tldr: they weren't that bad before (and, by extension, are still basically 'nothing' on the grand scheme of ecological harm). I'm about to cite some papers from the pre-AI era, so you can get a sense of what usage was like around 2021, before this discourse took off
I'm not aware of any large scale ecological malfeasance associated with data centers. i won't say "it hasn't happened" but i haven't heard of it.
large cloud providers (such as google/microsoft/etc) are actually significantly more energy efficient than traditional data centers. this is from a paper about water use (where it's an indirect measure) but as a direct measure of energy efficiency, you can see how much power is going to providing compute vs being wasted as heat
aside from some modest efficiency gains, among those "hyperscalers", power requirements per "unit of compute" and hardware across the industry are not substantially different than they were five years ago. the same major players in the industry are building and operating them. it feels different because these things are in the news and the popular discourse, but the industry was always going to grow*-- the world uses a huge amount of data.
the ecological costs of data centers have been wildly overblown. they are significantly less polluting than any heavy industry-- they're not using vast amounts of chemicals or doing intense manufacturing.
if you want to talk about water usage specifically, because this is the thing that people have latched onto, you need to look at the numbers in context. here are three numbers in context
[source: Mitton 2021, published in Nature] as reported in 2021, it was estimated that data centers were using 1.7 billion liters (449,092,489 gallons, four hundred forty nine million gallons) of water every day.
[source: USGS, 2015, estimated use of water in the united states] in 2015, the livestock industry (primarily beef) withdrew 2,000,000,000 gallons (two billion gallons) of water per day
[source: USGS, 2015, estimated use of water in the united states] in 2015, total united states freshwater water consumption was 281,000,000,000 gallons/day (two hundred eighty one billion gallons)
i wrote those numbers out in text not to be condescending but genuinely, staring at a number like that, it's hard to grasp the scale of what is going on. five hundred million gallons, approximately what data centers were using every day, is a quarter of what the livestock industry (2 billion gallons) is using every day, and both of those are less than half of 1 percent of domestic freshwater usage 280 billion gallons). even if the number of data centers has doubled since 2021 (it hasn't), it would not equal the livestock industry, and would come nowhere near irrigation, which accounts for 37% of US water use
tbh this is a subject i don't really know what to say about, aside from trying my best to point people at the numbers and compare them to other industries and hope that communicates a sense of scale. the opposition to data centers is not based in a good understanding of the scale of water and power usage across the board, and is just people venting their frustrations about technology they don't like. which is fine, be frustrated, but please, let's get real! the ecological impacts of this are less than so many other industries it never even crosses people's minds to think about
*the mitton article in Nature that I cite is a pre-AI boom paper, and it is already predicting a large buildout of data center needs from just "traditional" data center usage