An unprecedented degree of undersaturation?

Many people are familiar with Michael Mann’s famous Hockey Stick of global surface temperature over the last 2000 years, clearly showing the influence of anthropogenic climate change.  In a recent paper published in Earth and Planetary Science letters this week led by Sindia Sosdian from Cardiff University we show a series of “ocean carbonate system” hockey sticks showing how anthropogenic and future change in CO2, pH and aragonite saturation compare to what the Earth has experienced over the last 20 million years or so.

Surface water pH (A), atmospheric CO2 (B) and surface water aragonite saturation state (C) over the last 20 million years.  Bands encompass the mean and 1sigma uncertainty.  The different colours represent different scenarios (see the paper for more detail).  The plots on the right-hand side show historical (grey) and future projections from Winklemann et al. (2015). 

From the long-term trends in the above figure, pH (the –log10 of the H+ concentration) has increased and CO2 decreased, on the whole, over the last 20 million years (with some interesting structure that is for another post for another day).  CO2 and pH are clearly tightly coupled over this time, the reason is actually a bit complex, but can be simply thought of arising because CO2 is an acidic gas, so more CO2 = lower pH and vice versa.  Furthermore, the CO2 content of the ocean dictates the CO2 content of the atmosphere, so this tight coupling between pH and CO2 is not a surprise.  We are not sure why CO2 declined and pH increased through the last 20 million years but it most likely relates to a long term decline in CO2 outgassing from the mantle or a gradual increase in the weathering of silicate rocks in the Himalaya.

The key new record in this latest study however is the evolution of the saturation state of calcium carbonate (CaCO3) over the last 20 million years (in figure above expressed as omega aragonite – the saturation state of the aragonite polymorph).   When saturation state is greater than 1 CaCO3 can precipitate easily, when its below 1 CaCO3 dissolves.   Organisms that make their shells and skeletons out of calcium carbonate, like corals and shell-fish, require a high degree of CaCO3 oversaturation.  Similarly, carbonate structures like coral reefs exist in a delicate balance between dissolution and accretion, so any decline in saturation state can start to weaken and dissolve the reef. 

This really great figure below from Honisch et al. (2012; Science), firstly shows the close relationship between atmospheric CO2 (panel A) and surface ocean pH (panel B) in a model where you double CO2 on different timescales (warm colours – fast, cold colours – slow). In panel C the mean surface ocean saturation state of aragonite is shown.  Although it looks similar to the other two on short timescales, on long timescales of CO2 addition it becomes decoupled from pH and CO2 and doesnt change very much.  This is perhaps more clearly shown in panel D.  It’s not easy to explain why this happens and those of you interested should look at the Honish et al. (2012) paper in more detail (or see wiki or this Royal Society report)   

Our new record of aragonite saturation state shows that as predicted, there isn’t much of a trend over the last 20 million years, consistent with this idea of a decoupling of pH and CO2 from saturation state when CO2 change is slow – i.e. on thousands to million year timescales.

However, over the last 150 years or so, CO2, ocean pH and saturation state have all changed in tandem because the changes are so fast.  What the figure from our new paper shown above shows is that attempts to mitigate the effects of climate change by restricting CO2 rise at 2100 to <500 ppm (RCP2.5) or so keeps ocean pH and saturation state to well within the range of the last 5 million years or so.  The RCP8.5 scenario – the often called “business as usual” – risks tipping saturation state to lower values than have been seen in the last 14 million years, and maybe longer.

What does this mean for calcifying organisms? Well I guess we just don’t know for sure without further study. But the message is clear – if we continue to emit CO2 at the current rates we risk taking the Earth to a state not seen for many millions of years…