Ecological footprint pow on pos platform

since ergo s stablecoin will be used on cardano s platform, i have questions about a pow on a pos network. learning that ergo is a pow protocol.
An important reason of not wanting to support pow platforms is the absurd energy use of pow mining. it surprsises me often how that theme is so little discussed. I couldnt find any info on that topic on the ergo site or on the ergo forum. Do i miss something there? isnt that one of the important reasons of changing from pow to pos?
So first a technical question how can a pow platform being deployed on a pos network?
And secondly, what is the ecological footprint of ergo comapared to cardano s protocol and compared to bitcoin?