The fate of CO2 is that most of it ends up in the oceans.
In this post I am making a try at explaining physical ocean CO2 chemistry. There is a need for this explanation as some chemical oceanographers and their cabal seem to want to stay so confined in their test tubes as to never see “the forest for the trees.” Here’s a hint, chemistry in natural environments is controlled by the ecology not the other way around.
Ok So to start we all must agree that CO2 is highly soluble in water. That’s why we like our sparkling waters, mit gaz, and all manner of bubbly soft and not so soft drinks. It’s the CO2! Somewhat flavourable as it makes bubbly waters slightly tart.
Once CO2 meets the ocean it dissolves into the seawater. Once dissolved it follows two paths.
First we take H2O + CO2 and we end up with H2CO3 – that’s carbonic acid.
But that H2CO3 is not stable in seawater and much of it promptly breaks down into free hydrogen ions a and bicarbonate.
It follows this formulae H2O + CO2 → H2CO3 → H + HCO3 (that latter free HCO3 is bicarbonate.)
The single hydrogen now footloose and fancy free will be up to no good during their temporary separation from their more usual married H2 state. Think of him like your lying cheating brother-in-law.
So how about all the bicarbonate in the oceans, isn’t that an antacid, won’t it take care of the sour stomach the ocean is getting? The answer is yes but with a very large number of years required. And there is a dreadful feedback loop. But first lets talk about the acidification side of the process
This process is referred to as ocean acidification, Not that it makes the oceans fall into the category of “acids” but they become less basic or alkaline by become more acidic, thus acidification.
The pH of surface seawater has fallen from 8.2 to 8.1, (a pH of 7 is neutral in absolutely pure water). Today we have measured a decline of .1 pH units in the last century. This may not sound like a lot, but on the logarithmic scale of pH it translates to a 30 percent rise in acidity.
Ocean pH is projected to drop another .3 to .4 units if carbon dioxide levels reach 800 ppm – one of the scenarios projected by the Intergovernmental Panel on Climate Change by 2100 –raising levels of hydrogen ion, H +, 100 to 150 percent (Orr et al., 2005). As this happens and it will take “tens of thousands of years” for the chemistry of the oceans to return to pre-industrial levels, at least that is what the Royal Society of Britain proclaims.
As it happens ocean are naturally saturated with another base, carbonate ion (CO3−2) that acts like an antacid to neutralize the H+, forming more bicarbonate. So when more CO2 dissolves into the ocean the next reaction is that the additional CO2 joins up with the broken up bits of carbonic acid and more simple water and the formulae looks like this: CO2 + H2O + CO3−2→ 2HCO3-
As carbonate gets depleted, seawater becomes undersaturated with respect to two calcium carbonate minerals vital for shell-building, aragonite and calcite. Scientific models suggest that the oceans are becoming undersaturated with respect to aragonite at the poles, where the cold and dense waters most readily absorb atmospheric carbon dioxide. The Southern Ocean is expected to become undersaturated with respect to aragonite by 2050, and the problem could extend into the subarctic Pacific Ocean by 2100 (Orr et al., 2005).
Recently one ocean scientist tried again with me to derail biological fixation of CO2 and ocean acidification by claiming that since CO2 is released when cabonates precipitate by breaking bicarbonates there is no gain. True CO2 is released but this simplistic notion ignors the fact that incoming CO2 is also what makes bicarbonates in the first place. His one dimensional view being that one ought to argue each reaction in isolation and not as part of an ecosystem is beyond explanation. My view is that any carbonate that sinks is good for Mother Nature these days.
A tiny species of zooplankton, the pteropod, called “sea butterflies” because they have fluttery wings they use to swim around, may be in jeopardy. In an experiment that immersed a pteropod in seawater with low aragonite levels, part of the organism’s shell was eroded in as little as two days (Orr et al., 2005).
The Earth is involved in all this and over the course of hundreds to thousands of years, carbonate in the ocean is replenished through the chemical weathering of limestone rock and dead animals, such as pteropods, that use calcium carbonate to build their shells. The formation and dissolution of calcium carbonate depends on the saturation state (Ω) of water, or the ion product of calcium and carbonate concentrations. The solubility product in the equation, Ω = Ca2+ + CO3−2/K’sp, depends on temperature, salinity, pressure and the particular mineral. Shell formation usually happens when Ω is greater than one while dissolution happens when Ω is less than one.
OK now you know about carbonate and bicarbonate in the ocean. But here’s news. At the “normal ocean pH” that the oceans and we enjoyed 100 years ago carbonates were able to precipitation out of ocean water whenever the pH rose ever so slightly. The biggest force in the ocean is life, and that life is mostly plankton, phyto-plankton. Like all plants that photosynthesize using sunlight to help them capture and convert CO2 into more of themselves, plant or in this case plankton biomass, as those ocean plant plankton consume CO2 they make the ocean more basic.
When there is a bloom of an ocean pasture and its plankton that body of water becomes more basic/alkaline and calcium carbonate promptly falls out of its dissolve state and sinks as a precipitate. Just recently a wonderful paper about how the Bahamas are in fact a vast deposit of calcium carbonate that has been accumulating for 100 million years has been moderating the oceans and the planets CO2. That Bahamanian white carbonate that makes those beautiful white sands is in fact a million billion tonnes of CO2 that was once acidifying the oceans.
Read my post about the Bahamas Sahara Dust Produces Massive Bahama Carbon Sink
The problem of course is time. We are burning through hundreds of millions of years of fossil carbon fuels in just a century or maybe two if we last that long. We are overdoing it and overwhelming the oceans with so much CO2 that it doesn’t have time to naturally adapt.
Think of it as if your were on a holiday in the Bahamas. You will be there for a week and intend to drink plenty of tropical cocktails and wine over the course of the week. But then you decide what the heck you’ll drink the whole weeks worth of alcohol in one hour. You’d overdose and likely die, the same thing is happening with our CO2 and the oceans.
If our emission of fossil CO2 were taking a million years instead of 100 years there would be enough time, calcium carbonate to dissolve in large enough quantities to return the oceans’ pH to its natural state. Surely this is why ocean pH in the past did not fall as dramatically as the super high carbon dioxide levels in the past would demand.