Wow I just got done doing an entire module on environmental physics half of which was pretty much studying this topic. I would like to point out that although the main aspect in which humans do contribute to the environment is through CO2 emissions, it is rather a chain of feedback mechanisms that tends to harm the environment in the long run (CO2 itself is a very weak greenhouse gas). Examples such as:
-Warming due to CO2 leads to more evaporation of water and more water vapor in general in the atmosphere, Water is a much stronger greenhouse gas than CO2 and can cause even more warming.
-Warming leads to less ice cover. Ice cover normally reflects sunlight (by increasing earth's albedo) but without ice cover more radiation will be absorbed leading to more heating of the earth.
-The oceans themselves can absorb some of the CO2 but warming of the oceans means less CO2 can be absorbed my chemical reactions.
There are many more mechanisms but these are just a few. That said people often have the misconception that greenhouse gases are a bad thing, they are not. Without any CO2 (and other greenhouse gases), the earth's temperature would be approximately -10 degrees C making it uninhabitable, but the warming from these gases has made the temperature much more suitable for life to exist. But too much heating on the other hand would again make it uninhabitable (look at Mars and Venus, too much CO2 in their atmospheres). This is much more complicated of course affecting biodiversity, weather, etc. So we better not mess with the balance too much which the earth has kept for so long now.
