This cartoon shows each countries carbon emissions, China and the USA are by far leading the way.  If China do actually agree to the Durban protocol (when they decide the fine details) then a considerable drop in emissions could occur. This cartoon nicely highlights why Kyoto failed, as the worlds 2nd biggest emitter failed to sign up. Many of the coastal countries that depend heavily on the oceans for food and money have very low emissions yet are at the greatest threat from ocean acidification.  

Will we ever agree on cutting emissions?

So, another climate conference is over and still we are no closer to achieving a successful and viable framework for reducing global carbon emissions. Durban seeked to succeed where Cancun, Kyoto and every other climate convention had failed. Apparently they have drawn up a legally binding deal which will enforce the reduction of carbon emissions in both developed and developing countries, however the deal still has to be agreed and wont come into effect until at least 2020.  The fine details of the deal, for example by how much each country has to cut its emissions, has not yet been decided…..

For the first time ever, China has agreed to be legally binded into cutting its carbon emissions, if this is successful then the total global emissions will be significantly reduced.  However with so few details the deal is unlikely to succeed. One of the largest outcomes of the conference was that Canada withdrew from the only legal treaty committed to reducing greenhouse gas emissions- The Kyoto protocol.  The reasons for their withdrawal are thought to be that they are way off meeting their emissions targets, as in fact their emissions have actually risen.  In order to rectify this they would have to pay billions of dollars in carbon credits yet there would be no effect on the environment.  I think this shows that these legally binding deals are always going to have problems and no one will ever truly agree.  Carbon credits do little to actually help the environment as rich countries will just by credits and continue to pollute.  The fact that countries can pull out if they think they won’t meet their targets makes a mockery of the whole thing. 

Will we ever agree on how to combat climate change? For the health of our oceans I hope so, but somehow I very much doubt it.

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.

Natural trend or Anthropogenic change?

This blog post will explore some of the past changes in ocean pH to try and evaluate whether the current change is part of a natural trend or solely anthropogenic.  It is, however, hard to compare current CO₂ induced changes with those of the distant past because of continental drift, changes in the elevation and orientation of the continents and the location of mountains.  All of which alter the oceanic and atmospheric circulations (Goodwin et al. 2009). 

It is widely believed that the ocean is currently more acidic than it has been at anytime over the last 2my and δ¹¹B levels show that the predicted pH for 2100 has not been experienced since about 40mya (Pelejero et al. 2010).  Around 55mya, during the Paleocene-Eocene thermal maximum, there was a significant ocean acidification event.  There was a huge carbon input into the ocean, causing a 2km shift of the calcite saturation horizon towards the surface in less than 2ky (Doney et al. 2009).  The effects of which were catastrophic.  For a full review there is an excellent article by Zachos et al. (2008).  There were two epochs during the Cenezoic era which are thought to have seen extensive ocean acidification however, δ¹¹B levels have not yet been able to quantify this.

During the Cretaceous CO₂ levels were extremely high (up to 2000ppm) and it is believed that 100mya ocean pH was around 0.8 units lower than present (Kump et al. 2009).  Diatoms survived the event but calcified species were generally wiped out (Pelejero et al. 2010).  The early Aptian Oceanic Anoxic event happened around 120mya, a series of major eruptions along the Java plateau were thought to release enough CO₂ to cause a 2km shoaling of the calcite saturation zone.  This coincided with the demise of the nanoconids which were one of the most dominant species of the time and were heavily calcified (Kump et al. 2009).

Carbon isotopes suggest that there was a substantial decrease in ocean pH at the Triassic-Jurassic boundary (Hautmann et al. 2008).   Extensive volcanism was thought to release large amounts of CO₂, resulting in a temperature increase which initiated the release of further CO₂ from marine sediments (Palfy, 2003).  The CO₂ levels were high enough to inhibit the precipitation of CaCO₃ resulting in a subsaturated and acidified ocean (Hautmann et al. 2008).  Calacerous phytoplankton suffered very badly, whereas phytoplankton with organic walls actually benefitted from the acidic conditions (Van de Schootbrugge et al. 2007). 

A major ocean acidification event was thought to have occurred during the “super-greenhouse” state which followed the late Neoproterozoic and models have estimated that the pH could have fallen below 6.0 (Le Hir et al. 2008).  However, it is thought that this acidification occurred gradually, over a period of up to 30 my (Ibid.).  Calcifying organisms had not evolved during this time so it is hard to tell the impacts the acidification had on ocean biology and also how long the acidification lasted (Kump et al. 2009).

Several studies have looked back over the last 500my and reinforce the fact that ocean acidification played a major role in 5 mass extinctions, suggesting that the current ocean acidification event could contribute to the 6^th mass extinction.  It is clear that ocean pH has varied hugely in the past, with some acidification events being far more extreme than what we are currently experiencing.  This could suggest that the current acidification is part of a long term trend; however the rate of change suggests that humans are playing a significant role.

References:

Doney, S. Fabry, V. Feely, R. Kleypas, J. (2009). ‘Ocean Acidification: the other CO₂ problem’, Annu. Rev. Marine. Sci., 1, 168-192.

Goodwin, P. Williams, R. Ridgwell, A. Follows, M. (2009). ‘Climate sensitivity to thecarbon cycle modulated by past and future changesin ocean chemistry’, Nature Geoscience, 2,145–150.

Hautmann, M. Benton, M. Tomasovych, A. (2008) ‘ Catastrophic ocean acidification at the Triassic-Jurassic boundary.’, Neues Jahrbuch Geol. Palaontol. Abhand.,  249,  119–127.

Le Hir, G. Ramstein, G. Donnadieu, Y. Goddéris, Y. (2008). ‘Scenario for the evolution of atmospheric pCO2 during a snowball Earth’, Geology, 36,47–50.

Kump, L. Bralower, T. Ridgwell, A. (2009) ‘Ocean acidification in deep time’, Oceanography, 22, 4, 94- 107.

Palfy, J. (2003). ‘Volcanism of the Central Atlantic MagmaticProvince as a potential driving force in the end-Triassic extinction’, Geophysical Monograph, 136:255-267.

Pelejero, C. Calvo, E. Hoegh-Guldberg, O. (2010). ‘Paleo-perspectives on ocean acidification’, Trends in ecology and evolution, 25, 6, 332-344.

van de Schootbrugge, B. Tremolada, F. Rosenthal, Y. Bailey, T. (2007) ‘End-Triassic calcification crisis and blooms of organic-walled ‘disaster species’’, Palaeogeogr. Palaeoclimatol. Palaeoecol.,  244, 126–141.

Zachos J. Dickens G. Zeebe R. (2008) ‘An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics’, Nature, 451, 279–283.

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.