The Sixth Extinction: An Unnatural History

cope with massively fluctuating conditions? Well, it’s like that when you
ramp up CO 2 .”

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ROUGHLY one-third   of  the CO 2    that    humans  have    so  far pumped  into    the

air has been absorbed by the oceans. This comes to a stunning 150 billion
metric tons. As with most aspects of the Anthropocene, though, it’s not
only the scale of the transfer but also the speed that’s significant. A useful
(though admittedly imperfect) comparison can be made to alcohol. Just as
it makes a big difference to your blood chemistry whether you take a
month to go through a six-pack or an hour, it makes a big difference to
marine chemistry whether carbon dioxide is added over the course of a
million years or a hundred. To the oceans, as to the human liver, rate
matters.
If we were adding CO 2 to the air more slowly, geophysical processes,

like the weathering of rock, would come into play to counteract
acidification. As it is, things are moving too fast for such slow-acting
forces to keep up. As Rachel Carson once observed, referring to a very
different but at the same time profoundly similar problem: “Time is the
essential ingredient, but in the modern world there is no time.”
A group of scientists led by Bärbel Hönisch, of Columbia’s Lamont-
Doherty Earth Observatory, recently reviewed the evidence for changing
CO 2 levels in the geologic past and concluded that, although there are

several severe episodes of ocean acidification in the record, “no past event
perfectly parallels” what is happening right now, owing to “the
unprecedented rapidity of CO 2 release currently taking place.” It turns

out there just aren’t many ways to inject billions of tons of carbon into the
air very quickly. The best explanation anyone has come up with for the
end-Permian extinction is a massive burst of vulcanism in what’s now
Siberia. But even this spectacular event, which created the formation
known as the Siberian Traps, probably released, on an annual basis, less
carbon than our cars and factories and power plants.
By burning through coal and oil deposits, humans are putting carbon