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.

Can we do anything to reverse ocean acidification?

Up until now I have spent quite a lot of time looking at what is going on in our oceans and what is likely to happen in the future. The picture hasn’t been too rosy so far, but now I will see if there is anything we can do to combat ocean acidification or if we are just destined to disaster.  Iron fertilisation has been put forward as one of the main mechanisms that could help to reverse the acidifying trend.  Ice cores show that during past glacials iron fertilisation has occurred naturally (for example, dust storms coming from the Sahara towards the Canary islands) and has caused large amounts of CO₂ to be drawn out of the atmosphere (Lampitt et al. 2008). Put simply, it is thought that by adding iron to high nutrient low chlorophyll (HNLC) areas of the ocean it will stimulate the growth of phytoplankton blooms which will take up large amounts of atmospheric carbon dioxide and eventually deposit it into the ocean sediments (Cao and Caldeira, 2010).  There has been quite a large amount of speculation as to whether iron fertilisation will actually cause a reduction in atmospheric and oceanic carbon dioxide levels and several experiments have been run, each reaching different conclusions.  Martin was the first to propose the idea of iron fertilisation and he proposed that 1 ton of carbon added to a HNLC region could cause the sequestration of up to 100,000 tonnes of carbon. More recent, high resolution studies have suggested that iron fertilisation is far less efficient and would only result in a 10ppm reduction in atmospheric CO₂ (Lampitt et al. 2008).  The drop in carbon dioxide is only really found in the surface waters and doesn't extend deeper than about 70m (Cao and Caldeira, 2010).  Cao and Caldeira (2010) also found that even if iron fertilisation was successful by the end of the century it would only reduce the pH change by 0.06.  It is thought that iron fertilisation could produce carbon credits which would be doubly bad news for our oceans as not only would the iron do little to prevent the oceans from becoming acidic, countries would be allowed to increase their carbon dioxide emissions. Many private sector companies are already planning to enrich the ocean in order to gain carbon credits, the moderating of such activities is very difficult, especially away from territorial waters.  There are also several ecological impacts that need to be considered, it is probable that there would be an increase in harmful algae which could cause environments to become toxic.  Microbial shifts could also result in the production of other greenhouse gases such as DMS and methane, both of which are far more potent than carbon dioxide.  Due to monitoring difficulties it is also hard to know if plankton blooms could cause the carbon dioxide to be taken into the deep ocean and sediments.  Iron fertilisation initially sounds like a good idea and some believe that it could cause a large reduction in atmospheric CO₂. There are wide scale ecological effects that could result And the actual reductions in ocean pH and carbon dioxide levels are very low.  More work needs to be done to work out the potential side effects of iron fertilisation and to develop a greater understanding of the movement of carbon dioxide to the ocean sediments.

References

Cao, L. Caldeira, K. (2010). ‘Can ocean iron fertilization mitigate ocean acidification?’, Climatic change, 99, 1-2, 303-311.

Lampitt, R. Achterberg, E. Anderson, T. Hughes, J. Lucas, M. Popova, E. (2008). ‘Ocean fertilisation; a potential means of geoengineering’, Philosophical transactions of the royal society A, 366, 1882, 3919-3945.