It’s been eight years in the making, but Washington state finally has a rule that places limits on carbon pollution from its largest sources. It comes in response to reduction targets on greenhouse gas emissions first called for by the Legislature in 2008. That law called for limits that would get the state back down to 1990 levels by 2020, to 25 percent below the 1990 level by 2025 and 50 percent below it by 2050.