William Brangham:

He rolled back auto emissions standards. And he pushed to increase the use of coal and other fossil fuels.

Even though all the Democrats agree climate change is real, they have some pretty different ideas for how to tackle it. The signature rallying cry from the left has been the Green New Deal, a nonbinding resolution introduced in the House and Senate in February.

It calls for a sweeping overhaul of how the U.S. generates energy, and it sets an ambitious deadline for U.S. energy production to be net zero. That means no net carbon emissions in 10 years.

For now, there's no estimated price tag on this hugely ambitious idea. Of the 11 candidates who are currently in Congress, eight co-sponsored the Green New Deal, and another seven candidates not in Congress have said they support some version of it, including former Vice President Joe Biden.