Last time we looked at Dr. Everett’s testimony, we examined his claim that, because carbon dioxide levels have been higher in the past, increasing levels are not alarming now. His argument is flawed, because although CO2 levels have changed, they usually change only very slowly. Now, they’re changing abruptly. Graphs of Deep Time can be intuitively misleading, because they collapse time scales and it can be hard to compare the rates of change from one image to the next. For example, this next graph shows information that we have gathered from looking at gasses trapped in Antarctic ice. It’s obvious that the climate changes over Deep Time- but is it obvious from this graph how historical rates of change compare to modern rates?
Although the temperature (red, above) and CO2 levels (blue, above) oscillate on the 10,000 year scale, the rate of their change is very small- much smaller than what we see now:
Everett’s section IV is titled “Has this happened before?” It turns out that the sheer speed of the environmental change we are causing makes difficult comparisons between current events and historical ones. A recent review (published in 2009 and thus available to Dr. Everett before his testmony) writes:
“…no interval of Earth’s past is a perfect analog for today. …. It is the rate of CO2 release that makes the current great experiment [CO2 release] so geologically unusual, and quite probably unprecedented in Earth history.” (Kump et al. 2009) (PDF)
Another review taking a ‘paleoperspective on ocean acidification’ writes:
“… changes in seawater pH likely to be observed during this century will probably occur ~100- times faster than during glacial terminations, which are the periods of time when globally averaged surface seawater pH changed fastest over the last two million years. This rate of increase far exceeds the regulation capability of natural Earth system feedbacks to restore the system to pre-industrial conditions, suggesting that the perturbation in ocean chemistry from the release of CO2 from fossil fuels might last hundreds of thousands of years into the future. It also highlights the overwhelming challenge that faces the biology of the ocean in terms of adapting to changes which are several orders of magnitude greater than any seen over the past several million years ” (Pelejero et al. 2010) (PDF)
Paleodata also tell us that, rates aside, the size of the change is geologically quite large: already ocean pH is well outside the variations due to glacial-interglacial cycles. And even under Dr. Everett’s conservative projections, the pH will continue to drop.
Ocean acidification is so big, and so fast, that past events may not necessarily be an accurate guide to the present and the future. It doesn’t look like anything quite like it has happened before. But given that caveat, what does the geological record lead us to expect from it?
There have been a handful of geologically rapid acidification events- generally associated with atmospheric CO2 events. A canonical example (and arguably the best analog to today) is the Paleocene-Eocene Thermal Maximum, or PETM. Occurring about 56 million years ago and lasting for about 200,000 years, PETM temperatures spiked by 5-9 degrees C and the ocean became chemically hostile to carbonates. What happened to ocean life during this period?
“The PETM was marked by the largest deep-sea mass extinction among calcareous benthic foraminifera in the last 93 million years […] Other marine groups fared better, although shallow-water tropical carbonate platforms were transformed from coral-algal reefs to large-foraminifera-dominated platforms. Whether this was a response to acidification or warming has yet to be established, although one accompanies the other in CO2-induced global warming events. The PETM also involves dramatic changes among the calcareous plankton.” (Kump et al. 2009)
We can look back even further. There was a large CO2 release at the Triassic-Jurassic boundary, 205 million years ago. Plankton with calcium shells were negatively impacted. In fact, over the last 500 million years, extinction events have been associated with perturbations of the carbon cycle:
“In particular, there is a strong correlation between major extinction events on coral reefs, subsequent ‘reef gaps’ (absence of coral reefs for several million years) and rapidly increasing or high levels of atmospheric CO2, driving ocean acidification.” (Pelejero et al. 2010.)
It goes on. “Has this happened before?” Not exactly, no- but based on similar events in the past, the outlook is bleak.
* You might wonder how we know what the pH of the ocean was in the past- after all, there was no one there to measure it. The answer is that there is a minor contributor to ocean chemistry, boric acid, whose behavior is controlled by ocean pH. The behavior of boric acid, in turn, controls how boron gets incorporated into the shells of organisms called foraminifera. There unfortunately doesn’t seem to be a very good introductory discussion of this proxy on the internet- maybe I’ll put one together at a later point. For now, this PowerPoint seems to be the best resource outside of technical articles.
Lee R. Kump, Timothy J. Bralower, & Andy Ridgwell (2009). Ocean acidification in deep time. Oceanography, 22 (4), 94-107
Pelejero C, Calvo E, & Hoegh-Guldberg O (2010). Paleo-perspectives on ocean acidification. Trends in ecology & evolution (Personal edition), 25 (6), 332-44 PMID: 20356649