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Control of Atmospheric Carbon Dioxide Thomas J. Goreau
ABSTRACT A major part of human-induced climate change can be controlled by balancing the sources and sinks of atmospheric carbon dioxide (CO2). Natural sinks are decreasing because coral reefs, as well as rain forests, are being destroyed at an increasing rate. To balance CO2 flows it will be necessary to restore and protect both these tropical ecosystems and/or to make drastic cuts in fossil fuel use. If this is not done, the rural poor will have little choice but to destroy remaining forests and coral reefs. Stabilization of CO2 is technically feasible and cheaper than adapting to climate change. It is also extremely urgent because many coral reef ecosystems may already be near their upper temperature limits. NOTE: This paper discusses the possibilities of controlling atmospheric carbon dioxide by balancing the sources and sinks of the gas, both on land and in the sea, and the necessity of taking a complete look at the carbon cycle in order to stabilize it. The paper was published in Global Environmental Change, March1992, pages 5-11. Introduction The Intergovernmental Panel on Climate Change (IPCC) predicts that mean global temperature will rise about 3 + 1.5° Celsius if carbon dioxide (CO2) doubles in the atmosphere (1). Models that generate these predictions make provision for temperature increases derived from several different processes: direct heat absorption; heat added by greenhouse gases; and 'feedbacks'. Carbon dioxide is the major human-produced chemically active gas but climate models generally do not include temperature-induced changes in biological processes that strongly affect the total amount of CO2 in the atmosphere (2). Thus the models are limited by inadequate measurements of natural processes that constitute the major atmospheric CO2 inputs and outputs. Is global warming avoidable? espite uncertainty about magnitudes and rates of CO2 production, the greenhouse effect is very real: greenhouse gases like carbon dioxide prevent Earth from freezing (3). The addition of gases that are transparent to sunlight and opaque to thermal radiation will inevitably raise planetary temperature. Because heating of the ocean is slow, taking up to a thousand years (4), comparison of today's temperature with today's CO2 level is misleading. Total atmospheric warming is proportional to the product of gas concentration, residence time, and rate of molecular heat absorption (5). Humans change total warming by altering gas concentrations in ways that include the burning of fossil fuels. They also change the length of time gases remain in the atmosphere. For example, land management practices that decrease biological productivity reduce carbon recycling between biosphere and atmosphere, thereby increasing the atmospheric residence time of CO2 and amplifying greenhouse warming (6). Human actions have reduced primary productivity and strongly affected linked processes like respiration, decomposition, and soil erosion. These effects could be very significant in the global carbon cycle but they are poorly understood because data from tropical regions are inadequate. Warming might be amplified by several biogeochemical processes (7). Raising temperature increases plant and algal respiration faster than photosynthesis; increases animal respiration and plant consumption; and increases metabolism of bacteria and fungi that consume soil organic matter. Deforestation converts biomass and soil carbon into atmospheric CO2 via combustion and decomposition when low biomass, low-productivity land uses replace high-biomass, high productivity ecosystems. Ocean warming reduces solubility of CO2; reduces the rate at which CO2-rich polar water sinks into the deep sea; reduces nutrient upwelling needed for plankton uptake of CO2; and increases deep water acidity which dissolves marine limestone sediments. These positive feedbacks tend to amplify climate warming. Less attention has been paid to them than to possible negative feedbacks, such as increased cloudiness, that could partially cool Earth (8). The palaeoclimatic record implies that positive feedbacks could be very important. Antarctic ice cores show temperature has been 11 times more sensitive to CO2 over the last 160 000 years than is predicted for the future by climate models that do not take account of biological feedbacks (9). Fossils that are 130 000 years old lived when it was 2° Celsius warmer than now, sea levels were around 7 m higher than today, and crocodiles and hippopotamuses occupied the site of modern London. At that time the atmosphere had 27% less CO2 than today. We might eventually face larger changes than predicted because these ancient environmental conditions are underestimates of the full climatic impacts of excess CO2 that is already present in the atmosphere (10). All organisms possess physiological, genetic, and evolutionary adaptations to change. They are the secret of survival in the face of varying environments. Because life adapts to unpredictable changes in complex ways, our planetary ecosystem may not possess any optimal state. Life might continue if the atmosphere became much hotter, but individual species might be unable to adapt. Dinosaurs and cockroaches flourished when Earth had a hypertropical climate and oceans were 10° to 15° warmer than at present. Could we? The next few decades could produce climates beyond the range our species has experienced (11). Despite the fact that the human experience is dominated by Ice Age climates with temperatures that were up to 10° celsius lower than today, we should be biologically capable of limited adaptation to warmer climates - unless diseases and parasites evolve more quickly. The socioeconomic difficulties of adapting to climate change may be more intractable. For example, agriculture, cities and transportation infrastructure developed in climates that were cooler than those that are predicted. Environmental changes associated with a warmer Earth would cause spending for protection, modification or relocation of existing societal systems to reach billions o' dollars (12). Present climates are preferred because changes would mean the loss of many investments in non-portable structures. The poor of developing countries would also lose if forced to relocate. The cost of socioeconomic adaptations is highly uncertain, but it is probably vast enough to jeopardize economic development (13). Atmospheric carbon dioxide supply and demand When applied to greenhouse warming the 'polluter pays' principle suggests that producers of carbon dioxide should pay damages or clean-up costs (14). All major pollution-generated, climatically active gases - except CO2 - are removed mainly by physical and chemical processes in the upper atmosphere. Their removal rates are indirectly affected by human activities, but not controlled by them. The only way to alleviate the long-term environmental impact of these gases is to reduce emissions at their sources, and wait for the excess to dissipate over a period that might - in the case of gases like nitrous oxide and some chlorofluorocarbons - be several centuries long. Carbon dioxide is unique among climatically active gases because its major sinks are natural biological processes that are directly affected by human management of land and oceans. Ecosystem degradation reduces the biospheric capacity to remove and store CO2, but concentrations can be stabilized either by reducing emissions (supply side measures) or by increasing removal (demand side measures) (15). Human effects on carbon cycling no longer can be ignored in economic prices or environmental policy. The process of CO2 exchange links atmosphere, biosphere, hydrosphere, and geosphere. Carbon dioxide represents the essential raw material of life, and it is the major indicator of energy use by contemporary technologies. In other words, CO2 is a convertible currency for economic, energy, ecological, and climate systems. All major inputs, outputs, and storages need to be known at least as well as fossil-fuel use if CO2 is to be properly valued (16). Approaches that fail to recognize the complex exchange of CO2 are likely to attack isolated symptoms of global warming and to have only marginal impacts on causes. The entire problematique should be tackled in a coherent fashion so that future development may be optimal, as well as sustainable (17). The sustainable living standard that is ultimately achievable depends on whether carbon can be stored for the generation of renewable products, just as it depends on whether we choose to reinvest income or to consume it. Over a quarter of atmospheric CO2 is added or removed every year. Management alternatives for affecting CO2 are compared in Figure 1. Major sources and sinks are shown according to the percentage of atmospheric CO2 they process annually. Photosynthesis, decomposition, and respiration figures are given for terrestrial processes; marine ones are included in ocean-atmosphere exchanges. Only fossil-fuel combustion is now accurately measured. There is greater uncertainty about major sources and sinks than about fossil-fuel combustion. But they must be known at least as accurately as combustion before complete input-output accounts can be constructed. Minor sources of CO2, such as volcanic degassing, and minor sinks, such as formation of soil humus, chemical weathering of rocks, and burial in sediments, are too small to be shown at this scale. Moreover the minor sources of change would have to proceed for thousands to millions of years before they contributed appreciable concentrations of CO2. Figure 1. Supply and demand of atmospheric carbon dioxide expressed as a percentage of the gas cycled each year to or from the atmosphere by each process. Note: Supply-sources which increase CO2 have positive values, while demand-sinks which remove it have negative values. Sources: Data calculated using data from C.D. Keeling, 'The global carbon cycle What we know, and could know from atmospheric biospheric, and oceanic observations', in Institute for Energy Analysis, Proceedings: Carbon Dioxide Research Conference: Carbon Dioxide, Science and Consensus, US Department of Energy CONF-820970, Vol 2, 1983, pp 3-62; A. Solomon, J.R. Trabalka, D.E. Reichle and L.D. Voorhees, 'The global cycle of carbon', in J.R. Trabalka, ed, Carbon Dioxide and the Global Carbon Cycle, US Department of Energy, Washington, DC, 1985, pp 3-24; and P. Crutzen and M. Andreae, 'Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles', Science, Vol 250, 1990, pp 1669-1678. For CO2 to be stable, sinks and sources must balance. At present they do not - supply exceeds demand by about 0.36% of atmospheric CO2 per year. If attention is focused on fossil fuels alone, a very large decrease in use - around 60% - is necessary to eliminate the existing imbalance. But relatively small changes in biological or marine sources and sinks would accomplish the same goal. What are the prospects for affecting this part of the equation? Nearshore sediment burial accounts for half of marine carbon removal (18). This proportion might be growing because coastal plankton populations are increasingly being stimulated by nutrients derived from inefficient agricultural practices and inadequate sewage treatment, and because sedimentation of eroded soils has increased in coastal zones. Marine carbon removal cannot easily be controlled - transfer of CO2 across the sea surface is dominated by winds and waves which are beyond human control. Ocean biota store little carbon and rapidly recycle most of it back to the atmosphere. In any case, ocean fertilization has unpleasant side effects - biogeochemical, ecological, aesthetic and economic. These include massive blooms of algae and consumption of oxygen from water accompanied by the rotten-egg stink of hydrogen sulphide. Finally, marine carbon burial is much less efficient than land burial and it could reduce production of many economically valuable species of fish and shellfish. On the other hand, terrestrial ecosystems may be better candidates for manipulation of CO2. Compared to the oceans, they cycle more carbon to the atmosphere, are concentrated in smaller areas, and are readily affected by human activities. Terrestrial ecosystems are favourable storage sites for carbon because large areas have been deforested, degraded, and abandoned, or might benefit from reforestation for other reasons (19). If research leads to the development of economically useful tropical crops, reforestation and forest management could provide large savings by deferring future expenditures on soil, water and nutrient conservation, while generating jobs and industries that are based on renewable sources of foods fibres fuels and chemical raw materials (20). Can we outgrow carbon dioxide build-up? A major part of global warming could be averted if the addition of CO2 to the atmosphere were accompanied by equal removal. Emitters could pay for planting, growing, and sustaining enough biomass and storage to remove excess carbon permanently (21). It would require about 500 billion trees that consume 10 tonnes of carbon per ha per year, on 4 million km2 of land, to ensure removal of the current carbon increase (ie 3.5 billion tonnes of carbon per year) (22). The area could be as low as 2 million km2 in the humid tropics, if reforestation stopped, or as high as 8 million km2 if areas with long dry seasons or colder temperatures were planted. Tropical reforestation costs are around $400 per ha: therefore halting CO2 build-up by reforestation - and no deforestation - would cost around $80 billion (23). This estimate includes seedlings and transport, but omits other costs, which include land, labor, fertilizer, and water; and funds for research to select fast-growing, high-quality plants, to identify limiting nutrients, and to develop renewable products and raw materials from tropical plants. If these costs are added, the estimate might be doubled (24). Trees continue to remove excess carbon for around 40 years before they reach maturity. If planting took 10 years, costs would be about $16 billion per year or $3 per ton of carbon. Assuming that the costs are allocated to fossil-fuel users on a proportionate-use basis, in the form of carbon removal taxes or user fees, average annual per-person payments in the USA would be about $15. In Jamaica they would be around $2 and less in Brazil, India or China. Such sums are small fractions of existing per capita fuel taxes. To achieve intergovernmental equity, it would be necessary to allocate costs on the basis of historical emissions, not existing rates. Larger payments will also be needed if the goal is to move beyond balancing CO2 sources and sinks to reducing excess CO2 that is already 'in the pipeline' as a result of previous imbalances. Since there is adequate land available to absorb CO2 build-up for several generations, it is both technically feasible and economically affordable to stabilize CO2 and avert costs of unrestrained warming (25). Alternative technological fixes have been proposed. They include: shooting CO2 into space or the deep oceans, or into Earth's interior; transporting large amounts of dust to the atmosphere by plane or artillery shells in order to reflect light to space; and the use of high-altitude reflecting balloons or mirrors in orbit. These alternatives cost up to hundreds of times more than reforestation. Demand reduction of fossil-fuel use by means of high taxation might require tax rates of several hundred dollars per ton of carbon (26). Fuel costs would rise and standards of living would fall. Indeed, it is important to differentiate the taxation of fuels for purposes of paying the costs of biological removal from the far more expensive 'carbon taxes' that would prohibitively raise energy costs and discourage consumption, or energy taxes whose goals are unrelated to pollution abatement (eg revenue generation, debt relief, etc). The most effective way to reduce CO2 is not to emit it. Hence, it is essential to include improvements in the efficiency of energy use and non-fossil-fuel sources (27). Human population also might be limited to prevent energy, food, and resource demands from increasing (28). The sooner these steps are taken, the lower will be the total costs, the tax rate on fuels, and the land area required. The last mentioned is an important constraint: reforestation of already degraded land is capable of removing global CO2 build-up from the atmosphere only until available lands are forested. In other words, this strategy buys time to allow the replacement of fossil fuels by renewable sources of energy (29). Climate stabilization and Third World development The sustainable development of Third World rural populations is the sine qua non for success in stabilizing atmospheric CO2. If new trees are to survive, living standards in developing countries must be raised to levels that do not force deforestation in pursuit of fuel, food, and cash crops. This requires the generation of new jobs in landscape restoration, in sustainably managed harvesting, and in processing biologically derived products, as well as massive investments in research and education, and biological productivity monitoring (30). Such efforts provide many benefits besides carbon removal (31). Increased biological productivity would retain soil, water, and nutrients held by soil organic matter, thereby reducing land degradation and increasing future biological production, especially on fragile tropical soils. Greatly expanded research on the restoration of ecosystems and soils will be essential. Tropical research institutions should systematically apply the most modern scientific methods (such as molecular biology techniques) while saving or relearning traditional knowledge of forest peoples which is being rapidly lost. Productivity and carbon storage must be directly monitored worldwide, with a precision as great as our knowledge of fossil-fuel consumption. The problem is global: so is the solution. Short-sighted approaches to 'development' have already laid waste many renewable resources which we will need to restore. National socio-economic policies that reward deforestation should be replaced by incentives for reforestation, especially by small farmers on hill slopes. Educational reforms should focus on teaching children, from the earliest ages, about the natural history of plants, animals, soil, water, and air around them, and about environmental conditions or human actions which benefit or damage these ecosystems. Land reform, population stabilization, international peace, and common global purpose will be needed. Since environmental stabilization and global development are inseparable, the United Nations Conference on Environment and Development (Rio de Janeiro, June 1992) is an obvious forum for such efforts. Ocean warming and coral reefs How much time remains to allow for protecting natural resources before degradation becomes excessive? Mass coral bleaching episodes, which turn corals white, are a novel phenomenon of the past decade (32). Bleaching events, which are increasing in severity and extent across the tropical oceans, have starved corals and halted growth and reproduction for up to 10 months at a time. Sea-surface temperature records in the Caribbean show that bleaching has accompanied all of the high temperature extremes during the past decade, and that there has been a significant rise in temperature at most sites during that period (33). No other known stress explains the observed pattern. The 1990 Caribbean bleaching event caused widespread mortality and overlapped bleaching on the Pacific and Indian coasts of Australia (34). The 1991 event, the fourth of the last five years in the Caribbean, coincided with severe bleaching in Polynesia and other sites in the Pacific and Indian Oceans (35). Regional warming trends are not themselves proof of global warming, but increased bleaching over the last decade suggests that corals are vulnerable to any regional warming that may occur. Coral reefs have been subject to increasing human-induced stresses before the onset of bleaching (36). Coral reef ecosystems now show clear signs that they may be the first major tropical ecosystem to be severely damaged by global warming (37). Environmental monitoring and research support have not been adequate to assess the extent or pace of reef degradation. Most corals are now recognized to have been living just below their upper temperature tolerance limits. Only a small amount of warming is required to push them beyond their capability for resilience. Because of this extreme sensitivity, many Atlantic and Pacific reefs might be severely damaged in the next few years if sea-water temperatures continue to reach levels of the past several years. Even if temperature-resistant corals could be found, grown, and transplanted, several centuries would elapse before all those that are now threatened could be replaced. Moreover, they will not survive unless sea-level rise is slowed, coastal sewage is treated, and watersheds are reforested. Coral reef ecosystems are the most species-rich and productive in the oceans, and of major economic value to fisheries, tourism, and shore protection. A rapid halt to global warming and environmental degradation is essential if severe damage is to be prevented. Like the Antarctic ozone hole, mass coral bleaching was an unanticipated phenomenon which indicated that global climate change is more unpredictable, extensive, and expensive - and corrective action more urgent - than originally thought. If climate change is not halted, many coral reefs may be lost, together with the species and people that depend on them. The time left to save them is very brief, and failure to act on a global scale may condemn many of these ecosystems within a few years. If bleaching continues, tropical countries may face serious losses to fisheries, shore line protection, and tourism revenues. No further rise in tropical ocean temperatures is now acceptable, unless society is prepared to sacrifice this marine ecosystem (38). It is therefore crucial that the 1992 UN Conference on Environment and Development becomes an opportunity for action to halt climate change by stabilizing climatically active gas concentrations, not just fossil-fuel emissions. It is necessary to tackle seriously immediate and subsequent steps that will solve the interlinked problems of global change to which our descendants have been made hostage. The tendency to maximize immediate returns rather than long-term ones has been termed 'evolution's fatal flaw' (39) and it is the short-sighted pseudo-ethical justification that fuels many environmental, social, and economic ills. If we wish to safeguard our children's future, positive action, or 'geotherapy' is needed on a global scale to correct tendencies which have become increasingly maladaptive in a world with limits (40). REFERENCES: 1. J T. Houghton, C.J. Jenkins and J.J. Ephraums, eds, Climate Change: The IPCC Scientific Assessment, Report of the United Nations Environmental Programme, Intergovernmental Panel on Climate Change (IPCC), Cambridge University Press, Cambridge and New York, 1990. 2. D. Lashof, 'The dynamic greenhouse: feedback processes that may influence future concentrations of atmospheric trace gases and climatic change', Climatic Change, Vol 14, 1989, pp 21~242. 3. J.S. Lewis and R.G. Prinn, Planets and Their Atmospheres: Origin and Evolution, Academic Press, New York, 1984. 4. H. Craig, 'The natural distribution of radiocarbon and the exchange time of carbon dioxide between atmosphere and sea', Tellus, Vol 9, 1957, pp 1-17; B. Bolin, A. Bjorkstrom and B. Moore, 'Uptake by the Atlantic Ocean of excess atmospheric carbon dioxide and radiocarbon', in A. Berger, R.E. Dickinson and J.W. Kidson, eds, Understanding Climate Change, Monograph 52, American Geophysical Union, Washington, DC, 1989, pp 57-78. 5. R.A. Goody, Atmospheric Radiation 1: Theoretical Basis, Oxford University Press, Oxford, 1964. 6. T.J. Goreau and W.Z. De Mello, 'Tropical deforestation: some effects on atmospheric chemistry', Ambio, Vol 17, 1988, pp 275-281. 7. T.J. Goreau, 'Balancing atmospheric carbon dioxide', Ambio, Vol 19, 1990, pp 23~236; Lashof, op cit, Ref 2; J. Hansen and T. Takahashi, eds, Climate Processes and Climate Sensitivity, Monograph 39, American Geophysical Union, Washington, DC. 8. V. Ramanathan and W. Collins, 'Thermodynamic regulation of ocean warming by cirrus clouds deduced from observations of the 1987 El Nino', Nature, Vol 351, 1991, pp 27-32; A.J. Heymsfield and L.M. Miloshevich, 'Limit to greenhouse warming?', Nature, Vol 351, 1991, pp 14-15.
12. S.H. Schneider, 'The greenhouse effect: science and policy', Science, Vol 243, 1989, pp 771-781. 13. Ibid. 14. T.J. Goreau, 'The other half of the global carbon dioxide problem', Nature, Vol 328, 1987, pp 581 - 582. 15. Goreau, op cit, Ref 7; T.J. Goreau, 'A call for a tropical research action plan for sustainable Third World development and a stable global environment', in M. Bernard, ed, Energy Systems, Environment, and Development: A Reader, ATAS Bulletin No 6, United Nations, New York, 1991. 16. Goreau, op cit, Ref 7; T.J. Goreau, 'An analytical framework for environmentally sound technology assessment', in Environmentally Sound Technology Assessment, Vol ATAS Vll, United Nations Centre for Science and Technology for Development, New York, 1991.
20. lbid; T.J. Goreau, 'Technological options which minimize the loss of biological diversity', in Environmentally Sound Technology Assessment, Vol ATAS Vll, United Nations Centre for Science and Technology for Development, New York, 1991. 21. Goreau, op cit, Ref 14. 22. Myers and Goreau, op cit, Ref 19. 23. Ibid.
25. Myers and Goreau, op cit, Ref 19.26. W.D. Nordhaus, 'Economic growth and climate: the carbon dioxide problem', Journal of the American Economic Association, Vol 67, 1977, pp 341-346; J.M. Epstein and R. Gupta, Controlling the Greenhouse Effect: Five Global Regimes Compared, Brookings Institution Occasional Paper, Brookings Institution, Washington, DC, 1990, pp 1-40; M. Grubb, The Greenhouse Effect: Negotiating Targets, Royal Institute of International Affairs, London, 1989, pp 1-56. 27. Houghton et al, op cit, Ref 1; Schneider, op cit, Ref 12.
29. Myers and Goreau, op cit, Ref 19. 30. Goreau, op cit, Ref 15; Goreau, op cit, Ref 20. 31. Myers and Goreau, op cit, Ref 19 32. E.H. Williams and L. Bunkley-Williams, 'The world-wide coral reef bleaching cycle and related sources of coral mortality', Atoll Research Bulletin, Vol 335, 1990, pp 1 - 71 . 33. T.J. Goreau. R.L. Hayes, J.W. Clark, D.J. Basta and C.N. Robertson, 'Elevated satellite sea surface temperatures correlate with Caribbean coral reef bleaching', in R.A. Geyer, ed, A Global Warming Forum: Scientific, Economic, and Legal Overview, CRC Press, Boca Raton, FL, in press. 34. E. Williams, personal communication. 35. B. Salvat, 'Bleaching and mortality of scleractinian corals, events and causes, Society Islands, 1991, Intemational Society for Reef Studies Annual Symposium, International Society for Reef Studies, Berkeley, 1991, in press. 36. B. Salvat, 'Coral reefs and human societies', Global Environmental Change, Vol 2, No 1, 1992, pp 12-18. 37. R.L. Hayes and T.J. Goreau, 'The tropical coral reef ecosystem as a harbinger of global warming', Proceedings of the 2nd International Conference on Global Warming, World Resources Review, Vol 3, 1991 , pp 306-322; P.W. Glynn, 'Coral reef bleaching in the 1980s and possible connections with global warming', Trends in Ecology and Evolution, Vol 6, 1991, pp 17~179. 38. Ibid. 39. V.R. Potter, 'Getting to the year 3000: can global bioethics overcome evolution's fatal flaw?', Perspectives in Biology and Medicine, Vol 34, 1990, pp 89-98. 40. R. Grantham, Conference report of the Lyon CNRS Colloque on Modelling and Geotherapy for Global Changes, Global Environmental Change, Vol 2, No 1, 1992, pp 66-67. |
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