The New York Times called it one of the worst outcomes in a quarter-century of climate negotiations. United Nations Secretary General António Guterres said the international community "lost an important opportunity to show increased ambition on mitigation, adaptation and finance to tackle the climate crisis” at the recent UN Climate Summit in Madrid.But Harvard Kennedy School Professor Robert Stavins says global climate negotiators still accomplished something important last month at the COP25 conference—because of what they didn't do. Instead of approving lax rules full of loopholes that big polluting countries like Brazil and Australia were, negotiators held the line and pushed off a decision until next year's meeting in Scotland.Stavins, the A.J. Meyer Professor of Energy & Economic Development and director of both the Harvard Project on Climate Agreements and the Harvard Environmental Economics Program, tells host Thoko Moyo that getting workable economic solutions in place to combat the climate crisis is essential, because fundamentally the crisis was caused by economic activity. Stavins says his latest research shows that both carbon tax and cap-and-trade schemes can work, as long as they are well-designed.For more on Professor Stavins' thoughts on the COP25 summit and his research, check out his blog: An Economic View of the Environment. PolicyCast is produced by Ralph Ranalli and Susan Hughes.