Not using bitcoin and its ilk is big to (in absolute terms its only so big but for what it does its a massive cost in energy).
I mean, yes, and no, Bitcoin actually produces less CO2 than the alternative, it’s just that people won’t ever stop using the alternative so all of that CO2 will continue being created. People have this wrong idea that Bitcoin replaces credit cards, when in fact what it replaces is money. And Money is a CO2 hell, it’s made of cotton, so you need to add the CO2 cost of producing, transporting and processing the cotton, add the cost of manufacturing the inks, printing the actual money, transporting it again, then all of the CO2 cost in keeping that safe, moving it from one place to another, etc, etc, etc… Yes, Bitcoin and the like consume a lot of electricity, but most server farms are in zones where electricity is very cheap, and that usually means green energy (hydroelectric, wind and solar plants produce a lot of surplus energy, so they sell it very cheap, which is why you’ll see most server farms for Bitcoin are located near such plants), but Bitcoin could process Visa level amount of transactions with that same amount of energy, i.e. it doesn’t need a certain amount of energy to process a single payment it needs a certain amount of energy to process a block of transactions, regardless of block size, which means that theoretically Bitcoin could replace all of the money in the world using the same amount of electricity it’s using now. And the hardware for the server farms could theoretically be old GPUs that would otherwise become e-waste.
Having said that there are technical limitations and a long debate on how to better scale Bitcoin, and old cards will never be as profitable as new ones so it’s unlikely that old cards would get used for mining, but they could if Bitcoin (or others) were designed around that idea. At the end of the day my point is that most people don’t consider the scale of what Bitcoin is replacing.