Policy Implications of the Missing Global Carbon Sink

Policy Implications of the Missing Global Carbon SinkAuthor(s): W. Neil Adger and Katrina BrownSource: Area, Vol. 27, No. 4 (Dec., 1995), pp. 311-317Published by: The Royal Geographical Society (with the Institute of British Geographers)Stable URL: http://www.jstor.org/stable/20003601 .

Accessed: 17/06/2014 17:39

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The Royal Geographical Society (with the Institute of British Geographers) is collaborating with JSTOR todigitize, preserve and extend access to Area.

http://www.jstor.org

This content downloaded from 194.29.185.109 on Tue, 17 Jun 2014 17:39:03 PMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=rgs

http://www.jstor.org/stable/20003601?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


Area (1995) 27.4, 311-317

Policy implications of the missing global carbon sink

W Neil Adger, Centre for Social and Economic Research on the Global Environment University of East Anglia, Norwich NR4 7TJ and University College London, and Katrina Brown, School of Development Studies and CSERGE, University of East Anglia, Norwich NR4 7TJ

Summary Existing models of the global biogeochemical carbon cycle do not balance, so calibration of the models for unaccounted increases in carbon in the atmosphere, on land, or in the oceans is necessary. In the current parlance this is the 'search 'for the ' missing carbon sink '. The location of the sink has variously been hypothesised to be in forests, soils and oceans. We highlights the policy implications of this area of scientific uncertainty in the global warming debate, and informs scientists on the limits of perceiving the terrestrial biosphere as simply a sink or source of carbon.

Introduction

Human intervention in the global biogeochemical cycle of carbon is presently leading to increased atmospheric concentrations of gases thought to enhance the heat trapping capacity of the atmosphere. Although global warming per se is not presently detectable, the overwhelming body of scientific evidence points to future climatic changes with impacts on human and physical systems which are uncertain but in the

main negative (Houghton et al 1992; Warrick et al 1993). Present understanding of the trends in the mechanism driving the system, the

global carbon cycle, is low. Specifically the models of how much carbon is stored in the oceans, in vegetation and soils, do not tally with observations of increases in carbon in the atmosphere. These discrepancies have been defined as a ' missing carbon sink '. The search for the missing sink is an analogy for the activities involved. The search is in effect one for models which better represent the observed reality of different parts of the biosphere. Estimates of the sources of carbon dioxide in the atmosphere (fossil fuel use, land use change) exceed estimates of the sinks (soil, vegetation, the oceans and the atmosphere itself) by up to 3 billion tonnes of carbon per year. The discrepancy in the main is related to the two distinct modelling approaches of the ocean/atmosphere system and of modelling land use change.

Interventions and policies to reduce future potential negative impacts of climate change necessarily focus on energy, industry and land use related emissions of the so-called greenhouse gases and on strategies to enhance the sinks of carbon in oceans or on land. The objective of these policies is stabilisation of net atmospheric concentrations. However, the missing sink in the global cycle, which is hypothesised to exist in various parts of the terrestrial biosphere, distorts the policy prescriptions for mitigating climate change which involve land use. Land use activities make a significant contribution to the enhanced greenhouse effect, the mechanism by which climate change is being driven, through adding to the atmospheric concentrations of greenhouse gases. There are large uncertainties associated with the present land related sources and sinks of greenhouse gases, and how these will change in the future. It has been argued that till recent human history, the sources and sinks in the global carbon cycle were essentially in steady state, with sources equating sinks. In



312 Adger and Brown

the last century or so however, the use of fossil fuels as an energy source as well as land use change have seriously disturbed this cycle, leading to increased atmospheric concentrations of carbon and hence to the enhanced greenhouse effect (Houghton and Skole 1990). As the ocean sink for carbon is now thought to be less than previously estimated (Tans et al 1990) a missing sink of carbon exists in present understanding of the global cyclical process. This means that analysis of the causes and impacts of land use change have an added imperative.

Global warming mitigation policies can be delineated into energy related policies concentrating on reducing hydrocarbon sources of emissions; and land use related policies to reduce emissions through reducing present trends of land conversion and through enhancing sinks. These latter land use related strategies are in effect bioengineering mitigation strategies and are most critically affected in terms of feasibility and cost by the non-balancing of the models of the global carbon cycle.

Underlying the policy questions related to bioengineering is the fundamental issue of whether planetary scale management of the global atmosphere is an appropriate response to the global warming threat. The causes and impacts of global warming are highly uncertain and further technological fixes to such a global problem, when previous technologies have caused the problem, have to be seriously questioned. Concern over the potential impacts of global warming should not be interpreted as ' a mandate for multiple adjustments guided by an expansion in data collection, a more sophisticated use of Western science, more controls on technology, better institu tional design, and appeals to existing values ' (Norgaard 1994, 12). This paper explains the missing sink and discusses the implications for policy, particularly relating to land use emissions.

Carbon dioxide and the missing sink

Carbon dioxide is currently increasing at 0 5 percent per annum in the atmosphere, and now constitutes approximately 357 parts per million by volume (ppmv) compared to 280 ppmv in pre-industrial times (Houghton et al 1992). Estimates of the global biomass pool range from 550-830 billion tonnes of carbon (bt C) (see

Bouwman 1990), with the other major pools being the atmosphere, oceans and fossil fuel reserves. The interactions of these pools are shown schematically in Figure 1.

The estimates of emissions from natural sources are however, subject to great variation caused mainly by uncertainties in estimates of emissions from tropical deforestation and land conversion.

Approximately 40 percent of anthropogenic emissions, the flux figures shown in Figure 1, has remained in the atmosphere in the long term. On a yearly basis at present, the estimates presented by the Intergovernmental Panel on Climate Change (IPCC), derived from observational and model data, suggest a net imbalance in the budget of 1-6 bt C per year in the 1980s; the present rate of increase of the atmospheric pool of CO2 is less than half of that expected given the estimates of the magnitude of the major sources and sinks. There appears, then, to be a missing carbon sink which has been variously postulated to be in temperate forests, grasslands, soils or in the oceanic carbon pool (Tans et al 1990; Sedjo 1992; Fisher et al 1994).

Potential location of the missing sink The search for the location of this sink is an attempt to better represent observations of the biosphere through models of the carbon cycle. One possible resolution of the missing sink problem is suggested by Tans et al (1990). They model the terrestrial



Missing global carbon sink 313

0C

0

0~~~~~~ .3

0~~~~~~~

0 CD0

0

0. _

AC 0 C

0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

o ~~~~~~~CD OC O

~~~~ 0 ~ ~ ~ 0W

00CD

U~~~~~) 0,..C -,~~~~~~~~~~~~~~O)C

0.~~~~~~~~

U)~ ~ ~ ~~~~~~~U

...E S~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~o: 0.~~~~~~~~~~~



314 Adger and Brown

and oceanic carbon sinks by latitude, based on the observed north-south atmospheric CO2 concentration gradient. They conclude that terrestrial ecosystems are a considerably greater sink than the oceans and that there must be an extra terrestrial sink located in temperate latitudes to balance the carbon budget. The sink estimated by Tans et al (1990) is in the range of 2 0-3 4 bt C per year, greater than that of the IPCC assessment. Sedjo (1992) presents evidence which suggests that this temperate carbon sink is almost certainly in forests. Since forests contain possibly 85 percent of the above ground biomass carbon changes, in forest stock will have a large influence on the carbon pool. This is recognised in the context of tropical forests where deforestation is the principal source of emissions.

However, Sedjo's contribution has been to show, from cross-country forest assessments, that the land area under forest and the forest stock of biomass has been expanding in the northern hemisphere. In some areas of the northern hemisphere this has been occurring for over a century, but the bulk of the expansion has been concentrated in the past several decades, through both afforestation programmes and through reforestation by natural regression. This proposition is supported by a recent FAO assessment (UNECE and FAO 1992) which shows an increase of forested land in Europe of 1 9 million ha in the 1980-1990 decade, as well as felling rates of around only 70 percent of net annual incremental growth for Europe. Sedjo's estimate of annual carbon sequestration of the temperate forests of North America, Europe and the former USSR is around 0 7 bt C per year go some way to explaining the missing sink. The explanation of why forests are the most likely to be the location of the missing sink include consideration of active carbon sequestration in the world's climax forests due to recovery from past disturbance. Climax forests have traditionally been treated in models as being in net equilibrium. A further factor is the CO2 fertilisation: enhanced primary productivity of vegetation through photo synthesis in a CO2 rich atmosphere. Carbon dioxide fertilisation is frequently cited as a likely cause of changes in the net primary productivity of vegetation and hence of the strength of the terrestrial carbon sink (Melillo et al 1993). There may also be a fertilisation effect (enhanced productivity) due to the increased presence of nitrogen in forest ecosystems. The combination of factors, rising forest stocks in temperate regions, the impact of past disturbance in promoting carbon sequestration in mature vegetation, and fertilisation effects of increased nitrogen or CO2, therefore suggest that the conservation of present forests is of even greater importance as a strategy to mitigate future impacts of global warming (Dixon et al 1994).

However, the studies that have concluded that the missing sink is in the terrestrial biota of the northern hemisphere (Sedjo 1992; Kauppi et al 1992) have been severely questioned in a paper by Houghton (1993). His argument is based on a re-estimation of net carbon storage throughout the cycle of forest growth and harvest. Houghton believes that the decomposition of litter after harvesting and the decomposition of forest products releases a much greater proportion of carbon than previously estimated. The continuing debate shows that a definitive model of the carbon cycle is not an immediate prospect. Indeed, there are other factors which should be taken into account, such as the impact of climate change itself. It has been further proposed that the missing sink can be accounted for by an increase in the net primary production of vegetation throughout the terrestrial biota as a result of observed temperature and precipitation change in the last half century. Thus Dai and Fung (1993) estimate a cumulative carbon sink of 20 ? 5 bt C for the period 1950-1984, based exclusively on temperature and precipitation change and ignoring enhanced productivity of vegetation through CO2 fertilisation.




Policy implications It is clear that the problem of the missing sink is of critical importance for policy prescriptions. The failure of researchers to provide a full explanation of the global carbon cycle leads directly to significant uncertainties in the analysis of future concentrations of C02, and hence climate change impacts. In modelling past and future CO2 concentration changes, assumptions have to be made regarding the ' location ' of the missing sink. It may be that historical estimates of land use

emissions are in error, and that these may be ' corrected ' to side-step the missing sink problem. Alternatively, it may be that CO2 fertilisation accounts for the discrepancy in the budget. This uncertain science however, is ultimately translated into public policy. Uncertainties of scientific evidence are frequently ignored in this process, often with highly undesirable social consequences (see Thompson 1993 for example). If defective alternative models of the global carbon cycle are used for policy analysis, their different assumptions will affect the policy options chosen, by altering the relative importance of the other sources and sinks. Ultimately this will affect the feasibility and cost effectiveness of land use-related mitigation policies designed to reduce emissions. If for example, re-estimation of the CO2 fertilisation effect showed increased growth rates of temperate forest vegetation in the future, strategies designed to stimulate forest planting in anticipation of this effect would be a higher priority. The consideration of the missing sink also creates problems in the estimation of the global warming potential (GWP) associated with each greenhouse gas. The GWP for each gas is stated in terms of CO2 equivalent emissions and allows comparison of the emissions of the different greenhouse gases. Uncertainty in comparing the climate forcing of the other gases to that of CO2 exists because the relative radioactive forcing of the greenhouse gases change with increasing atmos pheric concentrations of the non-CO2 gases and of the reference gas (CO2). Nevertheless, GWPs, as defined by the IPCC (Houghton et al 1992) and alternative measures have been widely used in policy analysis, from site specific life cycle analysis of energy use, to compiling global indices of responsibility for global

warming (Hammond et al 1990). The fact that the missing sink exists, however has focused even greater deficiency and reliance on the GWPs. The assumptions used to account for the missing sink ensures that the estimation of the radiative forcing of CO2 (the numeraire in the equation of GWPs) may be biased, possibly up to 15 percent (Wuebbles et al 1992), thereby underestimating the importance of CO2 in any use of GWPs for comparative purposes.

In summary, the failure to balance the global carbon budget is an empirical problem which is not easily resolved given the present state of knowledge of the global carbon cycle. The implications of the missing sink are significant for researchers attempting to quantify future climate change, but especially for research ers in the area of identifying and reducing emissions or enhancing sinks of greenhouse gases. The uncertain science lends a greater emphasis on undertaking actions which are of a precautionary nature and those which are already socially and economically desirable.

Afforestation to sequester and store carbon The planting of forests on land not forested is often proposed as the only feasible large scale abatement option outside of the energy sector. If afforestation were considered as an option to offset the greenhouse effect, then future enhancement of growth rates and equilibrium carbon storage through climatic changes or through CO2 fertilisation would enhance the feasibility and cost effectiveness of this option



316 Adger and Brown

Table 1 Costs and carbon sequestration for afforestation options on UK set-aside land

Coniferous afforestation Broadleaved afforestation of UK set-aside land of UK set-aside land

Species mix 100% Sitka Spruce Oak (39 7%) Beech (17 1%)

Sycamore, Ash and Birch (43 2%)

Equilibrium Carbon 68 140 (tC ha- 1)

Annual rate of storage 3 7 1 28 (tC ha

- 1 yr- 1)

Total cost to afforest 0 6 million ha ?2383 m ?2230 m Sequestration cost per tC ?21*47 per tC ?58 08 per tC

Source: Adapted from Adger and Brown (1994) Sequestration rates are based on Adger and Brown (1994) and Dewar and Cannell (1992). Net present values of afforestation based on 3 percent discount rate and 50 year time horizon, with coniferous felling at 35 years.

relative to fossil fuel related policies. Afforestation policies are presently not attractive: if all the land diverted from agriculture in set-aside were afforested this would lead to an annual sequestration of less than one percent of the UK's net emissions of greenhouse gases, and would have a positive cost, unlike 'negative costs ' hypothesised to exist in the energy sector (Lovins and Lovins 1992). These costs are estimated in Table 1 for the UK example, showing that broadleaved afforestation, an option more likely to be socially acceptable, would cost almost triple the coniferous option in terms of abatement of net carbon emissions. The environ

mental and landscape impacts of afforestation on such a large scale in the UK or elsewhere are also not taken into account in these estimates. However, with increased knowledge the costs may be proved to be overestimates if active sequestration rates are higher in future.

Conclusion

The so-called ' missing sink ' is an abberation of the natural science models, in which the ' search ' for the location of the missing sink constitutes the re-examination of

models of various parts of the biosphere. The policy implications of the missing carbon sink debate seem to be that the confidence intervals of predictions of future global warming are wider than presently reported, and the potential dampening or even positive feedback phenomena in the greenhouse effect system are also highly uncertain. This uncertainty is due to the future processes of the sinks being unknown: although approximately half the historic emissions of CO2 have remained in the atmosphere and half have been absorbed by the sinks. There is no guarantee, that this will be the case in the future. Secondly, the global warming potentials of

CO2 may be biased so as to overestimate the importance of non-CO2 gases. This has implications for national strategies which aim to reduce the profile of total emissions of greenhouse gases under the Climate Change Convention. Thirdly, future enhanced vegetation productivity may make afforestation more cost-effective as a




strategy to reduce net emissions. A greater understanding of the ' location of the

missing sink ' could redirect abatement efforts towards the conservation of land based

resources, and make the carbon cycle implications of land use change an even more

significant issue. Incorporating uncertain science directly into public policy is inherently dangerous: the uncertainties need to be highlighted by those working in both spheres. The adoption of precautionary, conservation-oriented land use policies are not equivalent to pro-active bio-engineering ' solutions ' to global greenhouse gas

emissions. Policy formulation must acknowledge 'that we never know perfectly and that every way of knowing has its built-in weaknesses' (Norgaard 1994, 13). Hence

a cautious, or precautionary, approach would be taken to technological solutions in the atmosphere or terrestrial biosphere which have ill-specified social and environmental consequences.

References Adger W N and Brown K (1994) Land use and the causes of global warming (John Wiley, Chichester)

Bouwman A F (1990) 'Exchange of greenhouse gases between terrestrial ecosystems and the atmosphere' in Bouwman A F (ed) Soils and the greenhouse effect. (John Wiley, Chichester) 61-127

Dai A and Fung I Y (1993) 'Can climate variability contribute to the missing CO2 sink' Global

Biogeochemical Cycles 7, 599-609 Dewar R C and Cannell M G R (1992) ' Carbon sequestration in the trees, products and soils of forest

plantations: an analysis using UK examples' Tree Physiology 11, 49-71

Dixon R K, Brown S, Houghton R A, Solomon A M, Trexler M C and Wisniewski J (1994) 'Carbon

pools and flux of global forest ecosystems ' Science 263, 185-190

Fisher M J, Rao I M, Ayarza M A, Lascano C E, Sanz J I, Thomas R J and Vera R R (1994) 'Carbon

storage by introduced deep rooted grasses in the South American savannas ' Nature 371, 236-8

Hammond A L, Rodenburg E and Moomaw W R (1990) 'Accountability in the greenhouse' Nature 347,

705-6

HoughtonJ T, Callander B A and Varney S K (eds) (1992) Climate Change 1992: the Supplementary Report

to the IPCC Scientific Assessment (Cambridge University Press, Cambridge) Houghton R A (1993) 'Is carbon accumulating in the northern temperate zone? ' Global Biogeochemical

Cycles 7, 611-7 Houghton R A and Skole D L (1990) 'Carbon' in Turner B L, Clark W C, Kates R W, Richards J F,

Mathews J T and Meyer W B (eds) The Earth as transformed by human action (Cambridge University

Press Cambridge), 393-408 Kauppi P E, Mielikainen K and Kuusela K (1992) 'Biomass and carbon budget of European forests: 1971

to 1990' Science 256, 70-4

Lovins A B and Lovins L H (1992) 'Least cost climatic stabilisation' in Pearman G I (ed) Limiting

Greenhouse Effects: controlling carbon dioxide emissions Uohn Wiley, Chichester) 351-442 Melillo J M, McGuire A D, Kicklighter D W, Moore B, Vorosmarty C J and Schloss A L (1993) ' Global

climate change and terrestrial net primary production ' Nature 363: 234-40

Norgaard R B (1994) Development betrayed: the end of progress and a coevolutionary revisioning of the future

(Routledge, London) Schlesinger W H (1991) Biogeochemistry: an analysis of global change (Academic Press, London)

Sedjo R A (1992) 'Temperate forest ecosystems in the global carbon cycle' Ambio 21, 274-77

Tans P P, Fung I Y and Takahashi T (1990) 'Observational constraints on the global atmospheric CO2

budget' Science 247: 1431-8 Thompson M (1993) ' Good science for public policy ' ournal of International Development 5, 6, 669-79

United Nations Economic Commission for Europe and the Food and Agriculture Organisation of the UN

(1992) The forest resources of the temperate zones: main findings of the 1990 forest resource assessment

(United Nations, New York) Warrick R A, Barrow E M and Wigley T M L (eds) (1993) Climate and sea level change: observations,

projections and implications (Cambridge University Press, Cambridge) Wuebbles D J, Patten K 0, Grant K E and Jain A K (1992) Sensitivity of direct global warming potentials

to key uncertainties (Lawrence Livermore National Laboratory, California)



Documents

Policy Implications of the Missing Global Carbon Sink