Encyclopedias give you the background, including major events, ideas, problems, and facts. Read encyclopedia articles before you read scholarly journal articles, and you will have a better chance of understanding the specialized vocabulary that experts use with each other.
Encyclopedia of Global Warming and Climate Change by 
Encyclopedia of Science, Technology, and Ethics by 
Encyclopedia of World Environmental History by 
Environmental Encyclopedia by 
The Gale Encyclopedia of Science by Wikipedia is an encyclopedia, but an encyclopedia that anyone can edit. While the quality is going up, it doesn't work on the traditional publication model. It does not depend on academic experts or editors. If you use a Wikipedia article, use your critical thinking skills to evaluate it: does the author have authority? is it well-written? does the information compare similarly to other information you read? is there a well-formed reference list? does the reference list have a variety of sources? Sometimes Wikipedia is useful to get ideas and point you to other resources, but you probably don't want to cite it in your reference list or paper.
To learn more about evaluating sources you find on the web, in reference sources, and in databases, visit the Highline College Library Evaluate Information Guide linked below.