19th Centuries, the Earth's atmosphere was subjected to massive injections of industrial pollutants, seriously aggravated with the development of the internal combustion engine and its reliance on fossil fuels.  Centuries of industrial activity, during much of which little attention was paid to the carbon dioxide emissions being pumped into the Earth's atmosphere, compounded by the introduction and proliferation of the automobile, have damaged the Earth's atmosphere.  Another factor in environmental degradation was the wide-spread use of chlorofluorocarbons (CFCs), an organic compound used in aerosol sprays and, more importantly, in automobile air-conditioning systems.  Massive quantities of CFCs were determined to be causing irreparable damage to the Earth's ozone layer, which helps protect the planet from dangerously-high levels of ultraviolet rays from the Sun.

Another major cause of environmental degradation is the wide-spread practice of deforestation.  Plants produce oxygen.  They also protect the soil from harsh weather conditions, like torrential rain that, absent thick vegetation, would cause massive mudslides.  In much of the world, including South America, Southeast Asia, South Asia, and Russia, as well as regions of North America, enormous levels of deforestation caused by slash-and-burn agricultural practices and mankind's insatiable thirst for lumber have seriously diminished some of the planet's largest and most ecologically-vital forests.  Not only has this reduced the amount of mature vegetation needed to support the oxygen requirements of all living things, but massive mudslides have killed thousands of people throughout regions of Asia, for example, in Bangladesh, that would not have occurred had deforestation not been permitted.  Additionally, with deforestation comes a process known as desertification -- in effect, the spread of deserts unsuitable for human habitation.

Finally, as the earlier answer notes, population growth has put an enormous strain on the planet's nonrenewable resources.  While most people are familiar with the competition for such natural resources as oil and natural gas, just as important is the growing problem of fresh water resource availability.  As populations grow in regions like the Indian subcontinent and tensions remain in the Middle East, it can be anticipated that additional stress will be placed on scarce supplies of fresh water that can be expected to lead to conflict as well as further damage to the environment.