Short answers, pulled from the story.
Emissions trading allocates or sells a limited number of permits to allow discharge of specific pollutants over set time periods. Polluters must hold permits equal to their actual emissions, while those wishing to increase output buy permits from others willing to sell them.
The prototype phase launched the first cap-and-trade system under Title IV of the 1990 Clean Air Act in 1995. C. Boyden Gray conceived this acid rain related system while working with the Environmental Defense Fund.
Sulfur dioxide emissions from Acid Rain Program sources fell from seventeen point three million tons in 1980 to about seven point six million tons in 2008. This represents a decrease in emissions of fifty-six percent.
California and Québec successfully linked their systems in 2014. Ontario and Manitoba agreed to join the linked system between Quebec and California in 2015.
Firm-level causal evidence indicates EU Emissions Trading System reduced regulated manufacturers' CO₂ emissions by fourteen to sixteen percent without detectable losses in output or employment.