Dr Vincent Gray
Greenhouse Bulletin No. 111
Only a proportion of carbon dioxide emissions from combustion of fossil fuels appears to enter the earth’s atmosphere. The ratio of the annual emissions to the annual increment in the atmosphere is called the airborne fraction. Greenhouse Bulletin No 106 gave the latest charts for the annual emissions of carbon dioxide by fossil fuels and for the concentration of carbon dioxide in the atmosphere as measured since 1959 at Mauna Loa, Hawaii, and pointed out that for the 25 years from 1971 to 1996 a 54% increase in fossil fuel emissions had had no apparent influence on the approximately constant annual carbon dioxide increment in the atmosphere. The airborne fraction has declined.
Figure 1 shows this point in greater detail. It is a plot of the airborne fraction for each year between 1960 and 1995. The trend line is a second order polynomial, which shows a decline in the airborne fraction from 1971 to 1995 from 0.58 to 0.50, a reduction of 13%.
Figure 2 shows the annual amount of carbon entering the atmosphere, calculated from the Mauna Loa figures
Again, a second order polynomial trend line has been drawn through the points. It shows that the average annual increment of carbon in the atmosphere has now reached an approximately constant value of 3.2 Gigatonnes of carbon per year.
If it is assumed that the carbon entering the atmosphere comes entirely from fossil fuel emissions, then the remainder of the fossil fuel emissions must be absorbed by the earth. Figure 3 shows the results of making this assumption.
Again, a second order polynomial trend line has been drawn. It shows that the emissions absorbed by the earth have increased from 1.2GtCarbon in 1960 to 3.1GtC in 1995. The rate is accelerating. Where is the carbon dioxide going? Part must be absorbed by the ocean and part by the "terrestrial biosphere".
There is considerable disagreement amongst carbon cycle modellers on the amount of carbon annually absorbed by the oceans. The Intergovernmental Panel on Climate Change (IPCC) (Houghton et al. 1996) has consistently, in all its reports, summarised these disagreements by quoting the average annual ocean absorption over the period 1980-1989 as 2.0±0.8 GtC/yr. If the mean value, 2.0GtC/yr, is taken, then, from Figure 3, an average of 0.3GtC/yr would be absorbed by the "terrestrial biosphere" over 1980-1989. If a higher figure is taken, then there is little or no absorption by plants. If the lower figures are chosen, then plant uptake may have been as high as 1.0GtC/yr. It might also be noted that the confidence limits to the figure 2.0 are only ±90%. There is a 10% chance that the highest or lowest figures might be right.
Trends in ocean uptake depend on the particular model. The IPCC (Houghton et. al. 1995, Figure 1.7. page 47) selected the GFDL (Geophysics Fluid Dynamics Laboratory, Princeton) model as an illustration. It shows an ocean uptake of 0.92GtC in 1960, 1.41GtC in 1970, 1.81GtC in 1980, 2.29GtC in 1990 and (extrapolating), 2.78GtC in 1995. This would mean that the "terrestrial biosphere" absorbed zero carbon in 1960, 0.31Gt in 1970, 0.35GtC in 1980, 0.41GtC in 1990 and 0.52GtC in 1995.
The evidence is, then, that there may be a substantial global net absorption of the fossil fuel emissions of carbon dioxide by plants, and that this absorption is increasing each year, as the absorption by the atmosphere remains constant and the fossil fuel emissions increase.
Yet the impression given by the IPCC, and by the carbon cycle modellists, is that the land surface of the earth emits carbon dioxide. An emphasis has been placed on "land-use changes" and "deforestation", both of which are considered to give a net emission of carbon dioxide. Thus in the 1960 IPCC report a figure of 1.6±1.0GtC/yr was given as emissions from "deforestation and land use". These figures resulted from trends in forest inventories compiled by R.A. Houghton and associates at Woods Hole, Mass which go back to 1840.
One set of these estimates was used as an input to the IPCC carbon cycle model exercise (Enting et al. 1994). They assumed that there was a steady increase in net emissions from land use changes since 1850. The net emission figure for 1990 was 1.71GtC.
Although these figures were supposed to apply globally, the IPCC chose to restrict the figures for their future emissions scenarios (Pepper et al.; J.Houghton et al. 1992) to net tropical deforestation. The figure of 1.6±1.0GtC/yr) appears as "emissions from deforestation and land use" in the 1990 report (Houghton et al. 1990) but became "net emissions from changes in tropical land-use" in Climate Change 1994 (Houghton et al. 1995) and Climate Change 1995 (Houghton et al 1996).
Of course, this caused an imbalance in the carbon budget, which was originally attributed to a mysterious "missing sink". As the imbalance must be met by plant growth somewhere the IPCC has now accepted this obvious fact, but cannot let go of the concept of "emissions" from land use changes. They therefore (Houghton et al 1996) subdivide the terrestrial component of the carbon budget into "net emissions from changes in tropical land-use; +1.6±1.0GtC/yr, "Uptake by Northern Hemisphere forest regrowth, -0.5±0.5GtC/yr, "CO2 and N2 fertilisation",-1.3±1.5GtC/yr
This gives a net absorption by the terrestrial biosphere of 0.2 ± 1.9GtC/yr, close to the 0.3GtC/yr figure for the 1980s from Figure 3, and it is increasing. Again, note that the confidence limits are ±90%. There is a 10% chance that there is a net absorption of carbon dioxide by plants of 2.1GtC/yr or a net emission of 1.7GtC/yr.
This admission by the IPCC that some carbon dioxide emitted by combustion of fossil fuels is, on average being absorbed by plants, is thus leading to an overall increase in the net productivity of agriculture and forestry, and is therefore beneficial, has been ignored by the media, and has been unnoticed by the general public.
The Woods Hole researchers are unrepentant. Dixon et al. (1994) recalculated their estimates of changes in carbon fluxes by the world’s forests and now produce a figure of -0.9±0.4GtC/yr over recent years; a reduction on their previous estimates. However, they insist "if the missing C was to be accumulating in these Northern forests, the observed rate of growth must have been an underestimate of the actual rates by a factor of up to 4, which is unlikely". They have produced a model (Melillo et al. 1993) which admits the possibility that the terrestrial biosphere might become a net absorber of carbon.
Meanwhile the evidence mounts up that the plants are absorbing some of the extra carbon dioxide. The missing sink has been found.
Charles Keeling of the Scripps Institute, La Jolla, CA, whose carbon dioxide measurements at Mauna Loa, Hawaii are the first accurate ones, was one of the first to indicate, from a detailed model, from studies of the 13C/12C isotope ratio, and from the changes in seasonal amplitude of carbon dioxide, that there had to be a significant absorption by plants, particularly in the Northern Hemisphere (Keeling et al., 1989). He has followed this up by subsequent supporting evidence (Keeling et al 1995). Pieter Tans, and the NOAA team at Boulder, Colorado calculated a lower ocean uptake from measurements and postulated a significant carbon uptake by Northern Hemisphere forests (Tans et al. 1990). They have provided further supporting evidence (Francey et. al. 1995). The rejection of this evidence by the Woods Hole team led to the comment by Taylor (1993) "Until we can do such an analysis, predictions of future CO2 concentrations and the efficiency of policies directed at stabilising atmospheric CO2 must remain questionable".
Ralph Keeling , Charles’ son (1996), has calculated, by measuring global and hemispheric oxygen concentrations, that for the 1991-1994 period the global oceans and the northern land biota each removed the equivalent of approximately 30% of fossil fuel CO2 emissions, while the tropical land biota were neither a strong source or sink. If this is confirmed by future measurements, it should settle the matter.
Myneni et al (1997). (which included Charles Keeling) added further weight to these conclusions by detecting the existence of increased plant growth in the northern high latitudes from 1981 to 1991 by satellite observations.
To summarise. It is increasingly evident that a substantial proportion (maybe 30%) of fossil fuel emissions are absorbed, mainly in the Northern Hemisphere, as additional crops and forest growth, and that the "net emissions from land use changes" and the "missing sink" have to be abandoned. It even seems possible, then, that on balance, the increase of carbon dioxide in the atmosphere provides more benefits than misfortunes
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Francey, R.J., P.P. Tans, C.E. Allison, I.G. Enting, J.W.C. White & M. Troller. 1995. Changes in oceanic and terrestrial carbon uptake since 1982. Nature 373, 326-330
Houghton, J.T, G.J Jenkins,., and J.J Ephraums,. (Eds.) 1990 Climate Change: The IPCC Scientific Assessment, Cambridge University Press. (IPCC90)
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Myneni, R.B., C,D, Keeling, C.J. Tucker, G. Asrar, & R.R. Nemani , 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature 386 698-702
Pepper, W., J. Leggett, R. Swart, J. Wasson. J. Edmonds, I. Mintzer 1992 Emission Scenarios for the IPCC: an Update. Supplementary Report to Climate Change 1992.
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Keeling, C.D., R.B. Bacastow, A.F. Carter, S.C. Piper, T.P. Whorf, M. Heimann, W.G. Mook and H. Roeleffzen. 1989. A Three dimensional model of Atmospheric CO2 transport based on observed winds, 1. Analysis of Observational Data, 2 (Heimann & Keeling) Model Description and Simulated Tracer experiments. in Aspects of Climate Variability in the Pacific and the Western Americas, D.H. Peterson, Eds, Geophysical Monograph 55, American Geophysical Union, Washington, D.C . Pp 165-363.
Keeling, C.D., T.P. Whorf, M. Wahlen & J. van der Plicht 1995, Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980. Nature 375 666-670.
Tans, P., I.Y. Fung, T. Takahashi, 1990. Observational Constraints on the Global Atmospheric CO2 budget. Science 247, 1431-1438 Taylor, J. 1993 The mutable carbon sink. Nature, 366 515-516.
Vincent R. Gray
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