There are several major global environmenal issues.
Perhaps the one with the greatest potential impact in terms of human suffering is the lack of clean safe drinking water. Population growth, industry, and agriculture have resulted in many depleted or polluted aquifers just as global warming is reducing glaciers. Many rivers run dry before they run to the sea, destroying wetlands. Desertification, due to unsustainable farming and grazing practices, makes land unable to absorb what rain does fall, leading to alternating flood and drought cycles.
Global climate change and ozone layer deterioration is another major global environmental issue, further complicated by population growth that expands human habitation into flood and other extreme weather zones.
Finally, deforestation and species extinction are a global problem, especially in the tropics.
Clean water is a major global issue