The market for greenhouse gas emissions is a bazaar - dependent entirely on government regulation - selling various types of pollution. But can it reduce emissions? The answer so far: Not yet. Second of three parts.

Air pollution over the Indian subcontinent shows some surprising seasonal changes, say U.S. scientists analyzing 10 years of satellite data. During the monsoon season, rains wash some of the dust and soot from the air, but other man-made pollutants continue to build up.