General resources to get you started with selecting a topic of interest:
GREENR is a research database dedicated to environmental topics and arranges them by subject. It also clearly divides the search results into categories like journal articles, magazine articles, statistics, reports, news, organization reports, and more!) Use it to browse around for something of interest to you and maybe get a few sources along the way.
CQ Researcher includes brief overview reports on controversial topics, each with a wealth of links and citations to additional resources. Just click the "Browse Topics" and focus on the environmentally-related issues.