The ocean's acid test is not a test at all? Some personal experience sharing

Last Thursday, I went scuba diving for the first time ever in my life at the Fish Eye Marine Park in Guam. Guam is an island located in the western Pacific Ocean. Scuba diving is a truely amazing experience and I strongly urge you to try if you haven't done it before! Having submerged under the sea for about 45 minutes, I saw plentiful of beautiful tropical fishes and different kinds of spectacular coral reefs, felt like swimming with the fish in those aquariums!

After this scuba diving experience, I was thinking to myself maybe ocean acidification is not actually that bad. Or it hasn't had that much of an impact on those marine life yet, not for those in Guam at least! I am now more convinced that ocean acidification is a highly localised problem. As mentioned in my previous posts, ocean acidity varies with latitude and time. There are still plenty of precious corals and fish out there in the world's oceans! Perhaps we should stay a bit more optimistic towards our oceans future but be cautious at the same time to protect these beautiful sea life.

The Ocean’s Acid Test might just be another scary story?

It seems that most of the scientific literatures and media reports I’ve been looking at so far in this blog share the common believe and even try to persuade us that Ocean Acidification is going to be another global environmental disaster by the end of this century, caused by anthropogenic CO₂ increase. Have you been wondering if this is actually true or not? How reliable are these scientific experiments and findings? Or, they are just another ‘scary story’ or ‘horror film’ portrayed by the scientists and media, like climate change after all?

In this post, I’m not trying to dispute the scientific evidences and media reports that I’ve reviewed in this blog. Instead, I wish to provide a holistic view of this ocean acidification phenomenon and avoid leading you to any particular views e.g. ‘ocean acidification is detrimental’.

The first critique that I want to look into is a report produced by the Science and Public Policy Institute (SPPI), published on the 5^th January 2011. It mainly challenges the video ‘Acid Test:  The Global Challenge of Ocean Acidification’ produced by the National Resources Defense Council (NRDC) as a new propaganda film, which I’ve posted in my first entry. Through reviewing over 100 scientific literatures, the SPPI criticizes strongly of the NRDC documentary ‘reveals nothing of substance’ (Knappenberger, 2010), which has provided no empirical evidence of this ocean acidification catastrophic theory proposed by most of the scientists. They further argued that ocean acidification is impossible to cause catastrophic disruption to the world’s marine ecosystems; in the past geologic era, levels of CO₂ are 20 times higher than today has promoted coral development (SPPI, 2010). Ridgwell has further supported this argument that ‘ocean pH in the past was indeed lower than now during the Cretaceous, and probably lower than anything we will manage in the future’ (Ridley, 2011). In addition, ocean acidification induced by CO2 increase is actually beneficial to most marine calcifiers, especially coccolithophores and coral reef building (SPPI, 2010).

Matt Ridley, who is also an ‘ocean acidification critic’ made similar arguments of this ocean’s acid test challenge. In the article ‘The Threat of Ocean Acidification is greatly exaggerated’, Ridley responds to his critique on his book ‘The Rational Optimist’ in the New Scientist. In his book, he strongly emphasized that ‘ocean acidification may not be the widespread problem conjured into the 21^st Century’, which is completely opposite to projections made by Orr et al. (2005) and Hoegh-Guldberg et al. (2007) that I have previously reviewed in this blog. Ridley has also made the following main arguments:

1.   Ocean pH is not actually turning acidic, but less basic only. The current average ocean pH is about 8.1, which is alkaline and well above neutral (pH=7).

2.   Even ocean pH do experience significant temporal (daily) and spatial variation, some marine organisms, e.g. a four-decade-old-mussels learned to cope with this acidity fluctuation.

3.  There is no evidence of coral danger ‘may be extreme’ e.g. coral bleaching depends more on the rate of change than the absolute temperature (even his critic Hoegh-Guldberg accepts this point). It is therefore even more unconvincing to assume that a 0.1 unit of pH drop since the Industrial Revolution will cause a catastrophic effect on marine ecosystem (SPPI, 2010)

4.  Most literatures disregard the fact that the corals are able to recover quickly from episodic bleaching. In fact, corals are now more resilient and adaptive to thermal stresses.

5.  Ocean acidification increases bicarbonate ion concentration which promotes biological precipitation of carbonate ions by marine organisms (calcification) e.g. some coccolithophores deposit carbonate shells easier at slightly lower pH

Both the SPPI and Matt Ridley’s skepticism of the ocean’s acid test are logical and convincing. However, as Ridley said in his critique, views and opinion about the ocean acidification phenomenon depends on different interpretation of the scientific evidences. In other words, it also depends on the assumptions made about this phenomenon. And of course, assumptions differ among different scientists and many other literature authors, so assumptions can always be challenged and changed constantly. Although both critics argued that there is currently no empirical evidence to support the dominant ocean acidification theory, none of them were able to provide solid evidence and projections on the impacts of ocean acidification either. Since the ocean is an open system and constantly subject to climatic and other environmental changes, there are still significant levels of uncertainty in both the optimistic and pessimistic views about the oceans. Therefore, it is very hard to draw a conclusion of whether ocean acidification is beneficial, detrimental, or simply having no effect on the oceans.

However, in the environmental justice point of view, anthropogenic impacts on the world’s oceans should never be ignored. Because of the definite high uncertainties of the oceans future, the oceans should therefore be even more closely monitored to produce more certain projections, thus leading us to make sensible decisions about the future. 

A summary of the Acid Test so far

Today, I found this fairly recent video about the current scientific work and concerns of the ocean acidification produced by Cassandra Brooks from the Pacific Coast Science and Learning Centre. It summarizes what I've been posting in my blog entries so far very nicely, analysing the ocean acidification problem from the past to the present. Towards the end of this video, some interesting questions were posted in regards to the future of our oceans which I found it quite thought-provoking. For instance, the latest hatchery and oyster growing strategies of pumping seawater only during times with low CO2, but these strategies might be rather short term. Or, maybe ocean acidification is not as bad as it sounds!As said in this video, marine organisms might be able to adapt the acidifying ocean. But again, we don't know whether they will be able to based on our current scientific knowledge. The situation is simply too uncertain.

Indeed, there are many ocean acidification critics out there who simply think that all these are all exaggerated. So, before drawing any conclusions, I would like to explore critiques of this ocean acidification phenomenon in the following week. Let's see who to believe!

Iron Fertilisation: A way out of the Acid Test?

Having seen so much destructive impacts of the ocean’s acid test so far, is there a way to mitigate the situation? The answer is YES, there is. One of the most well-known geoengineering solution to mitigate ocean acidification is Ocean iron fertilisation (OIF), which is originated from the ‘Iron hypothesis’ proposed by an oceanographer John Martin in 1990.

How does ocean iron fertilisation work?

Martin’s Iron hypothesis is based on his research on the last glacial-interglacial CO₂ changes. He argued that the high concentrations of atmospheric CO₂ during the last and Holocene interglacial, i.e. 280ppm pre-industrial level was due to the high deficiency of the element Iron in the Southern Ocean, hence reducing the potential CO₂ uptake by phytoplankton in the surface ocean via the ‘biological pump’.  In contrast, during the Last Glacial Maximum (LGM), atmospheric dust iron supplies were 50 times higher than that of the last interglacial period. This iron enrichment had enhanced phytoplankton growth, which had utilised large amount of upwelled macronutrients to stimulate the overall productivity in the Southern Ocean, leading to the total decrease in CO₂ during the LGM.

The current geoengineering technology, ocean iron fertilisation is therefore based on the principle of using iron to enhance the productivity of the ocean, so as to increase the anthropogenic CO₂ sequestration in the ocean interior. In order to investigate the effects, efficiency and feasibility of OIF, a number of small-scale field OIF experiments and modeling studies have been conducted in different high-nutrient, low-chlorophyll (NHLC) regions across the world’s oceans, including the large parts of the Southern Ocean, the eastern equatorial Pacific and part of the North Pacific (Cao and Caldeira, 2010).

Figure 1 Effects of Ocean Iron Fertilisation 

Figure 1 illustrates one of those OIF field experiments employed in the northeast Pacific Ocean. Red and yellow colour represents regions with high concentration of chlorophyll a, thus a high biomass of phytoplankton and vice versa for blue colour. Area (a) has not been enriched with iron while area (b) has. It is clearly shown that with iron fertilisation, biotic community in the ocean has shifted from cyanobacteria-dominated to diatom-based i.e. larger phytoplankton productivity (Armbrust, 2009).   

How effective and feasible is OIF?

The effectiveness and feasibility of this OIF technology is largely questioned by the scientific community. For instance, the above OIF experiment at NE Pacific Ocean has only generated a slight increase in organic carbon storage in deep water despite of the expected enhanced phytoplankton bloom, with most of the organic carbon consumed and recycled in the surface ocean (Armbrust, 2009). Modeling studies produced similar conclusions and projections with the use of OIF. According to the model simulations by Cao and Caldeira (2010), a globally sustained OIF could not diminish atmospheric CO₂ concentrations to below 833ppm or reduce the mean surface ocean pH change to less than 0.38 units; compared to 965ppm with 0.44 units reduction in pH under the IPCC A2 emission scenario (Figure 2). Ironically, it has been reported that iron fertilisation cannot even mitigate ocean acidification but acidify the deep ocean further. As OIF stimulates carbon sequestration at the deep ocean, it is very likely that deep ocean pH will be further reduced with this addition of carbon. This can pose undesirable acidic environment for marine organisms in the deep ocean to survive, although its extent and impacts are still not very well known.

Figure 2 Minor effects of OIF in ocean acidification mitigation

The associated environmental risks are even more alarming to scientists, especially the impacts on marine ecosystems. Silver et al. (2010) reported that high levels of amino acid neurotoxin domoic acid (DA) have been produced historically through the iron enrichment experiments. The high levels of DA have been released by the increase in a diatom genus Pseudo-nitzschia. Pseudo-nitzschia spp. are present in coastal harmful algal blooms (HABs) worldwide and are recognised as contaminants to a wide range of fauna – from invertebrates to marine birds and animals (Silver et al., 2010).

In addition, Denman (2010) have highlighted the possible side effects associated with this OIF technology. He suggested that the increase in remineralisation triggered by iron fertilisation results in increased denitrification and the production of nitrogen dioxide (N₂O), the third most anthropogenic greenhouse gases. Furthermore, models have projected a decrease in dissolved oxygen or even anoxic in wide areas of subsurface ocean caused by increased remineralisation during OIF, making marine organisms, especially calcifiers harder to adapt to the changed environment. This increase in remineralisation process also reduces available macronutrients returning to the surface ocean, which causes a reduction in productivity i.e. biological carbon exports and the complexity of marine ecosystems. As illustrated above, OIF tends to favour the growth of diatoms, which eventually dominates that of all other phytoplankton groups, e.g. one of the dimothylsulfonioproprionate (DMSP)-producing phytoplankton species. Several OIF field experiments have shown a short-term increase in surface ocean DMS concentrations, followed by a decrease relative to the concentration outside the experimental area. Dimethylsulfide (DMS) is an important gas to stimulate the formation of cloud condensation nuclei. Therefore, a reduction in DMSP-producing phytoplankton might cause undesirable climatic impacts.

Despite of all the above negative consequences and concerns of OIF, it might still be considered as a viable mitigation measure in the long run. A striking research produced by Blain et al. (2007) have concluded that the efficiency of natural iron fertilisation was at least ten times higher than previous estimates from short-term phytoplankton blooms induced by the short-term iron-addition experiments. However, it has been stressed that the ocean natural system is very sensitive to iron, in which the addition of iron e.g. from dust  has to be occurred slowly and continuously as supposed to adding in large amounts purposely in order to achieve effective carbon sequestration.  The effectiveness of OIF is also highly constrained by the availability of macronutrients in local and surrounding water to support remineralisation.

To conclude, in a very large extent, ocean iron fertilisation is not a viable approach in mitigating ocean acidification; it is both economically and environmentally unsound. Deep ocean carbon sequestration using OIF technology not only causes undesirable environmental consequences, it also acts as a catalyst to encourage more anthropogenic CO₂ emissions in the form of carbon credits (Cao and Calderia, 2010). How would you weigh up the benefits of removing 32ppm of CO₂ and reducing a maximum of 0.06 unit of ocean pH in 100 years with the costs of potential marine ecosystems lost and increased N₂O production? I believe that the answer is very apparent here. 

The UN Climate Talk agreement: Can the oceans be relieved from the acid test challenge?

Followed by the failures of the Kyoto Protocol to call upon a legally-binding agreement for countries to reduce carbon emissions, the UN Climate Talk this year finally ends with some results today. You can click here for the Guardian news link. 

Seventeenth session of the Conference of the Parties (COP 17) has been taken place since the 28^th November at Durban, South Africa and finally comes to an end two days ago, with a commitment by all countries to accept binding emission cuts by 2020. A new climate fund is also agreed to be set up, carbon markets will be expanded and countries will be able to earn money by protecting forests. Hopefully, this deal will be able to force countries to cut their emissions enough to stay under a 2^oC temperature increase, the threshold of ‘dangerous climate change’.

In no doubt, this climate talk is a great step forward towards a legally-binding joint action of all countries to cut their carbon emissions. By adopting a lower IPCC emission scenario, it might increase levels of certainty in future climate projections. Hopefully, the oceans will be able to absorb and buffer upon this lower and constant anthropogenic CO2 without dramatic changes in pH and marine ecosystems. Nevertheless, neutralisation and CO₂ buffering in the ocean is a very long process, in a timescale of hundreds and thousands of years. The effects of this climate deal are therefore yet to be seen.  

Model the ocean’s future

Previously, we’ve looked at how the Paleocene-Eocene Thermal Maximum provides us an analogue to the future of our oceans, which projects a similar or even worse marine mass extinction in the future. This week, I’m going to explore what do climate and oceanic models say about our ocean’s destiny in the next 100 years.

Ocean-carbon cycle model projections

Orr et al. (2005) have used 13 coupled ocean-carbon cycle models to simulate changes in surface ocean pH, carbonate ion concentration [CO₃^2-] and calcium carbonate saturation under the Intergovernmental Panel on Climate Change (IPCC) IS92a ‘business-as-usual’ scenario over the 21^st century, which are adopted by the IPCC 4^th Assessment Report (IPCC AR4) Chapter 10.4.1-2. Their multi-model projections indicated large decrease in oceanic pH and carbonate ion concentrations across the world’s ocean during the 21^st Century, driven by direct geochemical effect of increasing anthropogenic CO₂ emissions only (Figure 1).  Currently, surface ocean pH is already 0.1 unit lower than pre-industrial values. It is further projected for another 0.3-0.4 units decrease by the end of this century under the IPCC IS92a scenario, which is equivalent to a 100-150% increase in [H⁺] (Orr et al., 2005). Their results also highlighted similar latitudinal variability pattern; surface water pH and [CO₃^2-] are higher in high latitudes and decrease significantly towards mid and low latitudes. However, according to their model projections, latitudinal differences in [CO₃^2-] between mid and high latitudes will be reduced towards 2100 under the IS92a scenario (Figure 1c). Carbonate ion concentrations i.e. aragonite saturation state will begin to decline and the undersaturation of aragonite is projected to extent throughout the entire Southern Ocean and into the subarctic Pacific Ocean by 2100 (Figure 2).

Figure 1 Projected (a) atmospheric CO2, (b) ocean surface pH and (c) [CO₃^2-] during 21st Century (Orr et al., 2005)



Figure 2 Projected levels of aragonite saturation (%) over the 21st Century (IPCC, 2007)

The authors also found that this aragonite undersaturation in the Southern Ocean could threaten high-latitude ecosystems within decades, not centuries as previous studies suggested; it is likely to be first detected during winter. A more recent paper by McNeil and Matear (2008) further estimated that this Southern Ocean wintertime aragonite undersaturation will occur by the year 2030 and no later than 2038 with atmospheric CO2 of ~450ppmv under the IPCC IS92a scenario.

Coupled atmosphere ocean-climate model projections

In order to simulate the effects of climate variability and climate change (changes in temperature and ocean circulation etc.) on ocean carbonate ion concentrations on top of the direct geochemical effects, Orr et al. (2005) have also analysed three coupled atmosphere ocean-climate models. Surprisingly, their analysis suggested that physical climate change alone will not alter high-latitude surface [CO₃^2-] substantially during the 21^st Century. This argument is also reaffirmed by the coupled climate/ocean-carbon cycle model results generated by Cao and Caldeira (2008). It is largely due to the air-sea CO2 exchange mostly compensates for changes in surface dissolved inorganic carbon (DIC) caused by changes in marine productivity and circulation.

However, high-latitude subsurface [CO₃^2-] is projected to be decreased significantly during the 21^st Century based on all the three coupled model results, with small uncertainties in changes in temperature, ocean stratification and marine biological production and re-mineralization (IPCC, 2007; Orr et al., 2005). Orr et al. (2005) further suggested that the biggest uncertainty comes from atmospheric CO2 trajectories, which is the only means to limit further decline in ocean [CO₃^2-].  Figure 3a showed the atmospheric CO₂ projections under the six IPCC emission scenarios, which exhibits high levels of uncertainty, especially towards Year 2100. As surface ocean [CO₃^2-] is highly dependent upon atmospheric CO₂, high levels of uncertainty in atmospheric CO₂ trajectories have a significant impact on predicting global pH changes as well as the timing and extent of aragonite undersaturation (Figure 3b and c).

Figure 3 Model projections of  (a) Atmospheric CO2, (b) Global Ocean pH and (c) Southern Ocean Saturation under six IPCC emission scenarios (IPCC, 2007)

It is concluded that the increased anthropogenic CO₂ emissions has a significant impact on ocean carbonate chemistry, i.e. ocean acidification, based on both ocean-carbon cycle models and coupled climate/ocean-carbon cycle models. Despite of the large uncertainty in atmospheric CO₂ level projections, the changes in oceanic carbonate ion concentrations is closely linked with atmospheric CO₂. Although more research is required to provide more reliable projections of future atmospheric CO₂, it is clear that the impact of ocean acidification will be more severe in the future under ‘business-as-usual’ scenario. In the following weeks, I will be moving on to discuss ways to mitigate this ocean’s acid test challenge. Stay Tuned!