Evidence For Carbon Contributions To Climate Change Environmental Sciences

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There is ample evidence to demonstrate that Carbon Dioxide (CO2) is the main driver of climate change and Greenhouse Gases (GHGs) contribute to radiative forcing and the enhanced greenhouse effect; due to man-made emissions, mostly CO2, the earth presently has an energy imbalance which contributes additional warming of the surface (Meunier, 2007). This makes reducing and controlling the levels of atmospheric CO2 by enhancing sinks and reducing sources a growing concern, particularly as emissions have been rapidly increasing since the Industrial Revolution (Figure 1), and can reside in the atmosphere for thousands of years (Lacis et al, 2010).

Figure 1: Graph to show the atmospheric concentrations of Greenhouse Gases from year 0 to year 2005. The red line represents Carbon Dioxide (CO2). Increases since 1750 can be attributed to the Industrial Revolution. Year 2000 levels of CO2 lie at around 1800 ppb. (Source: Forster et al, 2007)

The Ocean sinkThe Carbon Cycle and the Ocean Sink

Understanding of the global carbon cycle is important if we are to identify ways of reducing CO2 concentrations. The oceans are significant in this as they have absorbed approximately 30% of anthropogenic CO2 since 1750 and therefore have a key role as a CO2 sink (Figure 2) (Raven and Falkowski, 1999). The primary mechanism by which CO2 is transferred from the atmosphere to the oceans is through surface exchange; where the partial pressure of CO2 in the atmosphere is greater than in the ocean there is a net flux of CO2 to the oceans to maintain equilibrium. Critical to this is the speed in which the ocean can move dissolved inorganic carbon (DIC) to deep waters (Quéré, 2010), allowing surface waters to draw down and store more CO2 from the atmosphere. This is aided by the 'biological pump' and the 'solubility pump' (Houghton, 1997:28).

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Figure 2: Figure to show the global carbon cycle in the 1990's, with pre-industrial 'natural' fluxes in black, and 'anthropogenic' fluxes in red, measured in GtC/y-1. (Source: Denman et al, 2007)

The 'biological pump' relies on phytoplankton, the principle autotrophs, which absorb DIC through photosynthesis. Some is used to build biomass in the form of particulate organic carbon (POC), and some is transferred to fish through the food chain. Once the phytoplankton and fish die, the calcium carbonate shells of phytoplankton and the flesh of fish fall into the deep ocean, which transfers carbon; the biological pump. However, primary production is thought to be limited by iron availability, partly based on evidence from paleo-climate records that show diatoms of phytoplankton during glacial periods were stimulated by terrestrially derived iron blown to the oceans; suggested by John Martin as the 'iron hypothesis' (Blain et al, 2007; Boyd et al, 2000).

In the Southern Ocean a weakening of CO2 sinks has been identified which shows responses in the ocean carbon cycle as a result of climate change which can affect the uptake of CO2 (Quéré et al, 2009); CO2 is most soluble in cold waters, so an increase in temperature means lower CO2 solubility (Raven and Falkowski, 1999). Geo-engineering approaches therefore have been trialled in an attempt to enhance the 'biological pump' to sequester more CO2.

2. Iron Fertilization

It has been proposed that fertilizing oceans with iron could increase carbon sequestration by enhancing algal photosynthesis and diatom growth, therefore increasing the drawdown of POC (Watson et al, 2000; Barker et al, 2007); laboratory experiments suggest 30,000 to 110,000 tons of carbon could be removed from the air with every ton of iron added (WHOI, 2007), thus reducing the gases which contribute to warming the atmosphere. It has also been identified that the production of dimethyl sulfide might increase with plankton growth, which would increase cloud cover, helping to cool the atmosphere.

Twelve small-scale field studies have been undertaken since the early 1990's (Figure 3) to comprehend how the oceans might respond to inputs of iron and the potential effects on the carbon cycle, by adding small amounts of iron sulphate to the surface water (Barker et al, 2007) in high-nitrate, low-chlorophyll (HNLC) regions; the Subarctic Pacific Ocean, the Equatorial Pacific Ocean, and the Southern Ocean (Wong et al, 2006).

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Figure 3: Figure to show the locations of the twelve small scale experiments which have been conducted across the oceans. Each experiment is represented by a red dot. (Source: WHOI, 2007)

2.1 Advantages and Successes

Blain et al (2007) noted, in observations of natural iron fertilization in the Southern Ocean, that a large phytoplankton bloom was sustained over the Kerguelen plateau by an iron supply from iron rich deep-water. Similarly, artificial iron fertilization experiments have found results where blooms were enhanced, complementing this observation and verifying the 'iron hypothesis'. For example, Buesseler et al (2004) discovered, in the SOFeX experiment, that the flux of POC increased within the iron-fertilized waters (Figure 4); at the end of the experiment, fluxes of POC IN were greater than POC OUT. Most noticeable in the deeper water column (100m), it could be suggested that this was successful as more CO2 was drawn down into the ocean than was released.

Figure 4: POC flux data from SOFeX, measured at 50m and 100m. Fluxes are shown for 'IN' and 'OUT' for the start and end of the experiment (duration of 28 days). (Source: Buesseler et al, 2004).

Such success has led to policy proposals and commercial endeavours to use these techniques to enhance carbon sequestration (Trick et al, 2010), especially in the interest of selling carbon offset credits, for example the work of California based company CLIMOS. However, the Kyoto Protocol's Clean Development Mechanism does not identify ocean fertilization as a mechanism for creating carbon credits for international trade, and therefore they would probably have to be traded on the voluntary carbon market where individuals or companies with no obligations under the Kyoto Protocol can buy credits (Strong et al, 2009).

2.2 Potential Problems and Consequences

However, there are still many uncertainties and potential risks which have been identified with this technique which limit its potential for success.

2.2.1 Uncertainties

Firstly, the efficiency of transport of phytoplankton biomass to lower depths below the main thermocline has not been explored in much detail. Boyd et al (2000) illustrated, through satellite imagery, that a significant proportion of iron remained in the surface waters which suggests that sequestration of carbon by phytoplankton is not very efficient; much of the biomass is moved back into the upper water column through upwelling. This brings concern as to whether export of carbon to deeper waters will occur enough to allow more carbon to be drawn down at the surface, and suggests that estimates of the amounts of carbon which could be sequestered could be over stated (Barker et al, 2007). Boyd et al (2000) also believed it is difficult to confirm the iron hypothesis, as iron availability, light availability and silicate availability could, in combination or separately, all exert controls on phytoplankton growth and thus iron supply may not be the only mechanism to enhance primary productivity.

Suggestions that whales can act as a long-term reservoir of iron have also been made, as Krill eat diatoms of algae, and Baleen whales eat Krill; a study indicated high iron content in seven species of Krill and muscle tissue from two Baleen whales. The study suggests that Krill can store iron in their bodies, and that Antarctic Krill could hold 24% of the total iron in surface waters. The study advocates that an alternative process of enhancing iron in Southern Ocean waters could be to allow whales which have previously suffered exploitation to recover to enhance iron levels naturally in surface waters (Nicol et al, 2010).

2.2.3 Economic Costs

Ocean fertilization can only be deployed in 30% of the world's oceans; those HNLC oceans which are iron deficient (Barker et al, 2007). Therefore, the effect would be very small in terms of the global carbon cycle, particularly with rising emissions and the need for even more carbon to be sequestered. Other techniques would therefore need to be deployed in the other 70% of the oceans and over the land surface to make significant impact and consequently to contribute to mitigating climate change, thus it is possible that the impact on global carbon budgets would be small compared to the costs.

Alongside this, the time period and geographical extent of carbon sequestration on the large-scale is poorly understood from the small-scale experiments (Watson et al, 2000). Since the outcome of iron fertilization cannot be tested directly without altering large areas of ocean, global-ecosystem models are used to anticipate the potential effects and efficiency of the method. These models have recommended that if the Southern Ocean were constantly fertilized with enough iron to eliminate limitations to phytoplankton growth, it would still only sequester less than 1 of the 8 GtC/y which is currently produced through anthropogenic emissions (Strong et al, 2009). This emissions figure is expected to continue to rise, so the cost of implementing the technique would be large compared to the small amount of sequestration achieved.

However, the economic cost is not the only concern. There are also potential ecological impacts of ocean fertilization which could cause an unpredictable shift in the marine food web and the ocean carbon cycle.

2.2.4 Ecological Costs

Increasing blooms could cause chemical changes elsewhere in the oceans as they would take up other nutrients as well as iron, such as nitrate and phosphate, which could potentially deplete neighbouring waters of essential nutrients for phytoplankton growth (WHOI, 2007), subsequently affecting the whole food chain (Strong et al, 2009) in nearby and adjacent waters. The absorption of anthropogenic CO2 into the oceans could also affect ocean acidity, decreasing ocean pH, and posing a risk to sensitive coral reefs and other organisms (Cao and Caldeira, 2010).

Trick et al (2010) discovered that iron fertilization could furthermore threat marine ecosystems, as the addition of iron increases plankton of the genus Pseudonitzschia which can produce toxic acid known as domoic acid, which rises in concentration in conjunction with the algal blooms (Vastag, 2010). This toxic acid accumulates in fish, which are immune to it, yet it is known to cause poisoning of mammals and birds in the waters such as sea lions and pelicans, of which fish are their main source of food (Black, 2010).

Nevertheless, none of these effects have been directly observed by the small-scale experiments (Barker et al, 2007) and therefore to understand the long-term side effects and uncertainties about the risks, more large-scale experiments would need to be conducted (Strong et al, 2009).

2.3 Impacts on Global Warming

A study conducted by the University of California have suggested that phytoplankton have a key role in absorbing solar radiation, and therefore actually have a warming influence in the atmosphere by increasing sea surface temperatures; without phytoplankton absorbing this radiation which would otherwise be reflected, the global climate would be cooler (SCRIPPS, 2002). This counteracts the idea that global warming would be reduced by increasing phytoplankton to sequester CO2.

Similarly, increasing organic carbon export as a result of fertilization could lead to an oxygen deficiency (anoxia) in the deeper ocean, which could give rise to outgassing of Nitrous Oxide and Methane through enhanced dentrification. Methane is an important GHG, as it has a global warming potential of 72 times more than CO2. Therefore, this could contribute more to the greenhouse effect as the increase in GHGs could be larger than the decrease from the drawdown of CO2 (Raven and Falkowski, 1999) and would thus be adding to the problem of global warming and not resolving it. This suggests that we do not know enough about the carbon cycle amongst the oceans to begin to attempt to change its chemical properties, and should therefore gain more knowledge.

3. Conclusions

Iron fertilization has proven to be controversial; whilst many studies have proven the iron hypothesis, it should still be considered with caution since there is insufficient evidence that it will work, and sufficient evidence for the potential ecological risks, the possibility that phytoplankton growth could be stimulated by other factors, and the potential addition to global warming.

The experiments which have been conducted so far have not observed closely enough exactly how much CO2 is absorbed and transferred into the deep ocean or, fundamentally, how long it can remain sequestered there (WHOI, 2008) and its potential side effects. Larger scales, comparisons of amounts of iron addition, and deeper flux measurements are needed to reduce uncertainty and obtain greater understanding of how iron fertilization might affect the global carbon cycle. In consideration of the risks, perhaps the precautionary principle should be applied here which determines if scientific understanding is incomplete, decisions should be made with caution; the risks need to be better understood before iron fertilization should be considered as a climate change mitigation strategy. Perhaps alternative geoengineering techniques and mitigation strategies should be explored.




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