Covid-19 is certainly not going to change the world forever, but it is going to change quite a few things, in some cases for a very long time. Here is some of them. 1. The clean air over China’s cities in the past month, thanks to an almost total shutdown of the big sources of pollution, has saved twenty times as many Chinese lives as Covid-19 has taken. (Air pollution kills about 1.1 million people in China every year.) People will remember this when the filthy air comes back and will want something done…