Chinese President Xi Jinping is arriving at the White House for a state visit with President Obama today, where President Xi is expected to commit to implementing a cap-and-trade system to limit China’s greenhouse gas emissions in 2017. A cap-and-trade system is a market-driven strategy in which governments cap emission levels and then allow companies to buy and sell permits to pollute. Environmental groups are saying the announcement is a step in the right direction, but that more needs to be done.