It goes without saying that you should thoroughly research your essay. Relying on lecture notes alone is not acceptable; even the reading list provided by your supervisors is just a starting point. Peer-reviewed, scientific journals are the most reliable source of information. These texts exist as dusty tomes in college libraries but you will mostly encounter them online (cite them as if you were consulting the hardcopy). Remember that there is a big credibility gap between peer-reviewed journals and scientific magazines (e.g. New Scientist); note also that science is progressive and a journal article from the 1970s, for example, is likely to have been superseded by more recent publications. Text books are a good way of getting into a topic, but they are presented from the viewpoint of single author and date rapidly. An increasing number of websites provide very accessible, high quality data. Just bear in mind that they are rarely peer-reviewed: it is probably best to stick to the websites of reputable government agencies and research institutions (e.g. the Met Office, USGS). Avoid the temptation to cite lecture notes from other universities, unpublished dissertations, etc. Lecture notes should never be cited as a source.