Mike Moffat has a valid point. It isn't, as David Suzuki sometimes contends, that economists don't understand "externalities" or claim we should ignore them. The term is a bit of economic jargon, after all. No. The people who ignore/downplay externalities are those folk who, for example, say they want to help clean-up the environment but decry a carbon tax or whine when somebody sticks a turbine out in the lake.
The rest of us, in other words.