According to The Atlantic, the US military alone counts for 5.5% of global carbon emissions.
The world will likely move onto using rollups on other chains like Ethereum instead, which scale better, and pays gas to validators to get included into the blocks.
Seems like the Bitcoin community is looking into rollups on Bitcoin, but it'll be impossible to get trustless Bitcoin on them, even with BitVM, without a hardfork.
Okay, the banks might cause more carbon emission than blockchains. But, the banking system serves the whole world, providing services for 8 billion people. But Blockchain is still a niche thing.
You might say, no, Bitcoin is not niche, Blockchain is used by a lot of people! Then, you are wrong again. The media is talking about Blockchain all the time, but it is indeed a niche thing. No one is using, say Bitcoin, to buy real physical stuff, cryptocurrency is not taking part in the people's real life.
So, my point is, if Blockchain is adopted by the world and replace the banking systems, it will definitely emit much much much more carbon, I am saying orders of magnitudes more.
There is an argument to be made that Bitcoin could end up producing more carbon emissions if it already produces this much with a much lower transaction capacity than most banking systems. However, systems like the Lightning Network <https://wiki2.org/en/Lightning_Network> build on top of Bitcoin by settling transactions before they ever reach the blockchain, slashing fees and bundling possibly hundreds or thousands of transactions into one, so the number of payments that actually occur could be much greater than it seems.