I want to clarify something I said in the nitrogen post about carbon dioxide. I mentioned that anthropogenic carbon dioxide emissions still can't compare to natural carbon dioxide emissions. This may have given the impression that human contributions aren't the biggest problem since nature gives off more each year (by a factor of around 11 for just plants and animals alone). This is actually a common argument: we needn't worry about our emissions since nature is emitting a lot more than we are. This is in fact a deep misunderstanding. While it is true that plant and animal life give off far more carbon dioxide than we do, vegetation absorbs slightly more carbon dioxide than the total given off naturally. In other words, nature absorbs slightly more CO2 than it gives off, so each year planet Earth breaks about even. Human emissions, then, are on top of that balanced cycle, leading to an overall increase in CO2 levels each year.
Consider a large water tank connected to a pool by two hoses: one hose delivers water to the pool, the other hose brings water from the pool back to the tank. Let's say that each day the tank delivers 1000 gallons to the pool, and 1000 gallons are also returned to the tank throughout the day. A little boy playing with a bucket and a garden hose decides to fill up his bucket and dump it into the pool. His mother comes out and, after watching him dump bucket after bucket into the pool, warns him to stop or else the pool will overflow. "Don't be silly, Mom!" the boy yells as he fills up another bucket, "that water tank puts way more water in the pool each day than I do, how could these few buckets of water make any difference?"