> The cruise industry uses over double the emissions of the bitcoin protocol

Not to defend the cruise lines, but that quote appears bogus. The tweet you cite refers to a "carbon footprint", in response to a quote about particulate emissions (using sulfur dioxide as an example).

This [1] puts total cruise line CO2 emissions in 2017 (first source I picked, but I imagine the number won't have changed much) at 21K tons, whereas a recent Reuters article [2] put Bitcoin at 22,000K tons of CO2.

If the above data is correct, then Bitcoin is responsible for more than 1000x as many CO2 emissions as cruise ships.

[1] https://www.tourismdashboard.org/explore-the-data/cruise-shi...

[2] https://www.reuters.com/technology/how-big-is-bitcoins-carbo...

But cryptocurrencies don't appear to need proof of work.

Proof of work has demonstrated security over time. It costs a lot to do so - as soon as Proof of Stake or another system can provide the same level of security assurance, the market will rapidly shift there to capture the benefit without the costs.

I don't think the bitcoin community is capable of making a change that large at this point, other ideological objections that they have against PoS aside.

Another option would be continually adjusting the block reward to not over subsidize mining.

Block reward is effectively adjusted via difficulty retargeting every 2 weeks.

The amount issued doesn't change, but the expected value of a number of hashes changes significantly.

Block reward is only changed on halvenings. Difficulty retargeting changes to try to keep blocks and therefore bitcoin emissions per unit of time (10 minutes) the same.

The issue is that there is now too much electricity & therefore carbon emissions being spent on security the bitcoin blockchain. If the bitcoin community could adjust this down for slightly lower 51% attack defense they might be able to avoid the pressure from outside groups.

But the bitcoin community has shown it's self unable to make changes or establish any consensus outside of what is already believed.

If just mining was taxed, ordinary crypto transactions would be quite possible.

But regardless by allowing transaction outside of state control, crypto may indeed help some hard bitten business people but simultaneously helps a vast array of criminals, from ransomware gangs to corrupt third world officials expatriating bribes and theft.

Most of the discussion about "eliminating" and/or taxing cryptocurrency are about trying to reduce the use of cryptocurrencies within developed countries where they are not needed.

Nobody here is trying to stop people in authoritarian states from using crypto.

Bringing up the authoritarian states is just a strawman.

What happens when those developed countries become authoritarian? Or starts printing trillions of dollars?

I think climate change is the bigger risk here in context.