Krugman's Monday column was on how insidious pollution such as increasing CO2 concentrations does not get on the public radar screen the way oil spills, smog and burning rivers in Cleveland did in the 1960s.

This decline in concern would be fine if visible pollution were all that mattered — but it isn’t, of course. In particular, greenhouse gases pose a greater threat than smog or burning rivers ever did. But it’s hard to get the public focused on a form of pollution that’s invisible, and whose effects unfold over decades rather than days.

I think CO2 emissions are a threat. Something should be done to tax and/or cap greenhouse-gas emissions. But to say baldly that greenhouse gases are a greater threat than carcinogens in rivers and drinking water, or than smoggy clouds of nitrogen oxides, sulfur dioxide and ozone, which have killed thousands of people, strikes one as an overstatement.