Multi-gas approaches to climate change policies require a metric establishing "equivalences" among emissions of various species. Climate scientists and economists have proposed four classes of such metrics and debated their relative merits. We present a unifying framework that clarifies the...

In a model where firms face a continuous choice of how much to invest in environmental innovation, we show that an ever stricter environmental policy does not always lead to ever cleaner production methods and ever lower production of polluting goods. It does so when the abatement technology is...