The instigator in this process is often oxygen-starved seas. Stagnant, low-oxygen seas are often preceded by rapid atmospheric warming. The tipping point is 1,000 parts per million of carbon dioxide, he says.
(We've written about the end-Permian extinction 251 million years ago, the greatest of all extinctions, and one thought to have occurred in precisely this manner.)
But, this warning aside, Ward's book isn't really about human-caused global warming. It's about the long-term future of life on the planet.
Organic life has repeatedly caused the collapse of the biosphere, and on at least one occasion (snowball earth) has almost extinguished it entirely.
Consider the advent of photosynthesizing organisms more than 3 billion years ago. Oxygen, a byproduct of photosynthesis, was deadly to the other organisms living at the time. As it accumulated, it killed — another mass extinction. (Of course, atmospheric oxygen also allowed for the emergence of multicellular life and, eventually, humans.)
The major long-term problem facing life on Earth is not, Ward says, global warming, but the gradual drawdown of carbon from the atmosphere by photosynthetic organisms.
Depletion of carbon dioxide has, in the past, caused dramatic global cooling. Twice before — once 2.3 billion years ago and again 700 million years ago — photosynthetic organisms used up such a large quantity of greenhouse gases that Earth froze over — what's called "snowball earth" — nearly ending life on the third rock from the sun.
In the future, Ward says, life's hunger for carbon will inevitably lead to a paucity of what's a fundamental building block of life, adding that this is the single greatest challenge facing life on this planet.
With graphs, he illustrates the long-term decline of atmospheric carbon dioxide over the past 3.5 billion years.