Global warming is an environmental issue because it affects the natural environments of various parts of the world. It does so in ways that hurt humans directly and in ways that alter environmental conditions for nature.
Global warming brings about changes in the environment in many places. There are many examples of how it does this. It can melt the polar ice caps. This would lead to higher sea levels. Higher sea levels would flood low-lying areas of the world. This would be catastrophic for many densely inhabited areas. It would also be problematic for some forms of non-human life. The melting ice might make it harder for polar bears to live. Rapidly rising sea levels could drown coral reefs. Warmer waters will also affect those reefs.
On the land, global warming could do things like affecting the patterns of precipitation. It could increase rainfall in some areas or at some times of year and decrease it in other places or times. This could have a tremendous impact on the sorts of plants that will grow in certain places. This can affect human beings by impacting the kinds of agriculture that are possible in particular places.
Since global warming affects the environment in many ways, it is an environmental issue.