Ocean acidification feedback causes increased warming

27 03 2014

Mark Cochrane

Mark Cochrane

Another guest post from Mark Cochrane…… who continues bringing bad news after a few months of more research.

I’ve posted before on the “other CO2 problem”, namely ocean acidification but up until recently, ocean acidification and greenhouse gas-related warming of the planet have been seen as problematic but separate processes happening due to our fossil fuel-related binge of carbon emissions.

For those who don’t know, ocean acidification arises when CO2 is forced into ocean waters, sort of like in your soda or beer. We increase atmospheric concentrations of CO2, this forces some of the gas into solution in the ocean surface layer. The carbon dioxide then reacts with water molecules to form carbonic acid.

Acidity is measured in units of pH. The ocean’s pH has fallen by a little more than 0.1 pH units since the beginning of the Industrial Revolution which doesn’t sound terribly impressive but the scale is logarithmic, meaning that changes are exponential in terms of their magnitude. You might have some idea of what that means if you think in terms of the Richter scale of earthquake intensity which is also logarithmic, with each additional 1.0 meaning 10 times the shaking magnitude and 31.6 times the energy release. Back to the ocean pH, that measly 0.11 change equates to a 30% increase in the global ocean’s acidity levels.

That is a very significant change in such a short time for organisms like oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton. If we continue on the path of “Business as Usual” carbon emissions then we will make the ocean’s surface waters 150% more acidic by the end of the century. Nothing like this has been seen for more than 20 million years. This is what those acidity levels will do to current organisms living in the oceans.

Where this impacts the global climate is by messing up the oceanic sulfur cycle. Phytoplankton release something called DMS (dimethyl sulfide). DMS goes into the atmosphere above the oceans and forms cloud condensation nuclei (CCNs). In layman’s terms, the DMS seeds low level clouds over the oceans. These clouds act to reflect a portion of the sunlight from the Earth and thereby cool the planet.

So what we have is the following chain of events. Continued CO2 emissions raise atmospheric CO2 levels which in turn raise aqueous CO2 levels in the oceans. This then lowers the ocean’s pH (increased acidity) which stresses or kills phytoplankton. Less phytoplankton results in less DMS. Less DMS yields less clouds over the oceans. Less clouds means more sunlight warms the oceans and hence the exacerbates global climate change. Multiple experiments in sea water exclosures (mesocosms) have verified this impact. Which leads us to this recent scientific finding:

Global warming amplified by reduced sulphur fluxes as a result of ocean acidification

Global DMS emissions decrease by about 18(±3)% in 2100 compared with pre-industrial times as a result of the combined effects of ocean acidification and climate change. The reduced DMS emissions induce a significant additional radiative forcing, of which 83% is attributed to the impact of ocean acidification, tantamount to an equilibrium temperature response between 0.23 and 0.48K. Our results indicate that ocean acidification has the potential to exacerbate anthropogenic warming through a mechanism that is not considered at present in projections of future climate change.  (Six et al. 2013)

For those who don’t know 0.23-0.48 K (Kelvin) is the same as 0.23-0.48 C (Celsius).

This is yet another positive feedback that will likely make the rate of climate changes more rapid than current values given in global climate models. Those who have an issue with models should understand that if the models are ‘wrong’, they are at least as likely to be understating the rate of climate change as overstating it. The impacts of the rapid acidification though will be the real problem for life in the oceans. Eventually life in the oceans would adapt to the higher acidity but it is happening so fast that evolution cannot keep pace. There will be serious consequences for all life because of this on any time scale significant to human societies.