You need to cite any data you use in a paper if it is not data you generated yourself. You have to cite data sources, just as you would ideas from articles or books.
The OECD Factbook provides a global overview of todays major economic, social and environmental indicators.
Data to support your research in environmental studies can come from a variety of sources, and may be collected and produced by researchers and scholars in a variety of disciplines too. Depending on the nature of your topic, you may need data collected or disseminated by sociologists or demographers, biologists, earth scientists, chemists, economists, public heath researchers, etc.
Some strategies for finding data:
