When you’re in a hole, stop digging, supersonic edition

It’s fashionable in some circles to argue that the impacts of new supersonic aircraft can be offset, rather than mitigated via environmental standards. What do the numbers tell us? 

Aspiring supersonic transport (SST) manufacturers like Boom Supersonic, and even now one reported hypersonic start-up, are riding a wave of optimism that faster-than-sound aircraft will take to the skies in the near future. But due to a variety of factors—their high speed, small gauge (passenger capacity), inability to carry “belly” freight, and the need for refueling stops on many transoceanic missions—new commercial SST aircraft are likely to be fuel and carbon intensive. Ballpark, commercial SSTs may be five to seven times as carbon intensive per passenger as comparable subsonic aircraft and emit almost 100 million tonnes of CO2 annually by 2035. That’s as much as American, Delta, and Southwest Airlines collectively emitted in 2017.

Environmental regulators are currently discussing whether new supersonic aircraft should be required to meet existing noise, air pollution, and CO2 standards for subsonic aircraft, or if new SST-specific standards should be developed that would allow them to pollute more than subsonics. (For an overview of the most recent developments, see this blog post.) Consistent with the latter view, one aspiring SST manufacturer advocates for special rules for supersonics and offsetting the resulting emissions under the UN’s Carbon Offsetting Scheme for International Aviation (CORSIA). Is that likely to be enough?

CORSIA, agreed to by the International Civil Aviation Organization (ICAO) in 2016, requires airlines to offset most CO2 emissions growth from international flights from 2021 to 2035. CORSIA has become somewhat of a Rorschach test for those following efforts to curb aviation’s environmental impact. For some, CORSIA offers the promise of offsetting near-term emissions in a flexible, low-cost manner while industry develops longer-term technical fixes for aircraft and fuels. For others, it is an unwelcome distraction with significant potential loopholes, questions about additionality, and implied carbon prices that will likely be too low to promote new technologies that reduce emissions from aircraft and their fuels.

There’s good overlap between CORSIA’s international focus and the anticipated SST market. As we’ve shown in our most recent research, 87% of SST flights are likely to be international. (The balance will be largely on U.S. domestic routes, but these are on hold until the existing overland flight ban is lifted.) Furthermore, CORSIA applies to emissions growth after 2020. Since new supersonic aircraft will start flying in 2025 at the earliest, this means that no supersonic CO2 will be grandfathered into the 2020 baseline. This contrasts with subsonics, where about 80% of emissions are exempt starting on day one. So far, so good.

But CORSIA isn’t expected to cover all future emissions growth. In part that’s because ICAO hasn’t committed to extending the system beyond 2035, but it’s also due to exemptions for flights to and from developing countries and low-volume routes. In addition, states including Russia and India filed declarations of reservations to the basic agreement. We estimate that CORSIA will cover about three-quarters of international subsonic emissions growth during its “mandatory phase” starting in 2027, assuming that both China and the United States remain in the system. Using a database developed for this study, we calculate somewhat lower coverage for international supersonic CO2—about two-thirds—because some refueling stops will occur in lower-income Asian, African, and South American countries exempt from CORSIA.

The figure below was created from ICAO subsonic traffic projections and our own research assuming unconstrained SST growth. It categorizes the estimated 33 gigatonnes (Gt) of international aviation CO2 expected to be emitted through 2050, based on how it would be treated under CORSIA. Four wedges are shown: “base” subsonic emissions below 2020 levels not subject to CORSIA (blue); emissions required to be offset under CORSIA through 2035 for both subsonic and supersonic aircraft (grey); post-2035 subsonic emissions growth (orange); and uncovered supersonic emissions (green). Since there’s currently no consensus about the extent to which SST flights might replace subsonic business class, we assume here that half of SST traffic is substituted from subsonic business class and half is induced.


Chart of subsonic CO2 emissions from international aviation by CORSIA coverage, 2015-2020
Subsonic and supersonic CO2 emissions from international aviation
by CORSIA coverage, 2015 to 2050

As shown, CORSIA is expected to cover about 2 Gt of CO2 through mid-century, or about 6% of all international emissions from subsonic and supersonic aircraft through 2050. SSTs could add about 1.2 Gt of uncovered emissions, about 90% of those after 2035. To put these numbers in perspective, let’s refer to the Intergovernmental Panel on Climate Change’s most recent assessment. While highlighting considerable uncertainty, the report estimated that we can collectively emit somewhere between 420 and 570 Gt of cumulative CO2 starting in 2018 in perpetuity to allow a two-thirds chance of avoiding the worst impacts of climate change.

That implies a carbon budget of about 9 Gt for international aviation, assuming it’s allowed to emit relatively as much in the future as it did in 2015 (about 1.4% of anthropogenic CO2). As shown in the figure, subsonic aircraft are on track to emit triple that budget through 2050, even with CORSIA and without SSTs. Depending on where you stand, an additional 1.2 Gt of CO2 from supersonic aircraft may seem like either a rounding error or a prime example of the old saw that when you find yourself in a hole, stop digging. I lean to the latter.

Separate from CO2, CORSIA doesn’t cover the other environmental impacts of SSTs. These include noise pollution during landing and takeoff plus sonic boom generated en route. CORSIA also doesn’t cover aircrafts’ non-CO2 climate impacts from emissions of nitrogen oxides, black carbon, and water vapor in the stratosphere. These are typically believed to be as important as CO2 for subsonic aircraft but potentially much larger for supersonics

On balance, it’s clear that CORSIA alone is insufficient to fully address the environmental impacts of subsonic aircraft, and that new SSTs threaten to compound the problem. Since offsetting emissions alone won’t be enough, aspiring SST manufacturers would do well to support a robust policy framework to manage the environmental impacts of their aircraft. A modest first step is for SST manufacturers to commit to meeting existing noise, air pollution, and CO2 standards for new subsonic aircraft. We’re partway there, with some, but not all, manufacturers committing to meeting existing subsonic noise limits. Here’s hoping that the fledgling SST industry puts down its shovel and instead throws its shoulder to the wheel on environmental standards.