Traveling by plane is one of the worst things you can do for the environment. On average, one nonstop round-trip flight from the United States to Europe releases about one metric ton of carbon dioxide per person, or about what a resident of Honduras emits in a year.

Unfortunately, several American start-ups are proposing to make the problem even worse by reviving the bad old idea that the world’s jet-setters need to break the sound barrier to get where they’re going.

The aviation industry, recognizing its impact on the climate, belatedly adopted a global climate plan for commercial aviation in 2016 that is scheduled to take effect in 2021. But returning commercial supersonic transport aircraft, or SSTs, to the skies could upend those efforts.

Supersonics were all the rage when testing began for commercial flights in the 1960s, but the planes ran into problems because of high costs and pollution concerns. Ultimately, only the Concorde, a British-French collaboration, ever saw long-term commercial service after its inaugural flight in 1976. The development of that aircraft was famously expensive, costing about 10 times its initial budget.