Carbon Sinks

We do not inherit the earth from our ancestors - we borrow it from our children

What degree of proof about the human catastrophe from global climate change do we need

before we are motivated to act to prevent it? - Eric Chivian, MD

Carbon Sinks

There are two main carbon sinks, 1. The Oceanic Sink and 2. The Terrestrial Sink.

1. The Oceanic Sink

The oceans hold 98.5% of the carbon in the atmosphere-ocean inventory and therefore play a vital role in the natural regulation of atmospheric carbon dioxide levels. The ocean can hold enormous quantities of CO2 because unlike most atmospheric gases it reacts with water to form carbonate and bicarbonate greatly enhancing its solubility. CO2 uptake is controlled by two "solubility pumps" that keep CO2 concentration ~10% lower than at depth. First of all CO2 is approximately twice as soluble in the cold water at high latitudes than it is in warmer water. Cold water is also very dense, and in the North Atlantic high salinity also adds to its density. This density causes the water masses to sink to the deep ocean forming North Atlantic Deep Water (NADW), a crucial component of THC. This creates a flux of gas from the atmosphere to the ocean. The biological pump constantly remove CO2 at the surface and export Particulate Organic Matter (POM) to the deep where the carbon may be converted back to CO2 by bacteria, eventually returning to the surface. Or a very small portion of it may get buried in the sediments.

In model simulations where these pumps were shut down the atmospheric CO2 concentration rose from 280ppm to 720ppm (Sarmiento, 1993). This biological pump is particularly important south of 30'S. High latitude regions represent only 16% of the ocean surface and most of this is in the Southern Hemisphere. Though relatively small this region is responsible for ~ 35% of total CO2 absorption (Libes, 1992). This is because the region is like a 'window' in that it is the same temperature and density as ~ 84% of the ocean's volume found as deep water. If the pump is not functioning, the window is open and CO2 escapes to the atmosphere. If it's operating at maximum efficiency then the window is closed and biota strip CO2 from the deep water as it surfaces and the carbon stays in the ocean. It is estimated that atmospheric CO2 could be reduced to 165ppm if this pump operated at maximum efficiency. However phytoplankton are limited by Phosphorous, Nitrogen, Iron, Silicon, other micronutrients, or grazing by zooplankton (Sarmiento, 1996). Therefore the fate of much of the carbon stored in the deep waters by these pumps is regulated by what occurs in these high latitude regions before the water sinks. In all latitudes there is already some anthropogenic carbon being deposited in some areas of the deep by sinking biogenic detritus. But this is limited by recycling efficiency and nutrient availability in the surface waters (Libes, 1992).

Though some studies indicate that the oceans play a role in delaying climate change through their role in the carbon cycle and their absorptive heat capacity (Bryan et al, 1982) recent studies show we may experience a more rapid change in temperatures than the gradual change that had been predicted. This has been shown to have happened in the past through polar ice, ocean sediment and bog records which show very rapid changes in climate, believed to be dominated by the role of changes in NADW formation (Broecker, 1987). If changes are rapid the biosystem may not have time to adapt to changes in scenarios where little or nothing is done to reduce emissions (Hansen et al, 1988).

How Big is The Oceanic Sink?

This is a long and complicated question, and one that we shall go into in some depth. According to Table 1 the oceanic sink is estimated at approximately 2.0Pg/year, but the IPCC scientists assumed that ocean circulation would remain constant (Sarmiento, 1993). However many scientists now feel that this has been overestimated and that it is no more than 1.0 Pg/yr, other estimates place the number between only 0.3 to 0.8 Pg/yr. The problem arises because in order to know the net flux of carbon into the ocean we must know the gross fluxes accurately, which we do not (Zahn, 1994).

The oceanic sink is complex because the air-sea flux depends on biota which depend on nutrient availability, and it also depends on wind speed, turbulence, bubble injection, the CO2 gradient, upwelling, heating and cooling at the surface and deposition rates in coastal ecosystems which bury organic carbon. Furthermore the net sink provided by coral reefs need to be further examined (Wollast, 1993), especially considering recent evidence of large scale destruction to the coral reefs.

We must also consider the role that sea level rise may play in the carbon cycle. The se level is expected to rise by 1m in a double CO2 scenario due to thermal expansion alone, and if the effect of meltwater from continental ice sheets is included this rise could be much higher (Manabe et al, 1993). This could inundate low lying vegetation, thus adding carbon in the form of carbon dioxide and methane to the atmosphere. The Southern and Northern Hemispheres also differ in their response to climate change because of the deeper mixing in the open seas of the southern hemisphere which lack the continental barriers in the north, resulting in slower increases in temperature ( Stouffer et al, 1989) and an increased difficulty in predicting the ocean's response to global climate change.

To really complicate matters changes in the biological pump which transports carbon to the deep seas in the form of fecal pellets will be affected not only by changes in surface ocean temperature but also anything that affects the nutrient availability in the oceans. This could be from disruptions of THC due to global warming or from the anthropogenically increased transport load of nitrogen and phosphorous in the rivers from agriculture and erosion. This could increase deposition at the continental margins by way of the biological pump, thus enhancing the sink (Wollast et al, 1993), this is of course dependent on nutrient availability (Beran, 1995). The biological pump will not experience a fertilization effect from extra carbon as some terrestrial plants do because biota in the ocean are rarely carbon limited due to the high background concentration, they are limited instead by other nutrients including Nitrogen, Phosphorous, Silica, as well as Iron and other trace metals (Beran, 1995).

Limitations to the Oceanic Sink

An immediate limitation to the oceanic carbon capacity is the long time scales involved; it takes about a thousand years for the ocean to complete one mixing cycle. As a result the ocean simply cannot take up excess carbon fast enough to match the anthropogenic increase. Only 40-50% of the carbon added to the atmosphere since 1800 has dissolved in the ocean, with ~ 28% still remaining in the atmosphere. Several positive feedbacks also work to reduce the carbon capacity of the oceans. For instance the chemical capacity of the oceans to take up CO2 goes down as the concentration goes up (Sarmiento, 1993). There is also a positive feedback that occurs as we increase the CO2 in the atmosphere, this increases radiative forcing which increases surface ocean temperatures by an average of 3 or 6 degrees for double and quadruple atmospheric CO2 scenarios, respectively. This would reduce the ocean's capacity to absorb CO2 since gas is more soluble in colder water. This heating effect will be especially pronounced at high latitudes, which are already showing the greatest degree of warming relative to the rest of the ocean; which is of great concern considering this region's dominant role in carbon absorption (high latitudes are the sources of deep water circuation: the water from the surface, high in dissolved CO2 sinks to the bottom in these regions, carrying all the C)2 with them) (Sarmiento, 1993).

Another positive feedback in high latitudes is that reduced areal ice coverage at high latitudes will cause a reduction of reflected incoming solar raditiation (because ice has a much higher albedo (~reflectivity) than either water or land). As a result this will casue more absorption of solar raditiation, resulting in further increases in temperatures and further reductions in CO2 uptake.

Another problem is that an intensified hydrological cycle (increased heat causes more evaporation, more cloud formation etc) will increase runoff at mid and high latitudes, only partly balanced by increased evaporation. This will cause the surface salinity to decline in these latitudes. This combined with reduced wind strength in the lower atmosphere and increased surface temperatures may cause a stabilization of the water column resulting in a

partial or total collapse of NADW (North Atlantic Deep Water) formation (See graphs below from Sarmiento, 1996. Shows atmospheric CO2 concentrations at 350, 450, 550, 650, 750, and 1000ppm and the resulting effect on NADW formation.) (Joos, 1999).

This combined with the stability induced by the surface warming more than the deep, due to the lack of vertical mixing will cause a complete reorganization of THC (Skagseth, 1998). These factors work together to reduce carbon uptake by an estimated 50% (Joos, 1999). Some feel that the suppression of this THC would lead to a reduced ability to take up carbon while others believe that a slowing of THC will enhance biological processes, thus enhancing the uptake of carbon (Bryan, 1985). However even with this scenario this extra uptake by organisms and the subsequent export of carbon to the deep would at best only balance the reduction in carbon uptake caused by a weakening or collapse of NADW. Also productivity would only increase at high latitudes, it will likely decrease with increased warming and reduced nutrient supply at low latitudes. Furthermore as productivity increased respiration would soon follow as more zooplankton survive due to a larger food supply (Joos, 1999).

Enhancement of The Oceanic Sink

Two natural negative feedbacks operating in the oceans are that as CO2 rich THC comes in contact with sediments, some reacts with CaCO3 lowering the partial pressure in the mass and allowing it to absorb more when it returns to the surface. Also NADW formation currently releases considerable heat into the atmosphere. If this weakens or collapses this will lead to regional cooling as this extra heat will no longer be released into the atmosphere. This could initially increase the biological and solubility pump efficiencies in this area of the ocean, both serving to reduce carbon in the atmosphere. Eventually however the biological pump would be limited by lack of nutrients (Libes, 1992). These feedback mechanisms will take hundreds or thousands of years before they could become significant so are not effective means of remedying the problem on the human time scale.

One idea to artificially enhance the oceanic sink was the possibility of fertilizing the Southern Sea, which is believed to be iron limited. This would increase productivity, increasing carbon uptake and export to the deep. But models showed that if enough iron was added to completely deplete the major nutrients its impact compared with the magnitude of the fossil fuel source would not be that large. This in combination with other ecological impacts has led most scientists to abandon this proposal (Sarmiento, 1993). Another experiment that was recently carried out was the injection of CO2 into the deep waters as a means of disposal. This seemed plausible because of the size of the ocean, its enormous buffering capacity, and the fact that CO2 forms a solid hydrate at low temperatures and high pressures that could result in the sequestering of carbon from the geochemical cycle. Formation of this hydrate however is highly exothermic, along with volume expansion and rejection of salts this may generate local instabilities. The disposal would not be 'permanent' but it could be relatively long term (Brewer, 1999). However it must be remembered that most of the carbon that enters the deep by any mechanism will return to the surface on the time scale of THC, causing problems for future generations.

Oceanic Sink: Summary

In summary the oceans have only a limited capacity to absorb anthropogenic carbon dioxide increases which is dominated by several positive feedbacks that serve to further reduce oceanic carbon capacity as the CO2 in the atmosphere and ocean increases. These include disruptions of THC, partial or total collapse of NADW formation, increased surface temperatures, and the reduced chemical capacity of the oceans for carbon all of which serve to reduce carbon uptake. Biological changes at best will increase carbon uptake enough to partly balance some of these reductions and at worst will contribute to further reductions due to reduced surface alkalinity and buffering capacity caused by nutrient depletion in surface waters resulting from reduced ocean circulation. Some experiments such as iron fertilization and deep-sea injection of CO2 have been carried out to see if the oceanic sink can be artificially enhanced. However each method may pose various threats to the ecology of the areas, raises moral questions about our obligation to future generations, and neither seems plausible for the large-scale removal needed to offset global climate change. In time nature's own negative feedback mechanisms will come into play on a geological but not a human time frame. It seems that the only real solution to atmospheric CO2 increases is to reduce emissions.

2. The Terrestrial Sink:

Until recently tropical forests were believed to be a major source of CO2. It was thought that the destruction of the tropical rain forests released billions of tons of CO2 from rotting material, while removing possible CO2 sinks. However many cut areas are now growing back and consuming CO2 at a rate that appears to be greater than an old growth would. Consequently, the recent estimates indicating that tropical rain forests currently represent a significant net source of atmospheric CO2, may be incorrect.

Rodger Sedjo (a Washington DC scientist) maintains that as a result of the virtual elimination of forest fires and replanting programs in the US, Canada, Europe and the USSR since the 1920's, there has been a substantial re-growth of boreal(northern) forests. Increases in the forested areas are documented in 24 out of 25 European Countries. For example, Sweden has 50% more growing forests than it had 50 years ago, the forested area in New Hampshire has increased from 50 to 86%. In the last 30 years the USSR added forest areas three times the size of the British Isles. According to Auclair of the Washington DC based Science and Policy Association the missing CO2 sink correlates well with a time when the re-growth of the boreal forests was occurring. He claims that 1978 is the year that boreal forests slipped from net growth to net depletion. From 1890-1920 there was intensive cutting of our forests, and hence this was a period in which little of the fossil fuel generated CO2 was taken up by the boreal forests. The same may be true for the period since 1978. Reforesting the boreal forest may turn out to be very important for limiting CO2 increases in the future. However Auclair maintains that with the higher atmospheric CO2, and a somewhat higher temperature in our western North American forests, the amount of wood being deposited in the forests has been increasing in recent years. Recently evidence has also been found that tree roots increase in size when the carbon dioxide level increases. On the other hand, (Nature 521,520 (1993)) it has been suggested that warming northern climates could bringing about more rapid decay of the tundra, which could change a normal carbon sink to a carbon source, at least in next few decades.

What about the Mid-Latitudes?

In October, 1998 Tans (Science, Oct 15, 1998), an atmospheric chemist at the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder concluded that North America seems to be removing a substantial portion of the excess carbon in the atmosphere -- almost 2 billion tons annually. He suggested that re-growth on farmland and previously cut forests is a strong possibility for the source of the uptake. However, a recent study (Published by Daniel Markewitz and Susan Trumbore in Science, July 1,1999) showed that by the 1990s the 40-year-old trees that were planed in the nuclear testing zone in Arizona had taken up nearly 70 percent of all new CO2- derived carbon in above-ground woods and leaves while another 30 % had been accumulated below ground in the forest litter, tree roots and soil. That compares to less than 1%t of carbon retained by organically active soil matter underlying the litter. This appears to indicate that food- producing farmland probably sequesters an insignificant amount of carbon dioxide.

Sorry, I have no reference for the information on Terrestrial Sinks listed here - the notes came from a Chemistry 302 Atmospheric Chemistry Course at UBC. Other References are all listed below.

References

1. Beran, Max A. (1995). Carbon Sequestration In The Biosphere. NATO ASI Series 1: Vol 33.

2. Bonan, Gordon B, (1997). The Land Surface Climatology of the NCAR Land Surface Model Coupled to the NCAR Community Climate Model. Journal Of Climate Vol. 11, No. 6, pages 1307-1326.

3. Brewer, P.G, G. Friederich, E.T. Peltzer, and F.M. Orr Jr. (1999). Direct Experiments on Ocean Disposal of Fossil Fuel CO2. Science, 284: 943-945

4. Broecker, Wallace S. (1987). Unpleasant Surprises in the Greenhouse? Nature Vol 328. July 9, 1987. Pages 123-126.

5. Bryan, K, et al (1982) Transient Climate Response to Increasing Atmospheric Carbon Dioxide. Science Vol 215. Jan 1 1982.

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10. Libes, S.M. (1992). An Introduction To Marine Biogeochemistry. John Wiley & Sons, pgs 447-474.

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13. Sarmiento, J.L. (1993). Ocean Carbon Cycle. Chemical and Engineering News, May 31, 1993: pages 30-43.

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