From here, you need to figure out if most of the mining is done on, say, coal or, say hydro. My experience with large-scale mining is that it is often paired with hydro. This (probably biased) report at https://www.hydropower.org/greenhouse-gas-emissions claims 18g/kwh of CO2 from hydro.
So, the best-case lower bound would be 375MW1000 (to get to kwh) 24 (hours) * 365 * 18g = 59130000000 grams per year of CO2, or 59,130 tons of CO2 per year. Big emitters measure in kilotons, so 59kt. Note this does not cover other datacenter energy costs, but they are minimal compared to the mining.
For a rough comparison, Cruise ships (notoriously bad): Carnival in 2017 reported 10,539kt of CO2 emitted across their ships only.
The average US household produces 7.5tons of CO2 per year, so another way to look at it is that it is roughly 8,200 households of emissions.
Wikipedia estimates 90mm global users of Ethereum, so on average each user of Ethereum is emitting 0.00065555555 tons or 655 grams of CO2 per year for their Ethereum use.
Note that older miners won't be as power efficient. A reasonable rule of thumb is that 80% of mining power is with the most recent hardware. Also, there may well be 'dirty' miners out there, but most of the projects I'm aware of are looking for cheap energy, and in most places scalable cheap energy is hydro or possibly nuclear.
This is a gross oversimplification. How many of those 90M users would consider Ethereum to their primary medium of economic activity? What percentage of their economic activity is in ETH - 100%, 1%, 0.01%?
>The energy consumption downsides of proof-of-work systems are pretty well known. And those particular downsides don't apply to the systems you listed.
What? I have no idea what you're talking about.
Proof of Work energy requirements are not necessarily proportional to usage of the chain, while almost every single complaint about it has made invalid extrapolations under that assumption. It can easily be incredibly more efficient than the existing financial system.
OK.
https://digiconomist.net/bitcoin-energy-consumption
> Carbon footprint per transaction: 312.24 kg, estimated at 780,602 VISA transactions.