At the Vox blog Georgetown’s Arik Levinson asks:

Since the 1970s, US manufacturing output has risen by 70% but air pollution has fallen by 58%. Was this due to improved abatement technology or shifting dirty production abroad?

He answers the question with some very nice empirical research. Here are his conclusions:

What is the bottom line? Increased net imports of polluting goods account for about 70 percent of the composition-related decline in US manufacturing pollution. The composition effect in turn explains about 40 percent of the overall decline in pollution from US manufacturing. Putting these two findings together, international trade can explain at most 28 percent of the clean-up of US manufacturing.

Why should we care?

If the 75% reduction in pollution from US manufacturing resulted from increased international trade, the pundits and protestors might have a case. Environmental improvements might be said to have imposed large, unmeasured environmental costs on the countries from which those goods are imported. And more importantly, the improvements in the US would not be replicable by all countries indefinitely, because the poorest countries in the world will never have even poorer countries from which to import their pollution-intensive goods. The US clean-up would simply have been the result of the US coming out ahead in an environmental zero-sum game, merely shifting pollution to different locations. However, if the US pollution reductions come from technology, nothing suggests those improvements cannot continue indefinitely and be repeated around the world. The analyses here suggest that most the pollution reductions have come from improved technology, that the environmental concerns of antiglobalization protesters have been overblown, and that the pollution reduction achieved by US manufacturing will replicable by other countries in the future.