Arguments
OA not OK part 5: Reservoir dogs
Posted on 13 July 2011 by Doug Mackie
This post is number 5 in a series about ocean acidification. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.
Welcome to the 5th post in our series about Ocean Acidification. In the last posts, we introduced solubility (post 3) and pH (post 4). We noted that the pH of the surface seawater has dropped from about 8.25 to 8.14 since the pre-industrial revolution, leading to a 29% increase in H3O+. If CO2 emissions continue, a 150% increase in H3O+ is predicted by 2100. Concerns about this increase in acidity are not limited to the decrease in pH (or increase in H3O+), but include the resulting changes to carbon species in seawater.
Just as it is in biology, a species in chemistry is a classification. A chemical species describes different forms a substance can be found in. For example, the species of nitrogen in your garden soil and in stream water may be ammonium (NH4+), ammonia (NH3), and nitrate (NO3-). In the context of ocean acidification we refer to the different species of inorganic carbon – carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3–), and carbonate (CO32–) – and how it is possible to interconvert between them.
The equations below describe overall reactions (some are actually made up of several steps) of the inorganic carbon species in seawater. In equation 7, CO2 reacts with water to form carbonic acid (H2CO3). In equation 8, carbonic acid dissociates in water to give bicarbonate (HCO3–) and H3O+. In equation 9, bicarbonate dissociates to give carbonate (CO32–) and H3O+.
BUT, as we saw in post 1, each equation is in equilibrium and can occur in the left-to-right direction (as written) or the right-to-left direction. That is, the equations do not tell us if a reaction is thermodynamically spontaneous (i.e. which direction is favoured) or about the rate of reaction. These things need to be determined experimentally.
The experiments have, of course, been done. For now we will just note that at typical seawater pH values the reactions in Eq. 7-9. are spontaneous as written from left to right.
Overall equations 7-9 mean that in the oceans 91% of 'carbonate' is in the form of bicarbonate (HCO3–), 8% is in the form of carbonate (CO32–), and less than 1% is found as CO2 and H2CO3 (the way we calculate this distribution, the way these reactions have a feedback on each other, and the effect of pH on this balance is discussed in several later posts).
There is a large amount of these inorganic carbon species being held in the ocean. Figure 2 is modified from the IPCC 4th Assessment Report (2007) Figure 7.3 and shows the size of each carbon reservoir on Earth. (In the next series of posts we will discuss the movement of carbon between each reservoir (fluxes) that are in the original figure).
Figure 2. Carbon reservoirs as preindustrial size (blue circles) and modern size (red circles) and change, positive or negative (black circles) since the industrial revolution with the change expressed as a % change. Size is measured in gigatons of carbon = Gt C. (1 Gt = 1,000 million tons, i.e. billion tons). Numbers are expressed here as gigatons of carbon (Gt C) because this is the unit used by the IPCC. Other publications may use giga tons of CO2 (Gt CO2); multiply Gt C by 3.67 to convert to Gt CO2 and divide Gt CO2 by 3.67 to get Gt C. Another unit sometimes used is petagrams. 1 Pg = 1 Gt so 1 Pg C = 1 Gt C and 1 Pg CO2 = 1 Gt CO2.
The first number in each box gives the size of the preindustrial reservoir in billions of tons (gigatons, Gt) of carbon (thus, the preindustrial surface ocean contained 900 billion tons of carbon). The second number is the change in the size of the reservoir from preindustrial to modern times in billions of tons of carbon. NOTE that here 'modern times' means the mid 1990's – the IPCC is very conservative – and the fossil fuel reservoir is an estimate for 'recoverable' fossil fuels.
Thus, the modern surface ocean contains an extra 18 billion tons of carbon, which represents a +2% increase. In contrast, sea life and the surface sediment reservoirs have not changed appreciably in size.
Where has so much inorganic carbon in the oceans come from? It has come from the weathering of rocks – the subject of our next post.
Written by Doug Mackie, Christina McGraw , and Keith Hunter . This post is number 5 in a series about ocean acidification. Other posts: Introduction , 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.
It is divided schematically into five fields, representing the five main reservoirs of carbon in the global carbon cycle. As stated in the caption, the size of the blue circle represents the mass of carbon in each reservoir in pre-industrial times, while the red circle shows the mass of carbon in the mid-1990s (as noted in the post, things have changed quite a bit since then. Could someone put up some updated numbers? Even an approximation would help.) For some reason, unexploited fossil fuels are shown in a single black dotted circle rather than red and blue ones. In each field, the black circle indicates the scale of change from pre-industrial to mid-1990s. If the black circle appears in a blue circle (as it does for atmosphere and both surface and deep oceans), that reservoir has gained carbon. Soils and plants has a black circle in the red sphere, indicating a loss of carbon (think deforestation, though also other processes). The fossil fuel circle indicates the relative scale of estimated recoverable reserves vs total amount of fossil fuels extracted and exploited (i.e. less than 7%), making the point that there is still plenty of carbon we could potentially dig up and stick in places where its going to mess things around (i.e. the atmosphere and oceans). Numbers give the Gt of carbon represented by each circle and then the % change.
(Some have criticised the IPCC's figures for being considerably too rosy about the total reservoir of recoverable carbon from fossil fuels, but even less optimistic figures still give us plenty of scope to keep making more mess.)
So the quick take-away message from the figure is that between the industrial revolution and the mid-1990s, we took 283 Gt of carbon from places where it wasn't doing anything particularly bad for us and put it into places where it is.
Here's where the meat is :)
The Vegan in me is weeping quietly in a dark corner, Riccardo.
I apologise if it hurts your feelings. As a non-english speaking I may be wrong but the use of the word meat to indicate the essential part of something looks quite common to me. The Merriam-Webster Dictionary (entry #4) seems to confirm.
In any case, I hope the sense of my comment is clear.
Are deep ocean carbon increases due mostly to ocean currents or to something else? How does this affect the Carbonate Concentration Depth? Does the carbonic acid species last long and is it what disolves sea shells? (I'm not doubting that there is some basic chemistry that I'm missing. Please correct any misunderstandings I demonstrate.) (And forgive me, please, for not knowing what you think I know.)
When the oceans warm up, the uptake of CO2 will be reduced:
http://thinkprogress.org/romm/2011/07/12/267277/climate-change-reducing-oceans-carbon-dioxide-uptake/.
Is this reduced uptake already taken into account when an extrapolation of pH and the total dissolved carbonate species are calculated for a future date?
PS, for those interested, on http://www.eoearth.org/article/Ocean_acidification estimates are given for several parameters at different dates regarding the ocean acidification.
Too early to say with confidence.
McKinley et al. divided the North Atlantic into 3 zones and split the ocean pCO2 system into 2 parts. One part is temperature related and one part is chemical related. The temperature part is about a quarter the size of chemical part. The chemical part is further subdivided into 3 sub-components. In one subdivision of one of the ocean zones the trend for part of the study period of one of the chemical sub-components increases with time while the trend for the other two chemical subcomponents decrease.
From the conclusions:
From the press release:
Your question is not in any way difficult. I asked because that other DLB that is, as you say, not you, has a rather closed mind and I would not have wished to spend time answering questions for one who would not listen.
As I wrote above for TorB:
"If you put a pH electrode in a solution today you are measuring pH by proxy. Different proxies are used to determine past ocean conditions. Posts 11 &12."
Thanks for your answer.
One would also expect that a lower uptake of CO2 (very) slowly would get visible in the atmospheric CO2 concentration, at least when the emission rates don't change. The oceans contain a lot of water so I probably will be old when these effects will be better quantifiable.
second summary post