Ocean Acidification – comparing the present to the past

The two consequences of our burning of fossil fuels are the increasing concentration of CO₂ in the atmosphere with resulting warming effects on climate, and the increasing concentration of CO₂ in the surface waters of the oceans with resulting impacts on ocean pH.  About 1/3 of the CO₂ emitted dissolves into the ocean; its dissociation leads to a lowering of pH of the water.  The rate at which we are gaining knowledge of the rate, extent, and ecological effects of this acidification is growing rapidly, and keeping up with new findings is a challenge.  I am currently reading widely, and expect to learn more in my trip to various Florida universities over the next few days.  For now, let’s just discuss some aspects of the rate and projected extent.  The following is based extensively on a detailed review by Dr. Richard Zeebe, of the Department of Oceanography, University of Hawaii.  The article is currently on-line in pre-publication form, at the journal, Annual Reviews of Earth and Planetary Sciences.

Zeebe concludes his brief abstract as follows: “The results [of his review] allow evaluation of the current anthropogenic perturbation in the context of Earth’s history. It appears that the ocean acidification event that humans are expected to cause is unprecedented in the geologic past, for which sufficiently well-preserved records are available”.  The wiggle-words here are ‘for which sufficiently well-preserved records are available’, because as geoscientists go back in time the ability to measure such things as ocean pH with high precision has to decline.  His statement certainly holds true for the 65 million years of the Coenozoic period, that time since the end Cretaceous mass extinction and the end of the dinosaurs (plus a whole bunch of other creatures).  His statement probably also holds true for the Mesozoic, and perhaps even further back.  We are doing something unprecedented to the ocean.

In what ways is the current acidification episode unprecedented?  In its extent and its rate.  Understand that the CO₂ is dissolving from the atmosphere into the surface layers of the ocean.  Surface waters are typically warmer (and a bit more saline) than deeper waters, and their relative warmth keeps them floating above, and mixing only slowly with layers of deeper water.  The CO₂ concentration in the surface waters can equilibrate very rapidly with that in the atmosphere, but equilibration with deeper oceanic waters takes ~1000 years, because the processes by which the world’s oceans overturn their water is a very slow one.  Incidentally, the warming of surface waters may also act to slow down further the overturning of oceanic water, thus extending the time it will take to mix surface and deeper waters.

If we look at ocean chemistry during the Holocene, that 10,000 year period from the end of the Pleistocene glaciation to the present, ocean chemistry has been remarkably stable until very recently.  Atmospheric CO₂ varied between about 260 and 280ppm throughout this time, but began to rise higher with the onset of the industrial revolution.  It was at 315ppm in 1958, and is at 394ppm today.  The variation of about 20ppm over 10,000 years is far less than the 100ppm over the last 150 years.  Not surprisingly, ocean pH was also remarkably stable over these 10,000 years, although it has been decreasing recently.  Zeebe states that pH declined about 0.04 units during the Holocene, but since 1750 has fallen 0.1 unit.  He estimates that, under business-as-usual energy policy, it will decrease by 0.7 units by 2300 – a rate that is about 300 times faster.

Looking back beyond the Holocene, Zeebe notes that the current rate of acidification is about 70 times faster than the brief episodes of acidification that accompanied each of the deglaciation phases during the Pleistocene.  He states that surface water pH was lower during the early Coenozoic, pH of ~7.6, and rose very slowly, with possible short-term fluctuations at times of substantial planetary change such as the Paleocene-Eocene boundary, towards the pre-industrial range of pH = 8.1.  (Perhaps I should have noted at the beginning that the pH scale is logarithmic, and these small numerical changes mean a substantial change in water chemistry.)

Figure 6 from Zeebe 2012 Ann Rev Earth Planet. Sci. Comparing (a) the rate and total quantity of carbon released into the atmosphere in the PETM event and the present warming episode, and showing in (b) the calcite saturation state as a proxy for pH.  The onset of the PETM event has been aligned with the start of the industrial revolution (1750) to permit this comparison.

Zeebe concludes by examining specific events during the Coenozoic and the more distant past to see if there are short-term changes that may correspond to what is happening today.  Of various events, he identifies the Paleocene-Eocene Thermal Maximum, or PETM event, as perhaps the closest analog to what is now happening.  The Paleocene-Eocene boundary was a time of substantial vulcanism, and the PETM event has several characteristics in common with what is happening today.  First, it was a time of substantial injection of CO₂ into the atmosphere, and from there into the surface layers of the ocean.  Second it was a transient event, not a long-term steady-state situation when the surface and deeper waters would have come into equilibrium.  It also is a relatively well-studied event from geological, oceanographic and paleontological perspectives.  Nevertheless, it is not the same as what is now happening.  The difference is primarily one of rate of change.  Zeebe has previously estimated that during the PETM event, about 3000 peta-grams of carbon (that’s 3000 x 10¹⁵ grams) were released into the atmosphere over 6000 years.  Under business-as-usual projections, we are likely to release 5000 peta-grams of carbon over the 500 years since the start of the industrial revolution (1750 to 2250).  The figure in his review makes the difference very evident.

So putting all this together, the present episode of acidification appears unique over at least the last 65 million years in both the magnitude and the rate of change of CO₂ concentrations.  And it follows a period of at least 10,000 years of essential stability.  Perhaps it should not surprise us that biologists are now detecting a growing range of impacts on organisms and on biological processes, caused by these changes to ocean chemistry.  What are we doing to our only home?