text: Someday, China's leaders may, for their own reasons, alter their stance on climate. If so, and if Beijing can verify that it will adopt a price on carbon emissions, Washington should be ready to match it. For the foreseeable future, though, Beijing's verifiable preferred price on carbon is zero. Until that number changes, the main U.S. line of defense against the risks of climate change is not a carbon tax. It remains a free and productive economy.