Documentation to "My Case"

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JOSEPH STERNBERG, Emeritus, Naval Postgraduate School, Monterey, California.
Eos, Vol. 87, No. 48, 28 November 2006

Current concern is focused on the consequences of global warming as the atmospheric level of carbon dioxide (CO2) increases. But adding CO2 to the atmosphere has some potential benefits. As considered in this paper, published research supports the view that a moderate increase in the atmospheric CO2 level could prevent the initiation of another ice age. The amount of CO2 resulting from the use of the world’s fossil fuel reserves should be sufficient to provide this protection for thousands of years.

According to geochemical [Berner, 1991] and other studies, atmospheric levels of CO2 100 million years ago were several times as large as the pre-industrial level of 280 parts per million by volume (ppmv). The primary mechanism for removing CO2 from the atmosphere-ocean system has been the weathering of silicate rocks. Thus, calcium silicate reacts with water and CO2 to produce calcium and bicarbonate ions, which then flow into the sea. Sedimentation in the sea, forming calcium carbonate (CaCO3) from these ions, releases only half of the CO2 that was consumed by the weatherization, thus reducing the CO2 in the atmosphere-ocean system. CO2 is returned to the atmosphere-ocean system from volcanic outgassing and at mid-ocean ridges where tectonic plate spreading occurs. The net result of these mechanisms has been a falling level of CO2 and a cooling of the Earth.

Over geologic time, the land-sea distribution and changing solar luminosity are also believed to have been important in the evolution of the Earth’s climate. But the low level of CO2 is considered by many to be the key factor in the onset of ice ages about 2.5 million years ago [Crowley and Berner, 2001].

Computer modeling studies of the variation of the Northern Hemisphere ice sheet volume during the most recent ice age support the key role of CO2. Two forcing functions were used in these studies: the orbital induced insolation variations and the CO2 levels [Loutre and Berger, 2000]. They found that an increase in the CO2 level would prevent the initiation of an ice age from an interglacial period, with the required level depending on the amplitude of the insolation variations.

It is believed that between 2.5 and 10 million years ago, there was extensive ice cover over Antarctica and other polar regions but not the massive hemispheric ice sheets that have characterized the ice ages. But what CO2 levels existed during this period? A detailed evaluation of methods for estimating paleoatmospheric CO2 levels [Royer et al., 2001], including the geochemical model and four proxies for the pressure of atmospheric CO2, concluded that the highest precision for this period is provided by the measurement of the stomatal index of fossil deciduous oak leaves. The stomata are the tiny pores in leaves that permit the passage of atmospheric CO2.

The data showed CO2 levels for this period varying between 280 and 370 ppmv. This suggests that a CO2 level of around 400 ppmv or higher could provide enough greenhouse effect to preclude the initiation of another ice age.

This level is much lower than the atmospheric levels of CO2 in current forecasts for the next several hundred years. For example, in only six years, the atmospheric level of CO2 is expected to reach 400 ppmv.

Land use changes involving deforestation in the equatorial regions have been a measurable factor in accounting for the growth of atmospheric CO2. However, this should not be of long range significance compared with the use of fossil fuels. Fossil fuel reserves are estimated to reach at least 5000 gigatons of carbon. An extensive model to simulate the long-term response of the ocean-atmosphere system to fossil fuel CO2 emissions [Archer et al., 1998] was applied to the case where fossil fuel reserves are fully exploited in the next few hundred years, as currently forecasted. The atmospheric level of CO2 may reach many times the preindustrial level before absorption of a major portion of the emissions in the sea.

A large addition of CO2 to the ocean reduces the concentration of the carbonate ion decreasing the depth of the lysocline, the line below which the water is undersaturated with CaCO3. As a result, large bottom areas containing CaCO3 sediments previously deposited are subject to dissolution. The dissolution of these deposits by combining with CO2 slowly neutralizes CO2 in the ocean, and CO2 flows from the atmosphere into the ocean to restore equilibrium between the atmosphere and the ocean.

Archer et al. [1998] found that close to all of the available CaCO3 sediment inventory (estimated at 1570 gigatons C) would be effective in neutralizing atmospheric CO2 if fossil reserves of as much as 4550 gigatons C were used. More of the CO2 is neutralized during the slow process of balancing the flux to the sea from the terrestrial weathering of CaCO3 rocks and the burial rate of CaCO3 in the sea, a process that could take up to 50,000 years. Nevertheless, the final equilibrium CO2 levels in the atmosphere were found to be close to 500 ppmv above the 400 ppmv level that should preclude the inception of another ice age. Comparable results for CO2 levels after exhausting fossil fuel reserves were obtained in an earlier work [Sundquist, 1990].

Two factors are not included in the above model. Changes in the phytoplankton in the surface mixed layer brought about by global warming and ocean acidification could affect the level of CO2 in the atmosphere. Phytoplankton photosynthesis promotes the transfer of atmospheric CO2 to the sea by lowering the partial pressure of CO2 in the upper ocean. It has been estimated that the atmospheric concentration of CO2 is 150–200 ppmv lower than it would be if all of the phytoplankton in the ocean were to die [Falkowski et al., 2000]. At the present state of knowledge, though, possible changes in the phytoplankton are not predictable.

Second, the eventual removal of the pulse of CO2 in the ocean-atmosphere system by the slow weatherization of silicate rocks is not part of the simulation. Using the estimate for the magnitude of silicate rock weathering from the geochemical analysis [Berner, 1991], it would take some 80,000 years to reduce the atmospheric level of CO2 from 500 to 400 ppmv. So, enhanced CO2 levels could well be expected to persist in the atmosphere for a long time.

Another point that bears consideration is that it may be possible to maintain a CO2 level above 400 ppmv for much longer periods by using the combustion of methane to add CO2 to the atmosphere. According to the analysis of weatherization [Berner, 1991], the rate of weatherization increases as the CO2 level rises above the preindustrial equilibrium level of 280 ppmv. This increase in the loss of CO2 by silicate rock weathering could be compensated for by the addition to the atmosphere of 1.2 × 10-2 gigatons of carbon per year, about 0.002 of the current CO2 emission rate. Recently, it has been recognized that methane in the form of a hydrate occurs in the deep permafrost in Arctic environments and in the uppermost few hundred meters of sediments in continental margins, possibly exceeding the total of all other known hydrocarbon sources. If the estimate of the magnitude of this resource proves correct, the use of this fuel could maintain CO2 levels above 400 ppmv in the atmosphere for as long as 400,000 years.

In the near term (hundreds of years), a major reduction in the forecast rate of CO2 emissions into the atmosphere would be required to avoid exceeding the preindustrial level of CO2 (280 ppmv) by many times more than a factor of two. There are two possible near-term alternatives for substantial reductions in the rate of atmospheric CO2 emissions. One alternative would be an early shift to the increased world use of nuclear power. The other alternative is the capture of CO2 at large point sources such as power plants and industrial facilities [Intergovernmental Panel on Climate Change, 2005]. Until now, CO2 capture from process streams has only been used for specialized purposes. Increased technological development is needed in this area to reduce cost and the overall environmental impact. In the United States, large point sources represent over 50% of CO2 emissions. That leaves the question of what to do with the CO2 that has been captured.

A number of programs are looking at the possibilities for geological sequestration of captured CO2, but that would remove the CO2 from the ocean-atmosphere system, reducing the long-term level of CO2 in the atmosphere. Various studies have indicated that deep-sea injection of CO2 could keep the CO2 in the sea long enough to reduce the near-term peak in atmospheric CO2. Much more experimentation is needed to answer important questions about the local effects of deep-sea injections of CO2 in different forms. CO2 capture and injection in the deep sea may make it possible to limit the near-term rise in the atmospheric CO2 level while still increasing worldwide energy use.

Urgent steps are needed to limit the rapid growth of CO2 in the atmosphere, which can be expected to have serious consequences such as an increase in the seal level. But preventing another ice age is also an important objective. It may well be that the intense worldwide exploitation of fossil fuel resources came just in time to provide a way to warm the Earth enough to delay, for a long time, the occurrence of another ice age.

References
Archer, D., H. Kheshgi, and E. Maier-Reimer (1998), Dynamics of fossil fuel CO2 neutralization by marine CaCO3, Global Biogeochem. Cycles, 12(2), 259–276.
Berner, R. A. (1991), A model for atmospheric CO2 over Phanerozoic time, Am. J. Sci., 291, 339–376.
Crowley, T. J., and R. A. Berner (2001), CO2 and climate change, Science, 292, 870–872.
Falkowski, P., et al. (2000), The global carbon cycle: A test of our knowledge of Earth as a system, Science, 290, 291–296.
Intergovernmental Panel on Climate Change (2005), IPCC Special Report on Carbon Dioxide Capture and Storage, edited by B. Metz et al., Cambridge Univ. Press, New York. (Available at www.ipcc.ch)
Loutre, M. F., and A. Berger (2000), Future climate changes: Are we entering an exceptionally long interglacial?, Clim. Change, 46, 61–90.
Royer, D. L., R. A. Berner, and D. J. Beerling (2001), Phanerozoic atmospheric CO2 change: Evaluating geochemical and paleobiological approaches, Earth Sci. Rev., 54, 349–392.
Sundquist, E. T. (1990), Long-term aspects of future atmospheric CO2 and sea-level changes, in Sea- Level Change, edited by R. Revelle, pp. 193–207, Natl. Acad. Press, Washington, D. C.