I have no idea what I would say is the biggest challenge to the environment.  There are so many problems and I believe each is connected with the other (a perverse interdependence).  Global warming is due to many factors, and many claim it is the largest problem.  The hole in the ozone layer is not seen as large as an issue as it did in the 1980s, but it plays a role in many environmental issues.  Thus, I do not know which is the largest challenge or the most important issue regarding the environment.  However, I personally believe deforestation is a much larger issue than it is given credit for, mostly because there are better sources of paper and many trees are cut for paper.  Water and soil pollution are huge concerns of mine, as well.  Some of these issues of water and soil pollution could easily be changed (not necessarily fixed, but at least lowered) if those in the agricultural community would switch to a sustainable agriculture model.  In addition to lowering environmental costs, the farmers would save on pesticides and herbicides if they adopted a sustainable program.