Domestic and external pressures have driven the Chinese government to take firmer action to curb emissions from fossil fuels, especially coal. Engaging in a so called cap and trade initiative now, Mr. Xi will outline how he will halt the growth of China’s emissions by 2030. A cap-and-trade system, a concept created by American economists, means that governments place a cap on the amount of carbon pollution that may be emitted annually and companies can buy and sell permits to pollute. China, one of the world’s largest financiers of infrastructure projects, will agree to “strictly limit” the amount of public financing that goes toward high-carbon projects. China has been developing and carrying out smaller cap-and-trade programs for at least three years. President Xi Jinping of China arrived in the United States on Tuesday for a state visit at a crucial crossroads in the Sino-American relationship.