They used six criteria, including carbon emissions per capita, air quality, water quality and policies.
The results can be surprising. The states that are greenest are not necessarily the ones you would expect. Oregon and Washington being in the top 3 is scarcely a surprise from what I know of those states. But Vermont came out #1. Even New Jersey and New York came out greener than California, which is certainly not how people think of those states.
California took big hits on air quality due to certain areas having relatively poor air quality, despite that California on the whole has great policies for the environment.