The "greenest" plastic bag is the one that is never made.

Isn't plastic basically a byproduct of oil production? I'm not super familiar with the market, but my understanding was that gasoline was the main drivers of drilling, not the market for plastic.

It seems to me that recycling would just drop the price of plastic and encourage use elsewhere, but burying it underground would remove the byproduct.

Every petroleum product was a waste product at some point until they discovered a use for it. If no one used oil for anything but gasoline, I expect drillers would pump the distillation remains back down into old wells.

Petroleum cracking happens very long distances from the actual oil wells. I have never heard of this happening.

Because there is no waste...

Maybe, but they have to make a new plastic bag if they don't recycle your old one. Energy is required to make that new bag, which means CO2 is created that way too.

The best thing you can do is to not need so much energy in the first place. If you must travel use high efficiency transport. (shipping counts as travel). Make sure your housing is well insulated.

The next best thing store carbon: start forest and grass fires. A regular forest/grass fire will burn cold enough that a lot of carbon is converted to charcoal - permanently sequestrated carbon. Note that regular is key here. If you don't light a new fire every year fuel builds up until you get a hot fire.

There are several reasons to want to reduce the number and size of landfills. First, landfills are very oxygen-poor, which promotes the growth of anaerobic bacteria. The bacteria produce methane, rather than CO2 when breaking down waste. Methane is a much more potent greenhouse gas than CO2 (though it doesn't stay in the atmosphere as long). Second, greenhouse gases are released in the extraction of petrochemicals. Reducing the new plastic created would reduce this effect. Lastly, landfills can have an immediate negative effect on the environment surrounding them if solvents and heavy metals leach out of the landfill.

Quite a few landfills capture the methane and turn it into electricity as they can make a profit doing so.

But there's the problem that anaerobic decomposition in garbage bags creates methane, which is an even worse greenhouse gas than CO2.

Most of the carbon in plastics comes from oil, not from the atmosphere.