The oxygen provided Earths atmosphere by its oceans and rainforests is akin to what the species living in them get from their gills and lungs. Kill the oceans and the rainforests and most of the life on the planet suffocates. Right now they are dying
We could imagine the snow melt and glaciers that provide drinking water as a circulatory system. Once the glaciers and snow melt are gone the circulatory system no longer functions. The planet will have no pulse. We may live to see that in our lifetime but if that's the case, we won't live to see much else.
Perhaps the various oportunistic diseases, plauges, pestilence and famine headed our way will act as antibodies from the planets immune system ready to attack and flush away the nasty infection that has it ready to go into intensive care.
I'm going to focus just on methane hydrates. There are so many things going wrong its hard to see them all at once, but the problem posed by methane hydrates ought to be fairly easy to understand/
Right now there are by some estimates as many as three earth atmospheres of methane hydrates frozen in the planets ocean and tundra slowly begiining to outgas. Updated IPCC projections suggest that anything over 350 parts per million of greenhouse gases in the atmosphere will push us over the tipping point and doom us to a situation in which the poles begin to melt and our cities drown. We are presently at 450 ppm.
The warmer it gets the faster the hydrates outgas.
Methane hydrate doesn't stay around as long as CO2 but while its here its effect is 25 times worse. I'm expecting that if all that is true, and the hotter it gets the more methane is released, eventually we will be dealing with 3 million parts per million of methane and that won't leave a lot of room for oxygen.
Our present focus seems to be on drilling for it and using it as natural gas to alleviate our energy crisis. Government/Industry analysts figure that on the plus side a little global warming will provide a northwest passage and make getting to Prudhoe Bay easier.
Continuing as we are going, with people being a little two concerned with what the bottom line is, best case we should be able to get through the next few decades with nothing worse than our most of our major cities drowned, and humanity on the endangered species list. I'll allow that choice makes it possible that once it gets rid of us the planet should be able to recover, but it might be a better alternative if we got to stick around and see that.