As China scrambles to respond to the choking smog that has blanketed Beijing in recent weeks, authorities in several major cities are experimenting with so-called carbon-trading markets. The schemes allow air polluters to buy and sell official permits for their harmful carbon emissions, and create strong incentives to cut these climate-changing pollutants. Carbon trading is the latest effort to get control over run-away greenhouse gases in an economy still hungry for cheap energy. Shannon Van Sant reports from Beijing.