This claim appears every now and then. It is a good example of the problems with all of Monckton's arguments about climate change: i) it is based on an analogy, rather than an actual physical mechanism, ii) it ignores time scales, and iii) some of the "data" used to advance his argument is simply made up.
I've included the full responses from the three experts on the coral question (Jeffrey Kiehl, Charlie Veron and Nancy Knowlton) below. Their incredulity says enough.
Response from Dr. Jeffrey Kiehl
It is ironic that Monckton will accept that the geologic record clearly indicates that high CO2 leads to warm climates (thus CO2 is a driver for climate), but then uses the existence of life at these times to conclude that we need not worry. The point is that past warm periods developed over millions of years of time and lasted for millions of years. Thus, species could adapt to these changes. However, we also know that some species did not adapt. The concern about the future is that the rate of warming that is occurring and will continue to occur over the next century is unprecedented in the deep past. It took over 30 million years for CO2 levels to drop from 900 ppmv to their present levels, we are returning Earth back to this level in a mere 90 years. The accompanying rate of warming will also be unprecedented, certainly over the lifetime of our species. The issue is that our species and others will experience a rapid and large change that will have significant impacts on survivability. So both of Monckton’s arguments are flawed.
Response from Dr. John Veron
It is not possible for me to make any sense of Mr. Monckton’s assertions as they are not based on any scientific data or views that have ever been published. The levels quoted are higher than any spikes known to have existed. The time intervals quoted bear no relevance to the history of life.
Specifically:
Cambrian Era: Calcium carbonate (limestone) of the Cambrian, which abounds, has nothing to do with atmospheric carbon dioxide. Estimates of carbon dioxide levels at this time are not known with great certainty. There were no corals in the Cambrian, symbiotic or otherwise: they had not evolved then.
The Jurassic: There were high levels of carbon dioxide possibly reaching 2000 ppm for unknown time intervals with unknown effects on marine life. The spike immediately before the Jurassic caused the third great mass extinction. This extinction, which defines the Triassic/Jurassic boundary, was so drastic that it has been known
since the early 18th century.
Response from Dr. Nancy Knowlton
This paragraph completely ignores the fact that the seawater chemistry and the buffering capacity of seawater were very different during the times described from what they are today… The problem with CO2 emissions today is that the effects of burning fossil fuels on ocean pH first operate on the scales of decades to centuries, thus causing the acidification that has been observed. Eventually the pH of the ocean will be buffered again, but for hundreds of years ocean organisms will be affected by abnormally high acidity (low pH), and it is the damages associated with acidification over the “short” term (the next hundreds of years) that concern biologists. (see also Assertion 4)
Response from Dr. Lee Kump
One must carefully distinguish between conditions that were acquired and sustained over millions of years such as these, and abrupt events such as fossil-fuel burning that disturb these longer-term equilibria. Over long time scales the carbon cycle is balanced, and the oceans (and the life in them) can form limestone at essentially any atmospheric CO2 level. On these long time scales, rivers bring the building blocks for the calcium carbonate skeleton to the ocean; when CO2 levels are high, these compounds must accumulate to higher concentrations to overcome the increased acidity generated by the CO2, but this adjustment takes only millennia.
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