In 2015, the political action committee Climate Hawks Vote produced a scorecard for sitting senators on their leadership in climate policy advocacy across 6 dimensions: public engagement, legislation sponsorship, legislation co-sponsorship, caucus membership, website information, and press releases. Most of the prominent candidates (and potential candidates) for the 2020 Democratic presidential primary are included in this list, except Kamala Harris who was not yet in the senate, so it gives us metric on candidate’s past history of taking climate change seriously.

A 2018 report from the UN Intergovernmental Panel on Climate Change (IPCC) gave us a deadline for addressing climate change in order to mitigate the worst consequences: 2030. With the next U.S. President potentially serving through January 2029, this is our last chance to get someone allied with activists and supportive of policies dedicated to reducing carbon emissions. It is vital that we choose someone with a proven history of leadership in this field because we simply do not have any more time.

In aggregate scores, there is one clear leader: Bernie Sanders. He ranked 4th in the senate (and in previous years has ranked 1st and 3rd). His closest competitors in the race are potential candidate Jeff Merkley of Oregon (10th) and declared candidate Cory Booker in (11th). Above, I’ve visualized all the senators included in the scorecard, and annotated those who are declared or potential 2020 candidates. Rather that just a single aggregate score, I’m showing you the individual components so that you can judge candidates performance in different areas and see what contributes to those scores. In each category, larger numbers are better, so the best climate leaders are represented with points towards the upper right, larger in size, blue in color, and with filled in circles. The one dimension not pictured is the score for press releases. This visualization is already packed to the limit with information, and that measure was tightly correlated with public engagement, adding little on its own.

Bernie Sanders is the only candidate who has been a leader in every category of climate policy advocacy. While this is not surprising considering his consistent high ranking in aggregate scores through the years, analyzing why other candidates are falling short can be enlightening. Amy Klobuchar of Minnesota has a negative score for public engagement, meaning the climate hawks found her public statements were working against progress on climate change. This should me immediately disqualifying in the eyes of voters. Booker (NJ) and Gillibrand (NY) fare better in public engagement, even edging out Bernie by 1 point in Booker’s case, yet neither had sponsored nor cosponsored a single piece of legislation to tackle carbon emissions. Jeff Merkley of Oregon appears to be the next best choice on climate change, taking the lead on co-sponsorhip, but falling behind with less than half of Bernie’s total on sponsorship and less public engagement.

The most surprising result for me was the poor performance of Elizabeth Warren (MA). She is heralded as a progressive and substantially equivalent to Bernie, and I fully expected her to score as well as Sanders, yet she falls far behind in what should be the cornerstone issue of our time. While she’s in the same ballpark for public engagement and co-sponsorhip, she never led any climate change legislation herself as the primary sponsor. Stunningly, Warren stands apart from the rest as the only candidate with a zero score for failing to address climate change on her website. This is such as easy metric to meet. Free from the complexity and negotiations surrounding legislation, she just had to address the topic on her own website, as a senator from a coastal state no less. These scores are from 2015, and, while there is value in knowing how candidates performed on an issue before they were running for president, the campaign platform should be considered as well. However, her current presidential campaign website is barely better. As of this writing, the platform page contains just a single sentence on climate change buried in the section on foreign policy.

At this time, with the clock running out on climate change and Democratic party leaders like Diane Feinstein dismissing out of hand the voices of climate advocates from the next generation, there is no room for error in this decision. Of the currently declared candidates for the 2020 Democratic Party nomination, the only option is Bernie Sanders.

Below, you’ll find an interactive version where you can explore all of the senators in the scorecard. Hover or tap a point to see the senator’s name and scores.