California reached a critical emissions reduction milestone: The state's greenhouse gas emissions dipped below 1990 levels in 2016, according to data released Wednesday by the California Air Resources Board.

That's four years earlier than California's goal of reaching 1990 levels by 2020—a huge accomplishment and one that proves states can slash emissions and experience economic growth.

However, an even bigger challenge lies ahead for California. In 2016, the state legislature set a goal of cutting emissions by another 40 percent by 2030.

California emissions have dropped by roughly 13 percent since they peaked at 493.7 million metric tonnes in 2004. But it has not been a steady decline: Total emissions rose in both 2007 and 2012, and held steady between 2014 and 2015. So while emissions fell to 429.4 million metric tonnes in 2016—the latest year for which data is available and slightly below 1990's 431 million metric tonnes—there's no guarantee levels will stay there. For one thing, emissions from the state's primary source—the transportation sector—are rising.

Thanks, at least in part, to cheap fuel prices, transportation emissions rose by 2 percent in 2016.