We’ll be talking about ocean acidification. What I’d like to do is touch on three things, first the chemistry of ocean acidification, the consequences of ocean acidification, and some of the connections, not just in the planetary boundaries framework but outside that as well.
So let’s start off with chemistry. The first slide here shows the carbon dioxide budget globally, and it comes from the Global Carbon Project based in Australia.
And towards the left you can see the amount of carbon dioxide emitted by human activities like fossil fuel burning or cement production. That’s the arrow, the big arrow, going upwards. There’s another upward going arrow and that comes from the carbon dioxide emitted from land use change.
Of all of the carbon dioxide we emit to the atmosphere about half of it stays there, and that’s why carbon dioxide concentrations are increasing. But what happens to the other half? Well, about 25% or so goes into terrestrial ecosystems, that’s the downward going arrow to the trees, and the other 25% ends up going into the oceans.
So what happens when you dissolve carbon dioxide in the oceans? Well the carbon dioxide forms a compound with water called carbonic acid, which then dissociates – that means it splits up and forms two ions – a proton H+ and a bicarbonate ion HCO3-. That can dissociate again, giving off another proton and a carbonate ion. And each time a proton is added to water it becomes a little bit more acidic. So, is that a global issue? Well, yes it is.
This particular image shows three different plots of ocean pH as a function of time.
The one on the far left is a pre-industrial case. And the color coding is proportional to the ocean pH, so you can see that the red colors are about 8.2, green colors are 8, the purplish-blue colors are 7.8-7.9.
And you can see in the pre-industrial case most of the oceans were 8 or above in terms of pH. Present day it’s still around 8, but there are fewer areas which are above that. Predicted for the end of the century you can’t see any places at all with pH of 8.
Now that doesn’t sound like much, right? But keep in mind that a tenth, the 0.1 pH unit, is about a 26% increase in acidity of the oceans. So we’re talking about a lot. What are some of the consequences, uh, of ocean acidification?
Well this plot shows a lot of different kinds of marine organisms, and it’s not so important, you can take a pause and look at more detail, but what you want to – to concentrate on first is increasing ocean acidity is going to the left in these plots, and calcification rate, that’s how rapidly organ– organisms in the ocean turn carbonate into a – into a shell is on the vertical axis going up.
And most of these plots either have a little bit of a – an inverse U-shape or it’s just going down, which means that most organisms in the oceans don’t like it when it becomes more acidic, they slow down the rate at which they make a hard shell out of the carbonate in the oceans. What does that mean really?
Well if you’re a coral reef you’re under stress from ocean acidification and other things like ocean warming and different kinds of pollutants. And you can go from the kind of beautiful, vibrant, very bio-diverse reef you see on the right to the kind of bleached reef you see on the left as a result of those changes in the ocean, including ocean acidification.
What are the implications and how rapidly can this happen? Well the graph on the top shows predicted increases, in carbon dioxide concentration for the atmosphere under a number of different scenarios into the future.
The bottom shows the aragonite saturation, that’s the point at which the equilibrium shifts from making aragonite, a kind of calcium carbonate, a soluble or an insoluble compound in the ocean, and that you can see will happen by the early mid-century for the southern ocean under most all of these scenarios into the future. So the point at which aragonite becomes soluble, or coral reefs might have a very, very difficult time, uh, existing at all, will be about mid-century or so, not too long from now.
Now that’s not the only stressor that marine organisms are facing. It’s one of the more important ones, but if you look at some of the other ones, like ocean warming, and other kinds of pollution you can see that the oceans are facing multiple stressors.
And in particular, on this plot, if you looked at the hatched areas, much of the southern ocean and much of the north Pacific and Atlantic oceans, will be faced by a number of stressors including, uh, ocean acidification. And all of the organisms that live there will be in a much more stressed environment.
You can get more detail about this, and other issues as well, in a book, Managing Ocean Environments in a Changing Climate. It tries to put all of these different stressors together and tells you how they fit with each other.
Which moves us into the connections part of the presentation. So you’ve seen this picture before, or at least some version of the picture.
This is the image of planetary boundaries. It’s not by chance, that ocean acidification and climate boundaries are right next to each other, because the driver for both is the same, how much carbon dioxide we emit to the atmosphere.
So this next to the last slide shows some of the connections between planetary boundaries, ocean acidification and the couple of other things, like food production. So you can see on the top we can produce food from terrestrial sources or on the bottom we can produce food from marine sources.
And the things that connect them are our planetary boundaries. So you see chemical pollution, for instance, from production of food on land might negatively influence coastal areas where we farm or fish, uh, for food. And nitrogen and phosphorus cycling, and what we’ve been talking about so far in terms of ocean acidification, the amount of carbon dioxide that we emit into the atmosphere.
And the important thing with this slide is to realize that all of these processes are fundamentally connected with each other.
You can’t really pull one apart and look it all by itself. Having said that though we’ll like to do a little bit of a reminder of what the ocean acidification boundary actually is.
It’s written in terms of aragonite saturation, that is, it’s a chemical equilibrium, and this might be a poster child for planetary boundaries in the fact that that’s a very, very easy boundary to define, because it is a chemical equilibrium.
If you add a little bit more carbon dioxide to the oceans aragonite becomes saturated, or unsaturated, so this is probably the easiest to define of all the planetary boundaries, and you can see on this slide exactly what it is.