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Building cities outwards necessarily either requires removing farmland or destroying ecosystems, which is far and wide the largest contributor to extinctions. Since a significant goal of lower emissions is to save the environment and all its inhabitants, urban sprawl goes directly against this.

Building out fiber to remote areas doesn't help either as jobs continue to demand the worker be there during work hours. Look at what Yahoo did for an example.

I'm not saying building up is the answer either, as skyscrapers aren't exactly environmentally friendly either. If the answer were to be to retrofit existing buildings, and the process to build the transparent solar cells was sustainable and economical, then I'm all for it. Unfortunately, we've all heard the spiel before, and yet years have gone by and no one is building buildings with integrated solar windows. I'm not holding my breath that it starts now.