GeoJournal 7.4/1983

Carbon Dioxide and Climate - A Current View Crane, A.J., Dr., Cemtral Electricity Research Laboratories, Kelvin Ave., Leatherhead, Surrey, UK Bach, W., Prof. Dr., Centre for Applied Climatology and Environmental Studies, University of M~inster, Robert-Koch-Str. 26, D-4400 MLinster, FR Germany The forthcoming publication of the proceedings of a meeting on the CO2/climate problem (Carbon dioxide: Current views and developments in energy/climate research. Proceedings on the Second Course of the International School of Climatology, Ettore Majorana Centre, Erice, Sicily, 16-26 July 1982. D. Reidel Pub. Co., Dordrecht, October 1983) will be greeted by some with a decided lack of enthusiasm. The casual browser of the popular and multidisciplinary scientific journals over the last few years will probably have gained the impression either that the climate researchers have got it all wrong and that no threat to climate exists, or that continued use of fossil fuels is bound to lead to a climatic catastrophe. It is unfortunate that the attention paid to these extreme views of what is a highly complex geophysical problem has detracted from the valuable and soundly-based research work that has been in progress over the past two decades or so, and which was evident in the lectures presented at the Erice CO 2 course in July, 1982. Admittedly, uncertainty exists, and serious deficiencies in our knowledge and lack of data leave many issues open to a wide range of interpretation. This should not, however, be taken as a licence to publicize and promote extreme views and unlikely scenarios of the future to the exclusion of more reasoned discussion. In order to maintain a balanced perception of the CO 2 issue it is essential that the various interpretations of research findings and resulting changes of research emphasis, in addition to the more concrete developments, are continually kept under review. Only by such means will it be possible to develop energy programmes which properly take the problem into account. The Erice course contributed to this review process. Thus, in addition to new research results, fresh slants on familiar topics were introduced, all within the framework of review papers designed to give a firm grounding in the wide range of interdisciplinary aspects of carbon dioxide research. Measurements of the increase in atmospheric CO 2 concentration since 1958 have shown it to be equivalent to between 50 and 60 % of the CO 2 emissions from fossil fuel burning I). The fossil fuel CO 2 that has not remained airborne must have been taken up by other natural CO 2 reservoirs, principally the oceans and the biosphere. Considerable research effort has been devoted to the apparent inability of these reservoirs to perform this task. The CO 2 increase in the ocean is at present too small to be measured reliably using present techniques so investigation of the ocean's capacity as a sink for excess CO 2 has centred on oceanuptake models. Latest developments in this field 2), presented by A. Bj6rkstr6m, continue to indicate that the oceans are capable of accommodating only 30--35 % of the fossil fuel CO 2 release rather than the 40--45 % required if they act as the only sink for the CO 2 that does not remain airborne. Whether or not the biosphere has been able to take up the remainder has been the subject of much debate in recent years, largely as a result of the controversial findings of Woodwell and colleagues 3). Instead of acting as a sink for CO2, .through enhanced photosynthesis in a CO2-rich atmosphere, as postulated by the ocean modellers, biosphere inventory studies indicate that continuing deforestation and agricultural expansion render the biosphere a small net source of excess CO 2 4)

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at present despite regrowth in previously cleared areas, it having been an even larger source around the turn of the century 5). G. Kohlmaier has drawn attention to the possibility that the diminishing of sink strength attributable to regrowth, which is likely to occur as temperate forests reach climax states, accompanied by continuing deforestation in low latitudes, will lead to an increase in the net biospheric source of CO 2. The implication is that the atmosphere is retaining a significantly smaller fraction of the total CO 2 released and the currently estimated oceanic sink falls even further short of the required capacity. Despite the possibilities of additional sinks for excess CO 2 through eutrophication of coastal waters dissolution of carbonate sediments in limited regions 7), and CO 2 stimulation of photosynthesis 8), the question remains an open one. Further development of oceanic models to incorporate more realistic representations of mixing and progress in unravelling the tree-ring 13C isotope record of past biospheric changes would appear to be the best ways forward to an improved understanding of the fossil fuel CO 2 partitioning and the possibility of predicting future atmospheric concentrations for given release scenarios. Useful estimations of future CO 2 levels depend also on reliable assessments of past levels. Reviewing recent tree ring and ice core studies, C. Lorius concludes that the formerly assumed preindustrial(~1860) CO 2 level of 290ppm should be revised downward to around 260 ppm, a value consistent with there having been a large biospheric input of CO 2 to the atmosphere around the turn of the century.

6),

The incorporation of marine biospheric processes in oceanuptake models has until recently not received much attention on the grounds that increased CO 2 concentration is unlikely to enhance rates of organic carbon sedimentation, carbon not generally being a limiting factor. A strong case for inclusion, however, has been made by C.F. Baes who argues that this enables a wealth of data on the distribution of biologically-influenced oceanic constituents to be used for model calibration, a challenge which has already been taken up 2). Perhaps more importantly, Baes has stressed the major role that ocean biology plays in controlling the near-surface dissolved inorganic carbon concentration, and hence the atmospheric CO 2 concentration. It is clearly necessary to investigate the possible effect of climatic and ocean circulation changes on marine biological production since small changes in productivity could lead to large changes in atmospheric CO 2 concentration constituting a major secondary feedback process. New insight into the global climatic effects of increased CO 2 is being obtained through the climate modelling programme of the British Meteorological Office (BMO).Their approach to the problem, as explained by Gilchrist, is fundamentally different from, but complementary to, that of the Geophysical Fluid Dynamics Laboratory (GFDL) at Princeton. Since the changes in the global heat budget are small for the increases in CO 2 envisaged, the BMO philosophy is to regard the CO2-induced climatic effects as perturbations on the present-day climate. Emphasis is thus placed on maintaining realistic temporal and spatial distributions of sea-surface temperature in their general circulation model, essential for the understanding of processes likely to influence regional climates, but at the expense of an internally consistent equilibrium climate obtained by models which employ highly simplified, but model-dependent, oceanic characteristics. In practical terms, the relatively greater importance of regional climatic change is undeniable. Interestingly, despite the fundamental differences between the BMO and GFDL models, a substantial measure of agreement exists between many of their fundamental results9), a fact which engenders confidence in the ability of general circulation models to simulate the large scale climatic changes likely to result from increased CO 2. Further intermodel comparisons of the BMO and GFDL GCMs with those developed at Oregon State University, the Goddard Institute of

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Space Studies and the National Center for Atmospheric Research, would be very useful. Although recently G(;Ms have also been used for time-dependent studies, P. Michael questions whether at present this is likely to be the most desirable approach to studying the transient response to increasing (;02, arguing that G(;Ms are not yet ready to handle the disparate relaxation timescales of the different components of the climate system. Apart from increasing our knowledge about the working of the climate system, of essential value in itself, the benefits of research on the climatic influences of increased CO 2 will ultimately be measured in terms of its ability to help decision-makers reach conclusions about whether or not any actions should be taken to lessen (;02 emissions or enable society to adapt to the consequent climatic change. For such purposes the results of the climate models need to be translated into the effects that the climatic changes are likely to have on the natural environment, agriculture and forestry, water resources, ocean fisheries, human settlements, health and well-being. In addition to the modelling results the findings from analogue studies of paleoclimatic and recent instrumental periods can be used to develop composite climate scenarios which may be very useful to impact studies. Whereas research on the scientific aspects of the (;02 problem requires cooperation between the various disciplines of the natural sciences, the study of possible effects on society of climatic change necessitates understanding between climatologists and a wide spread of social scientists. W.W. Kellogg has pioneered this breakdown of institutional barriers with some success 10), but there is clearly a great need for the involvement of more meteorologists and climatologists in impact studies, not least to ensure that the climatological 'predictions' are treated with the appropriate care. Results from recent research on the response of the cryo. sphere to climatic warming were presented by' J. Oerlemans. In contrast to the dire warnings five years ago of a 5 m sea-level rise due to the collapse of the West Antarctic ice sheet l l ) , his modelling results indicate the more immediate possibility of a slight fall in sea level caused by increased snowfall over Antarctica as a

GeoJoumal 7.4/1983

result of an enhanced hydrological cycle. The effect of large-scale removal of Arctic sea-ice on the rate of deep ocean overturning is also an open question. Oerlemans has argued for an increasing rate, which would be expected to act as a negative feedback enhancing CO 2 uptake whereas others 12) believe removal of sea-ice would lead to a more sluggish meridional circulation in the Atlantic. Mention of two further issues with regard to future CO 2 level is pertinent here. First, it should be remembered that fossil fuel burning and other human activities produce, either directly or indirectly through photochemical reactions, infrared-absorbing trace gases other than (;0 2 whose build-up could potentially increase what H. Flohn has termed the 'effective CO 2 concentration' by 50 to 100 % 13). Knowledge of the sources and sinks and chemical reactions associated with these trace gases is very limited as A. Volz pointed out and hence their future concentrations are highly uncertain. Monitoring of their global trends is clearly necessary. Secondly, continual increase in CO 2 levels into the 21st century are not inevitable. In contrast to the numerous scenarios advocating expansion in energy use Lovins et al. 14) have clearly demonstrated that if nations adopt a 'least-cost energy strategy', i.e. if they use energy in an efficient way that saves them money, then their energy needs can be expected to go down, not up, despite economic and population growth. Implementation of such a policy on a world scale would not be easy, but not impossible 15). In fact many countries already show definite signs toward a more efficient energy economy. The obvious economic benefits to be gained by both states and individuals availing themselves of already available technology for efficiency improvements in a period of underlying increases in the real price of energy are plain to see. Such an economic strategy does not only have a strong political and social appeal but it also has the added advantage of reducing substantialiy the magnitude of any potential (;O2/climate risk.

Acknowledgement AJC acknowledges the permission of the Central Electricity Generating Board to publish this paper.

Referen ces 1) Bacastow, R.B., Keeling, (;.D.: Atmospheric carbon dioxide concentration and the observed airborne fraction. In: Bolin, B. (ed.), Carbon cycle modelling (SCOPE 16), 103-112, Wiley, (;hichester 1981. 2) Bolin, B., Bjorkstrom, A., Holmen, K., Moore, B.: The simultaneous use of tracers for ocean circulation studies. Tellus, in press (1983) 3) Woodwell, G.M., Whittaker, R.H., Reiners, W.A., Likens, G.E., Delwiche, G.C., Botkin, D.B.: The biota and the world carbon budget. Science 199, 141--146 (1978) 4) Clark, W.(;., Cook, K.H., Marland~G., Weinberg, A.M., Rotty, R.M., Bell, P.R., Allison, L.I., Cooper, C.L.: The carbon dioxide question: Perspective for 1982. In: Clark, W.C. (ed.), Carbon dioxide review: 1982. Oxford University Press, Oxford 1982. 5) Wilson, A.T.: Pioneer agriculture explosion and (20 2 levels in the atmosphere. Nature 273, 4-41 (1978) 6) Walsh, I.l., Rowe, G.T., Iverson, R.L., McRoy, C.P.: Biological export of shelf carbon is a sink for the global (;02 cycle. Nature 291,196-201 (1981) 7) US Dept. of Energy: Some aspects of the role of the shallow ocean in global carbon dioxide uptake. Garrels, R.M., Mackenzie, F.T. (eds.), Workshop, Atlanta, Georgia, 20--22 March,

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1980. CO 2 Effects Research and Assessment Progam, Report 014, 1981. Lemon, E.: The land's response to more carbon dioxide. In, Andersen, N.R., Malahoff, A. (eds.), The fate of fossil fuel CO 2 in the oceans, 9 7 - 1 3 0 , Plenum Press, New York 1977. Mitchell, J.F.B.: The seasonal response of a general circulation model to changes in (;0 2 and sea surface temperatures. Quart. J. R. Meteorol. Soc. 109, 113-132 (1983) Kellogg, W.W., Schware, R.: Climate change and society. Westview Press, Boulder, Colorado 1981. Mercer, J.H.: West Antarctic ice sheet and CO 2 greenhouse effect; a threat of disaster. Nature 2 7 1 , 3 2 1 - 3 2 5 (1978) Stewart, R.W.: Possible effects of the dynamics of the ocean associated with a carbon dioxide increase. In: Workshop on environmental and societal consequences of a possible CO 2induced climate change. Annapolis, Maryland, 2--6 April, 1979. US Dept. of Energy, CO 2 Effects Research and Assessment Program, Re port 009, 148-151, 1980. Lacis, A., Hansen, J., Lee, P., Mitchell, T., Lebedeff, S.: Greenhouse effect of trace gases 1970--1980. Geophys. Res. Lett. 8, 1035--1038 (3981} kovins, A.B., Lovins, L.H., Krause, F., Bach, W.: Least cost energy: solving the (;0 2 problem. Brick House Publishing, Andover, Mass. 1981. Bach, W.: Our threatened climate. Ways of averting the (;02 problem through rational energy use. Reidel, Dordrecht 1983.