World mayors are gathered in Copenhagen, Denmark, this week to discuss how their cities can fight climate change. We speak with one of the organizers of the summit, Mark Watts, executive director of C40 Cities. Also, the Nobel Prize in chemistry has been awarded to the three scientists who, over decades, developed lithium-ion batteries and created a portable technology revolution. And, Lisa Kern Griffin, former federal prosecutor and Duke University law professor, joins us to discuss if Congress can enforce subpoenas in the impeachment inquiry.