I don't think it's very environmentally great, but then nothing much we do is as humans. Probably the newer tech like ASIC will be more energy efficient which will be good. Factoring the environmental cost properly into the cost of generating power (taking into account the type of power generation i.e. coal, nuclear, solar etc) and passing that cost on to the power consumer is probably the best way for the market to correctly rate their particular scale of mining operations impact on the environment because it will be reflected in the bottom line.

Where I live solar is common and grid power is not especially cheap, so it become more sensible to mine during the daylight hours only and use just the solar power. Though it slows things down considerably, if your power costs are substantial it can make sense.