The energy consumption to mine is easy to estimate: You can just look at its hashrate (i.e. the total combined computational power used to mine Bitcoin and process transactions), and then make some educated guesses as to the energy requirements of the hardware that miners are using.

To calculate associated carbon emission you need to know energy mix i.e the makeup of different energy sources used by computers mining bitcoin. Mining is an intensely competitive business, and miners tend not to be particularly forthcoming around the details of their operations making it even harder to calculate.

Bitcoin can make use of hydropower which is not accessible to most. This is one of the reasons why some locations in china are heartlands of mining where production of such energy outpaces demand.

Another source of energy is natural gas which is a byproduct of oil extraction. It pollutes the environment but miners are leveraging this to mine. A minor player but some suggest natural gas from the US and Canada alone can run an entire bitcoin network.

Many journalists and academics talk about Bitcoin’s high “per-transaction energy cost,” but this metric is misleading.

Bitcoin’s energy consumption happens during the mining process. Once coins have been issued, the energy required to validate transactions is minimal. Simply looking at Bitcoin’s total energy draw to date and dividing that by the number of transactions doesn’t make sense, most of that energy was used to mine Bitcoins, not to support transactions

People sometimes assume that bitcoin will commandeer entire energy grids. It likely won't.

First, because the energy mix of bitcoin grows less reliant every year. Eg US is shifting to ESG focused miners and china recently banned coal-based mining.

Secondly, miners receive small fees for the transactions that they verify while mining, as well as whatever profit margins they can get when they sell the bitcoins they have mined, and the bitcoin protocol, is built to halve this issuance-driven component of miner revenue every four years. The financial incentive to invest will naturally decrease.