Trade, the exchange of goods and services across countries, is often viewed as an engine of economic growth. Benefits of liberalized trade include access to a larger variety of goods and services to consumers, easier access to foreign technologies, access to larger markets for producers, and increased efficiency in resource allocation. The impact of trade on the environment, however, is a contentious issue; air and water pollution, the degradation of natural habitats and loss of species, and global pollutants, particularly carbon dioxide emissions, are major concerns.