There are two major threats to the oceans today: global warming (GW; AGW for anthropogenic GW specifically) and ocean acidification (OA; AOA for anthropogenic OA specifically). They are both tightly linked, but we’ll look at them individually first, starting with OA.
Note: I can’t find an easy way to add super- and subscripts. CO2 is carbon dioxide; O2 is oxygen; HCO3- is hydrogen carbonate.
More CO2 in the atmosphere means more CO2 will be taken up by the oceans, thereby dropping its pH (dissolved CO2 is an acid, HCO3-). This obviously has a negative effect on ocean ecosystems – although how exactly is not known. The most severely affected organisms are any ones that build calcareous skeletons, including coral reefs, some sponges and major planktonic organisms, such as foraminifera.
The best way to look at the long-term effects of climate changes is to look at geology and palaeontology, since the Earth has been through all sorts of global climate types throughout its history. This is done with geochemistry and isotope analysis: organisms almost universally use lighter element isotopes in their biochemical processes. By looking at the isotope records left in sediments and in fossils, we can figure out approximate levels of the particular elements in the atmosphere. As an example, if plenty of light carbon is found in sediments, that means that this carbon was not been used by organisms, meaning that the biosphere was depressed.
Looking at the history of life on Earth, there have been many dramatic oceanic extinction events. In the Late Devonian, reefs worldwide were almost wiped out. The Permian – Triassic (P-T) boundary is defined by the largest extinction in the history of life, with over 70% of all marine life dying. At the Triassic – Jurassic (T-J) boundary, reefs again experienced a massive extinction. And of course, the Cretaceous – Tertiary (K-T) boundary was another extinction event that also affected the oceans.
What are the causes of these extinctions? The Late Devonian crisis was an anomalous event that led to widespread anoxia in the oceans and is not related to OA. The P-T extinction is associated with an enormous spike in atmospheric CO2, most probably from volcanism; a similar, but smaller, spike is seen at the T-J boundary.
Looking at what got extinct during those two events, we notice that particularly those organisms that build calcium carbonate skeletons were affected, so there is a clear correlation. The way this works is that these organisms take calcium and carbonate ions from the water and cause them to precipitate, forming the skeleton. In shallow waters where coral reefs are found, the concentrations of these ions exceed the equilibrium constant (the ratio below which calcium carbonate dissolves). Increasing acidity ruins this balance though – calcium carbonate will dissolve faster than it is produced (this is the same thing that happens in the deep sea, although pressure is the reason there).
Now let’s tackle global warming. Before starting, let me just lay out some facts (without turning this into a document on climate change – there’s much better websites for that). CO2 is a greenhouse gas (one of many). In 2008, the amount of CO2 in the atmosphere was 385 ppm, up from 280 ppm in pre-industrial times, and is currently increasing at a rate of 2 ppm per year. This drastic increase is a direct result of human activity, mostly from the burning of fossil fuels, the cement industry and land use patterns.
So, the Earth is warming. So are the oceans – on average, by ~0.40°C at the surface since 1980, and by ~0.1°C in the deep. Not only that, the increased warming affects ocean currents, which affects climate (which in turn affects the ocean currents, ad nauseum). The sea level rose by more than 15 cm during the 1900s. And of course, the extra CO2 in the atmosphere has increased the concentration of CO2 in the oceans, leading to OA (a decrease in pH of 0.1 in the past 200 years has taken place).
Although there have obviously been many dramatic climate changes during the history of the Earth, it is impossible to see how they affected the biosphere in the short term, so we really are facing an unprecedented scenario here.
Of course, the warming of the seas and oceans is not straightforward. The deep sea is especially protected from direct warming, but that that doesn’t mean that the ecosystem will be unaffected – as we saw before, the deep sea cannot be considered an isolated, self-sufficient habitat. So how does global warming affect it?
One ground fact that should always be kept in mind: the higher the temperature of water, the less gases it can absorb and keep. These include CO2 and, more importantly, oxygen.
First, let’s look at the pelagic realms. At the very top of the water column, the increased CO2 will increase primary production. At the same time, the increase in acidity will reduce the number of calcifying organisms (which, by biomass, are the most important up there). Also, by increasing the concentration of CO2, you decrease the amount of oxygen in the water, risking the expansion of anoxic zones. For the open pelagic realms, this adds up to a negative outlook. On the positive side again, the changes in climate will mean that there will be more storms – and storm waves bring lots of nutrients with them. And of course, on a way-too-obvious scale, the change in temperatures will affect the distribution of fish.
The OA and the anoxic zones apply for the deep sea as well, but the biggest threat is the decrease in organic material that will sink down from above. It’s not a matter of biology or of organisms needing to adapt. Global warming will lead to large patches of the oceans being deserted, simply because there is no food, and in the worst case scenario, no oxygen.
That may be a bit of an alarmist view, I admit. But at this point, it can realistically be considered as a possibility. But this is a biology blog, so let’s stick with the subject matter. How do rising water temperatures affect the organisms themselves? This is a difficult topic, since it goes through all levels of organisation, from the genetic to the ecological.
At the lowest level, temperature alters gene expression. This can potentially have drastic consequences: many species in the Antarctic re very sensitive to temperature changes and will simply die because their physiological systems simply shut down (as a result of changing gene expression). Another very important consideration: reproduction, especially the timing for laying eggs and larval growth, both of which are very temperature-dependent.
This may not seem like much, but on the ecosystem scale, all these effects add up, since oceanic ecosystems only function because of the very tight interlinking of all the species in it. If any of the lower links on the food chain get perturbed, the higher levels will not be able to cope, causing them to migrate away, or just die off – and this is not a simple “protect the environment!” message. Our fishing industry depends on the stability we find in the oceans today, a stability which has already started to disintegrate irreversibly.
In the open oceans, primary production in the commercially-important areas has been steadily falling every year. The only areas where it’s improving are in the Arctic, because the ice is melting and there are more storms. In the middle latitudes (45S to 45N), the global warming-caused changes in the ocean currents have reduced the amount of mixing between the deep and the surface, meaning that phytoplankton do not get the nutrients they need to grow, and the deep does not get as much food dropping down from above. That’s how simple the problem is.
And to see how quickly the deep sea reacts to surface changes, you only need to look at sediment cores taken from the deep sea and analyse the ostracode and foraminiferan assemblages in there. They serve as archives for the primary productivity (the more there are, the more productive the surface and therefore the deep) and act as proxies for the climate (by looking at species and numbers). And what you find are that there are continuous ups and downs, surges and catastrophies, and whole faunal turnovers within decades – that’s how fast the marine ecosystem. gets affected. Of course, looking only at microfossils doesn’t tell us much about the conditions for the megafauna, but it is very telling when you see such a sensitivity.
And looking at the geological fossil record, we see what happens in the mid- and long term: extinctions, both local and global. And these extinctions are almost always marked by changes in the climate, accompanied by changes in organism distributions. I’m just sayin’.
And for a visual representation, here’s a simple model for how coral reefs are affected by both warming and OA.