People who minimize or deny the threat of climate change (or ocean acidification, as in part IV of Dr. Everett’s testimony) will often demand that the change be “unprecedented” – that nothing like it has ever happened before in Earth history. (eg, here) The reasoning seems to be that if there have been ecological events like anthropogenic climate change in the past, then current events must not be alarming, since life on earth has each time survived and recovered:
“We know that the Earth has seen these conditions before, and that all the same types of animals and plants of the oceans successfully made it through far more extreme conditions. ” – Everertt (part V)
This has always seemed to me like it’s setting the bar a bit low: Do we only become alarmed when faced with the possibility of sterilizing the planet? And considering the amount of violence which earth life has withstood over the ages, it doesn’t seem a very strong statement that human impact is unlikely to wipe it out.
To back up his claim that ocean acidification is not a concern because it has occurred in the past, Dr. Everett shows us a graph of atmospheric carbon dioxide concentration over time, taken from the IPCC:
This graph does indeed show, in the IPCC’s words, “Pre-Quaternary climates prior to 2.6 Ma … were mostly warmer than today and associated with higher CO2 levels.” Does this mean that current increases in CO2, and their associated warming and acidification, are not alarming? Not at all.
We’ve already seen Dr. Everett slurring the distinction between quantities (like the concentration of CO2 in the air) and their derivatives (the rate at which a quantity like concentration changes). Here the distinction shows up again, because it is not merely the concentration of greenhouse gasses, or the temperature of the earth, or the pH of the oceans that is important to ecosystem health, so much as the rate at which those things change. The reason is that organisms are adapted to their environments, and when their environments change, the organisms must re-adapt. But adaptation isn’t instantaneous; there is an upper limit to the rate at which organisms can evolve.* Dr. Everett has pointed this out already, albeit as criticism of ocean acidification experiments:
“… studies have not been long- term enough to discover adaptations over multiple generations. I believe this is key because these genera have genetic information about past events and this may well take several generations for stabilization. In any scenario, there will be ample time for this to happen. In a laboratory it happens with the throw of a switch. ” (part III)
The problem is that there is no particularly good reason to think that organisms can adapt to their environments as quickly as human activity changes those environments; we may well lack “ample time” (eg, here). Reconstructions of past temperatures don’t just show relatively high temperatures after the industrial revolution, they show an abrupt increase in the rate at which the temperature changes. That’s why they’re shaped like a Hockey Stick. One of the most striking illustrations of the speed of ocean acidification comes from a slide about 80% of the way through this presentation by Andrew Knoll. It’s a graph of the rate of change in ocean pH over time. It’s pretty small and constant up until the industrial revolution- at which point the bottom drops out not only of ocean pH, but its rate of change as well.
So that’s the past. What about the future?
We don’t have a crystal ball to tell us the course of future carbon emissions, so we’re limited to exploring the consequences of various emissions scenarios; part II of Dr. Everett’s testimony was devoted to (unconvincingly) attacking a commonly used emissions scenario. Let’s compare the rates of carbon buildup under a few scenarios we’ve discussed.
Even under Dr. Everett’s preferred scenario, in which rates of carbon accumulation remain at current levels, the rate is far above preindustrial levels. Under the projection of rate data and the IS92a scenario, future rates will be even higher.
To give you a better idea of just how large and abrupt a geochemical shift we’re looking at, I used the above graph** to create this comparison of the size of carbon emissions over time. It’s dramatic.
Even if a population adapts to an ecological shift, it’s not necessarily good news. The price of evolution is natural selection, the death of individuals which are no longer fit in the altered environment. In a rapid, catastrophic event, so many individuals may die that the remaining population lacks the genetic diversity to withstand further environmental change. One example is a recent experiment in which a population of stickleback fish was raised in a significantly colder environment than it was used to. Although the fish adapted, the population lost much of its genetic diversity in the process. I can’t put it any better than this writer:
“Genetic diversity is critical to maintaining populations, and a period of such strong natural selection will dramatically reduce a population’s diversity. Even if a population can adapt to one sudden shock, it may so deplete their genetic diversity that there won’t be any convenient alternative genes in the population when the next hit comes.
The next year brought the coldest winter that part of Canada had seen for several decades, and despite all their adaptations, all three of the experimental populations were wiped out. It may be that it was just too cold, or perhaps the increased ice cover on the ponds reduced the oxygen levels in the water to below what the fish needed. Either way, it’s a grim prospect for conservation biologists if a population that seems, by all accounts, to be surviving and even adapting to the changes in its environment can suddenly hit an unpassable barrier and go extinct.”
Dr. Everett tries to dismiss concerns over ocean acidification by pointing to historic periods of high carbon dioxide levels. However, the speed of current makes it difficult to draw analogies with past events- some authors have estimated that environmental pressure on vulnerable organisms is at the greatest point in the last 65 million years. But to the extent that we can make comparisons, one thing stands out: despite what Dr. Everett says, acidification harms calcifying organisms. We’ll look more at what the geological record has to say on the subject next time.
* The upper bound on the rate of evolution is often a stumbling block to creationists as well, who will demand an observation of “a cat giving birth to a dog” as proof of common descent. In fact, if one generation was completely uncorrellated with its previous generation, if cows could grow on corn, then there would be no such thing as heredity- and thus no such thing as evolution! In fact, it is this constraint which is responsible for the nested heirarchy we see in biology.
** The curves on the graph are the rates of change in carbon dioxide over time, in ppm per year. That means that the area under a curve is the amount of CO2 added to the atmosphere between two points in time. This is easiest to see in Dr. Everett’s projection of a constant rate, which makes a rectangle with sides of 90 years and ~2 ppm/year. The area of the rectangle is thus 90 yr * 2 ppm/yr = 180 ppm of CO2 added. It’s just like driving in a car: if you drive at 90 miles per hour for 2 hours, you are 90 miles/hour * 2 hours = 180 miles from where you started. For irregular curves, it’s a little more complicated, but the principle is the same.
Ridgwell, A., & Schmidt, D. (2010). Past constraints on the vulnerability of marine calcifiers to massive carbon dioxide release Nature Geoscience, 3 (3), 196-200 DOI: 10.1038/ngeo755
Barrett RD, Paccard A, Healy TM, Bergek S, Schulte PM, Schluter D, & Rogers SM (2010). Rapid evolution of cold tolerance in stickleback. Proceedings. Biological sciences / The Royal Society PMID: 20685715