There were many environmental issues in the 1990s. Some of them continue to be issues today while at least one has largely been solved.
Early in the decade, a major issue was that of acid rain. This is caused largely by emissions from things like power plants. Regulations that were created late in the '80s have largely solved this problem today.
But other issues from the '90s continue to be important today. One of these is deforestation that is going on around the world. The most important one, however, is global warming. This issue first appeared on the global "radar screen" in the '90s. It is, of course, the major environmental issue facing us today.