The carbon dioxide pollution caused by BTC mining is almost negligible compared with the traditional industrial carbon dioxide production!
Compared to the exploitation of oil, gold mining, BTC mining is very environmentally friendly!
Yes i also cannot see any effect of BTC on global warming, there are a lot of other ways which are responsible for global warming and if it will not be control the situation will get more serious, but anyhow bitcoin or any other altcoin is not responsible for global warming, mining tool are contributing negligible amount of global warming.