Conclusions

So, this blog has tried to explore the issue of ocean acidification. It has been somewhat jumbled and so today I am going to try and bring it all together. 11bn tonnes of carbon dioxide have been absorbed by our oceans in the last 200 years, causing a 30% increase in ocean acidity.  CO₂ dissolves in sea water and forms carbonic acid, this dissociates into HCO₃^- and H⁺, this H⁺ is what’s making our oceans more acidic. I think the most important thing that I have learned from this is that the problem is not that by the end of the century our oceans will be more acidic than they have been in the past 30 million years; it is the rate of the change. This rate is currently 100 times greater than anything we have experienced over the last 650,000 years.  Currently it is expected that by 2100 the pH will have fallen by 0.5 to 7.7.  If oceans acidify slowly, marine species have time to adapt.  At the current rate of change however, species are caught by surprise resulting in many becoming extinct.  The greatest threat is to calcifying organisms due to the shallowing of the aragonite and calcium carbonate saturation horizon, calcification can only occur above this horizon, with dissolution occurring below it. Once corals and other calcium carbonate based species are lost, wide scale extinctions will occur along the food chain.  At the PTB it is thought that the loss of coral and other such marine species was the start of the 5^th mass extinction, this suggests that the losses we are seeing today could cause the initiation of the 6^th mass extinction.  The problem is very real and we can already see the consequences in many of the world’s coral reefs, namely the Great Barrier Reef, and it is believed that all of the world’s reefs will have been lost within 40 years.  I thought it was interesting that even large marine mammals are being affected, and in ways that you would not immediately think of, for example clown fish are finding it much harder to smell the specific anemone that they use as a habitat due to the acidic waters.  The increase in size of lobsters in acidic conditions shows that some species can thrive in these conditions; however they may still be at threat due to changes in the food web.  Boron isotopes work by looking at the ratio of borate ion to boric acid.  The levels of which are affected by ocean pH as borate ion is preferentially incorporated into marine carbonates.  They have been quite successful in helping to reconstruct past pH but in the future further work needs to be done in order to reduce the uncertainty surrounding the isotope fractionation index and to improve the analytical techniques that are used. Towards the end of the blog I have briefly considered if there is any ‘quick-fix’ to improve the health of our oceans, if we are to stop our oceans from becoming undersaturated in aragonite the ocean pH mustn’t fall by more than 0.2 (relative to preindustrial levels).  Iron fertilisation is one of the main theories that has been proposed to combat ocean acidification and it is thought that by dumping large amounts of iron into HNLC areas of the ocean the productivity of phytoplankton will be greatly increased, resulting in algal blooms which will draw large amounts of carbon dioxide out of the atmosphere.  There is much uncertainty surrounding the potential success of iron fertilisation, it is thought that the drop in CO₂ is only found in the surface waters and many studies suggest that only a 10ppm reduction in CO₂ would occur.  There are also possible negative ecological effects that iron fertilisation could have and microbial shifts could cause an increase in other, more harmful, greenhouse gases (For example, methane and DMS).

If I had to try and sum up this blog, the 4 key findings would be:

-          Ocean acidification is a real problem that can’t be ignored, the effects on calcifying organisms is evident and the 30% increase in ocean acidity since preindustrial times is worrying, to say the least.

-          The use of boron isotopes as a proxy needs to be improved further (in terms of the fractionation index and analytical techniques) to increase our understanding of past ocean acidification in order to allow us to see how the current situation varies from past natural variability.

-          Calcifying organisms are most at threat; however some species can thrive in acidic conditions.  More work needs be done to see how the food chain will be affected.

-          Iron fertilisation and other combating schemes need to be improved or drastic cuts in atmospheric carbon dioxide levels need to occur in order to try and slow the rate of change. The unsuccessful Durban climate convention suggests that the latter is unlikely in the near future.

Several weeks ago I posted an article which was very sceptical about ocean acidification.  After spending the last term writing this blog and researching ocean acidification, I will now attempt to explore some of their claims and see if there is any truth behind them.

One of their main claims was that ocean biodiversity will flourish under increasingly acidic conditions.  Whilst the last post I wrote suggested that some species of mussel and lobster will be able to survive in acidic oceans there has been no conclusive evidence to suggest that biodiversity will actually prosper.  In fact if you try searching species that will thrive in acidic waters in Google or Web of Science you get very few useful returns. However, if you search species which are threatened by acidifying waters the literature is vast.  As I have said previously, the species most at threat are those which calcify, due to dissolution and a shoaling of the carbonate and aragonite saturation horizon.  These species are often low down in the food chain, and so if they go extinct their predators will also be affected.  The evidence for dissolution is abundant, not only from scientific experiments but also just by looking at some of the world’s coral reefs. I posted this image before which shows how rapidly a shell can deform in acidic water. 

This cartoon shows the deformation of a shell in an acidic ocean over a period of 45 days (NAOO).

Anthony et al. (2011) created a model to see how ocean acidification affected the resilience of coral reefs.  Their main findings were that reefs which suffer from overfishing and eutrophication were more vulnerable to acidic waters.  They also found that as CO₂ levels rise, the resilience and growth rates of coral reefs will fall and mortality will increase.  The Great Barrier Reef has seen a rapid reduction in calcification rates and growth of large corals (Hoegh-Guldberg, 2009).  Hoegh-Guldberg (2009) has also shown that ocean acidification not only reduced the productivity of corals but also made them more sensitive to bleaching.  More recent work by Kurihara et al. (2008) and Mayor et al. (2007) has shown that ocean acidification reduces egg production in marine shrimps and also causes an increase in unsuccessful hatchings in copepods.  Earlier on in this blog I also pointed out larger marine animals were being affected by ocean acidification.  Whale numbers are falling due to an increase in ‘noise’ in acidic waters, which makes communication and navigation trickier.  Clown fish are losing their sense of smell, due to the acidic conditions, which means that they can’t smell their habitats. 

I believe that we cannot deny the fact that ocean acidification is having a detrimental impact on marine organisms.  The effects are most widely felt by calcifying organisms yet larger marine mammals such as Whales are not out of harm’s way.  Caution needs to be given and some people probably over emphasise the effects of ocean acidification, as it is evident that some species can prosper in acidic waters. However, if their prey cannot adapt and goes extinct then they will die too, whether they are adapted to acidification or not.  More work needs to be done to assess the impact of rising CO₂ levels on a wider range of species to see how many other species have been able to adapt.  A significant proportion of ocean species will be affected by ocean acidification but it cannot be claimed that every species will be negatively affected.  Similarly, at the moment the evidence is too large to suggest that creatures are unscathed from rising carbon dioxide levels.

References

Anthony, K, Maynard, J. Diaz-Pulido, G. Mumby, P. Marshalls, P. Cao, L. (2011). ‘Ocean acidification and warming will lower coral reef resilience’, Global change biology, 17, 1798-1808.

Hoegh-Guldberg, O. (2009). ‘Climate change and coral reefs: Trojan horse or false prophecy?’, Coral Reefs, 28, 3, 569-575.

Kurihara, H. Matsui, M. Furukawa, H. Hayashi, M. Ishimatsu, A. (2008). ‘Long-term effects of predicted future seawater CO2 conditions on the survival and growth of the marine shrimp Palaemon pacificus’, J Exp Mar Biol Ecol , 367:41–46.

Mayor, D. Matthews, C. Cook, K. Zuur, A. Hay, S. (2007). ‘CO2-induced acidification affects hatching success in Calanus finmarchicus’, Mar Ecol Prog Ser, 350:91–97.

The future of our oceans

Ocean pH is expected to drop by about 0.4 by the end of the century resulting in a 60% fall in calcium carbonate levels.  To prevent our oceans becoming undersaturated in aragonite it is believed that we shouldn’t let the pH drop by any more than 0.2 units in comparison with the pre industrial levels.  The change in ocean pH is almost entirely dependent on atmospheric CO_s levels, if they continue to increase, the oceans will continue to acidify.  It will be hard to reverse the acidifying trend by the end of the century as ocean chemistry works on much much larger timescales (Raven et al. 2005).  The figure below shows that with all scenarios of atmospheric CO₂ levels ocean pH falls, if the projected CO₂ levels reach around 950ppm then the ocean pH will fall to about 7.75 and the southern ocean will only be about 70% saturated in aragonite.  If the pH falls this low, it will result in a tripling in the number of H⁺ ions since the industrial revolution.  This rate of change is faster than anything seen over the last few hundred thousand years and at least 100 times higher than anything seen since then (Raven et al. 2005). 

 Figure 1.  Atmospheric CO₂ concentrations, global ocean pH and the surface saturation state of aragonite for IPCC emission scenarios for 2000-2100. (IPCC, 2007).

References:

IPCC (2007). ‘Climate change 2007: the physical science basis.

Raven , J. Caldeira, K. ELderfield, H. Hoehg-Guldberg, O. Liss, P. Riebesell, U. Shepherd, J. Turley, C. Watson, A. (2005). ‘Ocean acidification due to increasing atmospheric carbon dioxide’, The Royal Society: London. 

Why is ocean acidification a problem?

Why should we be concerned about ocean acidification?  Well, one of the main threats is that ocean acidification will cause wide spread extinctions of ocean fauna.  This will then have consequences that will work up the food chain and could potentially be catastrophic.  The rate of change of ocean pH is particularly problematic as it means that many organisms (especially those which calcify) will not have enough time to adapt (Guinotte and Fabry, 2008).  The increase of carbon dioxide into the oceans causes an increase in H⁺ and a decrease in CO^2-₃ which in turn causes a fall in the calcium carbonate saturation state.  Fewer CO^2-₃ will make it harder for calcifying organisms to make calcium carbonate.  Biogenic calcium carbonate is required by a huge amount of marine organisms- most notably the majority of coral species, foraminifera and coccolithopores.  Calcite and Aragonite are also used by calcifying organisms and the abundance of both is influenced by CO₂ levels.  The positions of the calcite and aragonite saturation zones are crucial as CaCO₃ precipitation can only occur above this horizon and below it dissolution will occur (Guinotte and Fabry, 2008).  As oceans acidify the horizons are moving towards the surface, with those of the North Pacific shoaling at a rate of 1-2m/year (Feely, 2007).   Orr et al. (2005) predict that the surface waters of the southern ocean will become undersaturated in Aragonite by 2050 and by 2100 this could include the subarctic pacific as well as the whole of the southern ocean.  Calcifying organisms appear to be incapable of maintaining their shells and skeletons when the oceans are undersaturated in aragonite.  Once the organisms exist below the aragonite/calcite saturation horizon dissolution occurs and so, they become deformed (Fabry et al. 2008).

This cartoon shows the deformation of a shell in an acidic ocean over a period of 45 days (NAOO).

A reduction in calcification and increase in dissolution is not the only detrimental influence that ocean acidification will have on these creatures; ion capacity, buffering capability; acid base regulation and metabolism are all also affected (See Fabry et al. 2008).  In my next post I will take a closer look at how the earth’s coral reefs are responding to these changes in calcite, aragonite and biogenic CaCO₃ levels.

References:

Fabry, V. Seibel, B. Feely, R. Orr, J, (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, ICES journal of marine science, 65, 414-432.

Guinotte, J. Fabry, V. (2008). ‘Ocean acidification and its potential effects on marine ecosystems’, Annals of the new york academy of sciences, 1134, 320-342.

Orr, J. Fabry, V. Aumont, O. Bopp, L. Doney, S. Feely, R. et al. (2005). ‘anthropogenic ocean acidification over the twenty first century and its impact on calcifying organisms’, Nature, 437, 681-686.

What is ocean acidification?

Changes in Ocean Chemistry

Increased levels of atmospheric CO₂ is having two serious impacts on the earth, the first is the well known phenomenon of global warming and the second is the subject of this blog- ocean acidification.  For over 650 000 years prior to the industrial revolution the concentration of CO₂ in the atmosphere was somewhere between 180 and 300ppmv (Siegenthaler et al. 2005).  Due to an increase in anthropogenic emissions, mainly through fossil fuel burning, the atmospheric carbon dioxide levels are now between 380 and 390ppmv resulting in a rate of change that is almost 100 times faster than anything the earth has experienced over the last 650kyr (IPCC, 2007).

Over the last 200 years the oceans have absorbed approximately 1/3 (11bn tonnes) of all the carbon dioxide that has been released into the atmosphere (Sabine et al. 2004). Dissolved organic carbon occurs in the ocean in 3 different forms- bicarbonate ions, aqueous carbon dioxide (including carbonic acid) and carbonate ions (Fabry et al. 2008).  The current pH of the ocean is 8.2 and 88% of the carbon occurs as bicarbonate ions.  Carbon dioxide dissolves in the sea water and forms carbonic acid which quickly dissociates to form H⁺ and HCO₃^- . The H⁺ may react with CO₃^2- to form bicarbonate.   This means that by adding CO₂ to the ocean we are increasing the amount of H₂CO₃, H⁺ and HCO₃^- and as an increase in the concentration of H⁺ ions causes pH to fall we are making the oceans more and more acidic (Ibid.).     Since the pre industrial times the ocean pH has fallen by 0.1, which has caused a 30% increase in acidity (Boyd, 2011).  Over the next century it is believed that the pH will fall by approximately 0.5 units, making the oceans more acidic than they have been over the past 30 million years (Calderia and Wickett, 2003). If the change in CO₂ is gradual a buffer system operates and so, interactions with carbonate minerals increase which helps to reduce the sensitivity of the ocean and stabilises the pH (Caldeira and Wickett, 2003).  If the change in CO₂ occurs over a short timescale (less than 10⁴ years) the oceans don’t have time to shift their equilibrium and so, the pH is altered.    The change in pH is most prominent towards the poles (i.e the North Atlantic and Southern Ocean) as more carbon dioxide is taken up at colder temperatures.

References

Boyd, P. (2011). ‘Beyond ocean acidification’, Nature Geoscience, 4: 273-274.

Calderia, K. and Wickett, M. (2003). ‘Anthropogenic carbon and ocean pH’, Nature, 425: 365.

Fabry, V. Seibel, B. Feely, R. Orr, J. (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, Journal of marine science, 65, 3: 414-432

Sabine, C. Feely, R. Gruber, N. Key, R. Lee, K. Bullister, L. Wanninkhof, R. Wong, C. Wallace, D. Tilbrook, B. Millero, F. (2004). ‘The oceanic sink for anthropogenic CO₂’, Science, 305, 5682: 367-371.

Siegenthaler, U. Stocker, T. Monnin, E. Luethi, D. Schwander, J. Stauffer, B. Raynaud, D. (2005). ‘Stable carbon cycle-climate relationship during the late Pleistocene’, Science, 310: 1313– 1317.