August 30: California limits greenhouse gases from cars on this date in 2006

The reason the U.S. auto industry mostly agreed to meet a 54.5 mpg fuel economy standard by 2025 finalized this week isn't because of some conversion to full-on environmentalism, or as political payback to either Democrats or Republicans. The real reason lies in the hands of Arnold Schwarzenegger in this photo, signing California Assembly Bill 32, passed on this date in 2006, that made California the first state to limit greenhouse gases, including those from vehicles. No single law makes a bigger impact on what you'll drive in the future.

There's been five years of legal and political wrangling since that bill was signed, but in a nutshell: Federal law gives California a power other states don't have to regulate pollution. Since carbon dioxide from cars and trucks is a pollutant, California can control how much they emit -- and the only real way to limit CO2 from vehicles is through burning less gas. After several rounds of legal battles that went to the U.S. Supreme Court, automakers faced either letting California set its own fuel efficiency standards -- which could apply to about half of their sales -- or embracing federal rules that, while tough, would be less onerous and nationwide.