One strange effect of Donald Trump’s election to the presidency has been to vault California Gov. Jerry Brown, one of America’s most fervent climate warriors and a failed presidential candidate himself, to national leadership on what he calls “the existential threat of our time.” As Trump promises to roll back President Obama’s climate agenda, California’s aggressive efforts to cut carbon emissions, along with Brown’s globe-trotting courtship of like-minded Canadian provinces, Chinese cities and other jurisdictions…