the risk of mass death like PT repeating itself

"Those who ignore the lessons of history are doomed to repeat them."
In the history of the past billion years, there was an event ("PT") which killed every living member of most species of life on the surface of the earth, through levels of poison (H2S) and radiation high enough to kill every human. What do you say when the world's most prominent expert on that event says we are firmly on track to repeating what happened then? It's not a simple story, but it is a serious one.
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For me, the story began when the Geosciences Directorate of NSF hosted and promoted a talk by Peter Ward, author of Under a Green Sky, who told this story.
As a first step, I studied and restudied his book; I have posted a condensed summary of what it actually says here:

http://drpauljohn.blogspot.com/2012/01/condensed-summary-of-under-green-sky.html

But being a very critical and independent scientist myself, I do not take his story at face value.

Ward basically forsees two steps, which he thinks will happen if we
let CO2 to get as high as 1000ppm (which
is consistent with current policies to expand fossil fuels):

1. Due to melting ice and rise of temperature at the poles, we may
shift the pattern of ocean currents to today's
"mixed ocean" to a "stratified ocean." Climate deniers often talk
about how CO2 was much higher at the time of dinosaurs
(really, paleozoic and mesozoic time) than it has been since mammals
started ruling the earth. That is true -- but it
also led to a stratified ocean at those times, and repeated (10 or 12)
mass extinctions due to certain instabilities in that
kind of ocean. Of course, that won't happen immediately, and those who
don't care whether everyone dies even as soon as 10 or 20
years from now would regard it as quite irrelevant to their values.
(But this is a century-off kind of risk.)

2. A "trigger," which he claims is due to the RATE of increase of CO2.
Since CO2 is now increasing at a RATE
a hundred times what it did in the past extinctions (all of which
occurred at peaks of CO2, 10 of 12 at peaks after
rapid increase of CO2), he says that will also follow as the night the
day if we get to a stratified ocean now.

In 2009, I saw his impressive graph of extinctions versus CO2 ... but
was not convinced. He said that the RATE
of CO2 matters because the ocean can buffer slow changes in CO2; rapid
changes lead to acid, a critical trigger.
But CRS said we should worry about pH as low as 7.7 (not so acid) by
2100 in the worst scenarios.

This week, I went back to the actual technical paper which is the real
origin of the kind of theory Ward was proposing:

http://geology.gsapubs.org/content/33/5/397.full

Seminal work from Pennsylvania... I wish I had tracked it down in
2009, when I was working in Specter's office, attached to EPW work. And there is also a paper on the history of acidity in the
oceans which goes with it.

The bottom line: acidity isn't really the trigger here, at least not
in H2S production in the ocean.

The triggers -- once there is a stratified ocean -- are simply further
current changes, supporting anoxia and warmth, the two
triggers for greater H2S production by creatures "under the chemocline."

Anoxia is already the prime suspect in the death of coral reefs, more
than ocean acidity. We are getting lots of it from sources like
agricultural runoff,
in the age of fertilizer. That plus warming and further current
changes could get to an H2S event without waiting for a low pH.

Can we predict THEM? Or change THEM? That is not so clear.

HOWEVER -- the two URLs I cited last time are very important, too, as
I discussion.
One can find Kung's 2005 paper in google scholar (I searched on Kung
PSU, and immediately recognized
which was the relevant one, cited by Ward, whose URL I sent you yesterday).
When I did that -- google scholar lets you see links to all 158 papers
which have cited Kung's paper since then,
many giving hard data confirmation from other avenues, a few
skeptical. I read through all of them yesterday;
for some I only read abstracts because of "$30 per click", but they
seemed unlikely to change the story which emerged from the others.

It seems clear that MOST upwelling regions of the ocean suffered from
the anoxia (and then H2S) problem
Kung discussed. But not all. Kung's original paper discussed this. The
most intense skeptic (who said his model is better than Kung's) said
that that volume of H2S would not be enough to zap the ozone layer (on
top of poisoning everyone); however, he said it would cause an OH-
deficit which would result in methane causing the same effect.

As I think about the implications... if ONLY the north pole became
de-iced and warm, that should logically be enough to
allow a stratified ocean in the northern hemisphere but not the south.
That would still be a rather serious problem.

Ward talked at the end about the need for more understanding of
thermohaline currents, how they can change radically
(e.g. in older earth history)... and that does seem to be the key to
understanding when stratified oceans become mass killers.

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Best of luck. We need it.