The chart below is adapted from a similar graph in Dr. Peter Ward’s book, “Under a Green Sky.” It simply plots all the mass extinction events of the last 500 million years against the best estimate of carbon dioxide levels (CO2) at the time. According to his analysis all major extinctions occurred when CO2 levels exceeded a thousand parts per million (ppm).

The cause for concern is that the current CO2 level — approximately 393 ppm — is projected to reach a thousand ppm in approximately one hundred years at the current rate of increase. What is unknown is how quickly such a chain of events could occur, and precisely what they are. In the above cited book, Ward offers a hypothesis about the changing ocean chemistry as CO2 and temperature rise significantly.

It is still hard to project the rate of change, due to the extremely fast change of CO2 levels in modern times. Dr. James Hansen, a leading climate expert points out in his book “Storms of My Grandchildren” that at the current rate CO2 will increase one hundred ppm in approximately 40 years. During past periods of abrupt change — the most recent one occurring approximately 50 million years ago — it took roughly a million years for CO2 to change by one hundred ppm. Thus it is now changing about 25,000 times faster than in known geologic history.