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.