from the different rocket types will need to be evaluated, they said. Soot and other particles generated by satellites burning up when they fall out of orbit is also a growing, poorly understood source of emissions in the middle-to-upper atmosphere. These and other topics will need further research to produce a complete picture of space industry emissions and their impacts on Earth's climate and ozone.
The study was supported by NOAA's Earth's Radiation Budget initiative.
Maloney, C.M., R.W. Portmann, M.N. Ross, and K.H. Rosenlof, The climate and ozone impacts of black carbon emissions from global rocket launches, Journal of Geophysical Research, doi:10.1029/2021JD036373, 2022.
Aerosol emissions from spaceflight activities play a small but increasing role in the background stratospheric aerosol population. Rockets used by the global launch industry emit black carbon (BC) particles directly into the stratosphere where they accumulate, absorb solar radiation, and warm the surrounding air. We model the chemical and dynamical response of the atmosphere to northern mid-latitude rocket BC emissions. We initially examine emissions at a rate of 10 Gg per year, which is an order of magnitude larger than current emissions, but consistent with extrapolations of space traffic growth several decades into the future. We also perform runs at 30 and 100 Gg per year in order to better delineate the atmosphere's response to rocket BC emissions. We show that a 10 Gg/yr rocket BC emission increases stratospheric temperatures by as much as 1.5 K in the stratosphere. Changes in global circulation also occur. For example, the annual sub-tropical jet wind speeds slow down by as much as 5 m/s, while a 10%-20% weakening of the overturning circulation occurs in the northern hemisphere during multiple seasons. Warming temperatures lead to a reduction in the northern hemisphere by as much as 16 DU in some months. The climate response increases in a near linear fashion when looking at larger 30 Gg and 100 Gg emission scenarios. Comparing the amplitude of the atmospheric response using different emission rates provides insight into stratospheric adjustment and feedback mechanisms. Our results show that the stratosphere is sensitive to relatively modest BC injections.