I am personally not up to speed on the scientific evidence, but I've heard a lot of people are worried about microplastics in the ecosystem, especially getting into human bodies via the food chain. Breaking plastics down completely could potentially fix this.

And to me as an idealist taking the super-long view, I think a "leave as little trace as possible" approach to the environment is preferable to one where we keep creating and using something literally called a garbage dump.

Pumping oil from inside the Earth and then converting it into plastic and then into CO2 has a much bigger impact than putting the plastic into a garbage dump. The oil to make the plastic came from a hole in the ground. A garbage dump is a hole in the ground. You've essentially done nothing. If you decompose the plastic into CO2, then that has a major impact on the atmosphere.

The estimate I've seen is that 25 million tons of plastic are added per year. That much carbon is added to the atmosphere every 6 hours.

Plastic in the oceans is not a worrying amount of carbon, it's just a worrying amount of plastic.

Even if you burned every single kilo of plastic produced, that's still "only" 380 million tons a year, or about 4 days worth of CO2.

This is a good point I hadn't thought of. One adjustment to your calculations is that the CO2 emitted from plastic would weigh over 2 times the weight of the plastic due to the additional oxygen atoms, but that doesn't change the calculus substantially.

Perhaps we should put plastic in to the landfills, and not the oceans?

Absolutely, but if the choice is "burn" the plastic if you could (via an oxidative process like digestion, rather than combustion) in place in the oceans or just leave it there forever, then adding to the carbon in the atmosphere is not the thing to worry about.

Oil deep underground somewhere in the middle of nowhere is not remotely close to landfills close to surface, next to understand waters and relatively close to cities

It is more efficient to burn the already refined petroleum product (plastic) than to dig up more unrefined petroleum products (crude) and put them to use

A landfill also emits e.g. CO2 and CH3, it's not that it's a sealed process.

The CO2 is from decomposing food, not from plastic. The CH3 can be harvested, or it will breakdown on its own after a decade.

Well it will be from plastic once a hypothetical plastic-eating microbe makes it into the dump.

Also, what do you think CH3 breaks down into?

> What is the point of just releasing the CO2 into the atmosphere with no benefit?

To prevent other carbon from being pumped up from deep beneath the ground