A primary source is the first place research appears. In sciences that means published journal articles (also called papers). Researchers do experiments and when they have interesting results they write them into a paper and publish it in a journal or a repository. The purpose of reading the primary source is to get the original data, not someone else's interpretation of the data (a secondary source). Books, encyclopedias, dictionaries, review articles, and textbooks are all secondary sources because they have already processed the information in the primary sources for you.
A scholarly research article is an article that has been published based on research data. This is not to be confused with the regular kind of article you'd find in a newspaper or magazine. Scholarly sources are often peer-reviewed. Peer-reviewed means that when a scientist sent their article to a journal to get it published, other scientists (their peers) read the article and comment on it. The idea behind peer-review is that other scientists can judge what is worth publishing or not. A journal without peer review would accept any articles submitted or would leave the choice up to an editor.
Data and statistics on environmental issues can be most often found with government institutions and not-for-profit agencies. Some large organizations have been provided below. It can be helpful to think of what kind of organization would be most interested in your topic of research, for example for information about the Arctic the Canadian Government would have some resources, but also Russia and Denmark.
More detailed help can be found in the following guides: