To date, 17 countries of the G-20—which account for 67 percent of global greenhouse gas pollution—have officially joined the Paris Agreement, bringing it into effect far sooner than anyone expected. If these countries follow through with their commitments to reduce emissions, it will represent unprecedented progress in the global effort to curb climate change. Unfortunately, U.S. President-elect Donald Trump has proposed a number of policies that would have negative climate implications. In light of this, the G-20 summit in July 2017 provides an important opportunity for other major powers to resist backsliding and even to make some progress in meeting the global climate challenge.