Cause and effect essay deals with exploring why things happen and what they may lead to. It's not compulsory to discuss both causes and effects of some issue in one paper, you can concentrate on either causes or effects of an event, action or phenomenon, depending on your objectives and discussed topic. But of course, there can be topics where discussion of both causes and effects is necessary. Thus, the issue of global warming, unless you explicitly decided to concentrate solely on causes or effects, needs a discussion of both.