Delta Airlines — the world’s largest airline by revenue — says it’s going carbon neutral.

Air travel has come under increasing scrutiny for its contribution to global climate change, with the UN predicting carbon dioxide from airplanes will triple by 2050. There’s even the Swedish word flygskam — meaning “flight shame” — inspired by that country’s teenage environmental activist Greta Thunberg, who herself does not fly.

Delta has committed to spending a billion dollars over the next ten years to mitigate all its emissions it causes, in part on carbon offsets.

There are a couple ways air travel could get cleaner, according to Sola Zheng, associate researcher at the International Council on Clean Transportation.

“One is alternative jet fuel and the other is electrification,” she said.

But those technologies could be decades away from powering commercial planes. So Delta and other airlines have to look elsewhere to reduce their carbon footprints now — by buying carbon offsets.

“It’s sort of a tool for someone who can’t reduce emissions themselves to pay someone else to do it,” Zheng said.

“If you were to plant trees that otherwise wouldn’t be there, you’ll remove CO2 from the air,” said Jason Bordoff, director of Columbia University’s Center on Global Energy Policy. “If you can prevent trees from being cut down that might have otherwise been cut down, you can keep carbon from going into the air.”

Bordoff said there are lots of programs to offset greenhouse gases, like investments in wind and solar farms and cleaner chimney designs.

But the key is that they have to result in a permanent CO2 reduction that wouldn’t have happened otherwise.

“Would this forest truly have been cut down without that carbon offset? That’s a difficult question to answer,” said Ethan Elkind, director of the Climate Program at UC Berkeley School of Law.

Elkind said that’s why offset programs need outside auditors to verify the actual carbon reductions. But there’s also concern that focusing too much on offsets could slow advances in clean tech.

“It really needs to be a whole suite of measures,” he said. “We don’t want to let carbon offsets become an excuse for not doing the other things we should be doing at the same time.”