These permits set an enforceable limit, or cap, on
the amount of greenhouse
gas pollution that
the company
is allowed to emit. Over time,
the limits become stricter, allowing less and less pollution, until the ultimate reduction goal is met. This is similar to the cap and trade program enacted by the Clean Air Act of 1990, which reduced the sulfur emissions that cause acid rain, and it met the goals at a much lower cost than industry or government predicted.