http://cis.org/GreenhouseGasEmissions
Overall, our findings indicate that the average immigrant (legal or illegal) in the United States produces somewhat less CO2 than the average native-born American. However, immigrants in the United States produce about four times more CO2 in the United States as they would have in their countries of origin. The estimated 637 metric tons of CO2 U.S. immigrants produce is 482 million tons more than they would have produced had they remained in their home countries. This 482 million ton increase represents about 5 percent of the increase in annual world-wide CO2 emissions since 1980. These figures do not include the impact of children born to immigrants in the United States. If they were included, the impact would be even higher.