When discussing the energy expenditure of the Bitcoin network one should note that Bitcoin is not only a proof-of-work but a reusable-proof-of-work system.
The electricity used for mining new blocks is paid for by issuing new bitcoins and by fees for including transactions. The first part of that reward is for making something that will never be destroyed. If you don’t lose the private keys, bitcoins minted now will be usable for centuries to come.
The Suez Canal was opened in 1869 and has been useful for world trade ever since. Once it was built it can be used forever. That means that whatever energy cost or dollar cost was necessary at the time has been paid back many times over in the 1.5 centuries since.
So if you are worried about the energy cost of Bitcoin, don’t think about the CO2 cost per transaction. That metric is largely meaningless, since there can be any number of real world payments settled by one Bitcoin transaction.
If you think about the CO2 cost per bitcoin mined, remember that once a bitcoin is mined it may be used forever. Also note that this cost has been going down massively over the first decade of the project. Since a large majority of bitcoins are mined in that first decade, society can now use a bitcoin mined at much lower CO2 cost years ago.
Anyone familiar with the concept of proof-of-work will know that Hal Finney proposed “reusable” proof of work as an improvement on the original idea. And Bitcoin is the RPOW idea without a central server.
Fossil fuel is not reusable. Once you burn it, you need to wait millions of years for new oil to form.
Bitcoin is reusable. A coin spent in one transaction can be spent to someone else in another transaction. That is important for debating the energy cost. If you get something for eternity, it does not matter much how much energy you needed to burn for mining that bitcoin in the first place.
Correction: Energy and CO2 cost of mining one bitcoin has been changing massively over the last decade, but the change was in the other direction. The bitcoins mined now cost much more energy than those mined 10 years ago, which is a good thing on the whole since most bitcoins were mined in the first years of the project. The way the bitcoin supply is created actually comes with a large advantage in saving energy.