W h y    d o e s    a t m o s p h e r i c    C O 2    r i s e ?

Jan Schloerer



     Contents   (Version 3.1, October 1996)

     1.  Why does atmospheric CO2 rise ?
     2.  Carbon fluxes and reservoirs
         2.1  Natural carbon fluxes
         2.2  Anthropogenic carbon fluxes
         2.3  Carbon reservoirs
     3.  Fluctuations of the CO2 rise
     4.  References
     5.  Acknowledgements.  Administrivia



1.  Why does atmospheric CO2 rise ?

Time and again, some people claim that human activities are only
a minor source of atmospheric carbon dioxide (CO2) which is swamped
by natural sources.  Compared to natural sources, our contribution is
small indeed.  Yet, the seemingly small human-made or `anthropogenic'
input is enough to disturb the delicate balance.   "Anthropogenic CO2
is a biogeochemical perturbation of truly geologic proportions"
[Sundquist] and has caused a steep rise of atmospheric CO2.

The vexing thing is that, in the global carbon cycle, the rising level
of atmospheric CO2 and the human origin of this rise are about the only
two things that are known with high certainty.  Natural CO2 fluxes
into and out of the atmosphere exceed the human contribution by more
than an order of magnitude.  The sizes of the natural carbon fluxes
are only approximately known, because they are much harder to measure
than atmospheric CO2 and than the features pointing to a human origin
of the CO2 rise.

>From its preindustrial level of about 280 ppmv (parts per million
by volume) around the year 1800, atmospheric carbon dioxide rose to
315 ppmv in 1958 and to about 358 ppmv in 1994  [Battle] [C.Keeling]
[Schimel 94, p 43-44].   All the signs are that the CO2 rise is
human-made:

*  Ice cores show that during the past 1000 years until about the year
   1800, atmospheric CO2 was fairly stable at levels between 270 and
   290 ppmv.  The 1994 value of 358 ppmv is higher than any CO2 level
   observed over the past 220,000 years.  In the Vostok and Byrd ice
   cores, CO2 does not exceed 300 ppmv.  A more detailed record from
   peat suggests a temporary peak of ~315 ppmv about 4,700 years ago,
   but this needs further confirmation. [Figge, figure 3] [Schimel 94,
   p 44-45] [White]

*  The rise of atmospheric CO2 closely parallels the emissions history
   from fossil fuels and land use changes [Schimel 94, p 46-47].

*  The rise of airborne CO2 falls short of the human-made CO2 emissions.
   Taken together, the ocean and the terrestrial vegetation and soils
   must currently be a net sink of CO2 rather than a source [Melillo,
   p 454] [Schimel 94, p 47, 55] [Schimel 95, p 79] [Siegenthaler].

*  Most "new" CO2 comes from the Northern Hemisphere.  Measurements
   in Antarctica show that Southern Hemisphere CO2 level lags behind
   by 1 to 2 years, which reflects the interhemispheric mixing time.
   The ppmv-amount of the lag at a given time has increased according
   to increasing anthropogenic CO2 emissions. [Schimel 94, p 43]
   [Siegenthaler]

*  Fossil fuels contain practically no carbon 14 (14C) and less carbon
   13 (13C) than air.  CO2 coming from fossil fuels should show up in
   the trends of 13C and 14C.  Indeed, the observed isotopic trends
   fit CO2 emissions from fossil fuels.  The trends are not compatible
   with a dominant CO2 source in the terrestrial biosphere or in the
   ocean.  If you shun details, please skip the next two paragraphs.

*  The unstable carbon isotope 14C or radiocarbon makes up for roughly
   1 in 10**12 carbon atoms in earth's atmosphere.  14C has a half-life
   of about 5700 years. The stock is replenished in the upper atmosphere
   by a nuclear reaction involving cosmic rays and 14N  [Butcher,
   p 240-241].  Fossil fuels contain no 14C, as it decayed long ago.
   Burning fossil fuels should lower the atmospheric 14C fraction (the
   `Suess effect').  Indeed, atmospheric 14C, measured on tree rings,
   dropped by 2 to 2.5 % from about 1850 to 1954, when nuclear bomb
   tests started to inject 14C into the atmosphere [Butcher, p 256-257]
   [Schimel 95, p 82].  This 14C decline cannot be explained by a CO2
   source in the terrestrial vegetation or soils.

*  The stable isotope 13C amounts to a bit over 1 % of earth's carbon,
   almost 99 % is ordinary 12C [Butcher, p 240].  Fossil fuels contain
   less 13C than air, because plants, which once produced the precursors
   of the fossilized organic carbon compounds, prefer 12C over 13C in
   photosynthesis (rather, they prefer CO2 which contains a 12C atom)
   [Butcher, p 86].  Indeed, the 13C fractions in the atmosphere and
   ocean surface waters declined over the past decades [Butcher, p 257]
   [C.Keeling] [Quay] [Schimel 94, p 42].  This fits a fossil fuel CO2
   source and argues against a dominant oceanic CO2 source.  Oceanic
   carbon has a trifle more 13C than atmospheric carbon, but 13CO2 is
   heavier and less volatile than 12CO2, thus CO2 degassed from the
   ocean has a 13C fraction close to that of atmospheric CO2 [Butcher,
   p 86] [Heimann].  How then should an oceanic CO2 source cause
   a simultaneous drop of 13C in both the atmosphere and ocean ?

Overall, a natural disturbance causing the recent CO2 rise is
extremely unlikely.



2.  Carbon fluxes and reservoirs

First we look at natural carbon fluxes, next at fluxes of anthropogenic
carbon, and finally at carbon reservoirs.  Carbon enters and leaves the
atmosphere largely as CO2.  The remaining carbon fluxes involve various
organic and inorganic carbon compounds.

Unless stated otherwise, the following figures are adapted from [Schimel
95, p 77, 79].  Natural carbon fluxes and, except for the atmosphere,
carbon reservoirs are hard to measure, their estimates vary somewhat
across the literature.  We omit many details like, for instance, atmo-
spheric carbon monoxide with a lifetime of about 2 months [Novelli],
or methane with a lifespan of 10+ years.  Their roles as atmospheric
carbon reservoirs are minor, both eventually end up largely as CO2
[Prather 94/95].  For more on the carbon cycle see [Butcher] [Denman]
[Melillo] [Schimel 94/95] [Siegenthaler] [Sundquist].

        Gt    =  gigatonne  =  10^9 metric tonnes,  which is the mass
              of one cubic kilometre of water.  Instead of Gt, some
              authors use   Pg  =  petagram  =  10^15 grams
   1    GtC   corresponds to   ~3.67 Gt CO2
   2.12 GtC   or  ~7.8  Gt CO2  correspond to 1 ppmv CO2 in the
              atmosphere.  ppmv = parts per million by volume



2.1  Natural carbon fluxes

                                                 GtC / year

    Atmosphere  -->  terrestrial vegetation       120  Photosynthesis
    Terrestrial vegetation  -->  atmosphere        60  Respiration
    Terrestrial vegetation  -->  soils & detritus  60
    Soils & detritus  -->  atmosphere              60  Respiration

    Atmosphere  -->  surface ocean                 90
    Surface ocean  -->  atmosphere                 90

    Surface ocean  -->  deep ocean                 90  Inorganic carbon
    Surface ocean  -->  deep ocean                 10  Organic carbon
    Deep ocean  -->  surface ocean                100  Mostly inorganic


These fluxes are averages for 1980-1989, with anthropogenic carbon
omitted.  Fluxes can vary from year to year.  The above irreverently
lumps land animals with soils and detritus, and it skips many other
details as well.  For instance, both volcanic CO2 and CO2 removal via
silicate weathering are in the order of 0.1 GtC/year and play a role
on geologic time scales only [Butcher, chapter 11] [Sundquist].

Some, like [Schimel 95, p 77], condense the first four of the above
carbon fluxes into a shorthand (in GtC/year):

    Atmosphere  -->  terrestrial vegetation   60  Net primary production
    Soils & detritus  -->  atmosphere         60  Respiration

Net primary production is  "total photosynthesis minus respiration"
of the photosynthesizing biota.  For terrestrial vegetation, net
primary production is roughly half of total photosynthesis.  The
shorthand omits about 60 GtC/year which plants first take up via
photosynthesis and then return to the air via respiration.  The
60 GtC/year estimate for terrestrial net primary production is
based on a reassessment of earlier estimates ranging from 45.5 to
78 GtC/year. [Butcher, p 250-251] [Melillo, p 452]

Organic carbon compounds stemming from terrestrial net primary
production are ultimately decomposed via respiration by micro-
organisms and animals [Butcher, p 46].  Microorganisms are likely
to do the lion's share, yet an estimate of the respiratory carbon
flux from land animals would be interesting (I couldn't spot one).
While the biomass of animals contains only perhaps 1 or 2 GtC
[Schneider, p 102], some, and not only warm-blooded, animals spend
much more energy in respiration than for growth [Butcher, p 48].
(True, not all fungi are microorganisms.  But you have to stop
somewhere :)



2.2  Anthropogenic carbon fluxes

   Carbon dioxide sources                           GtC / year

      Fossil fuel burning, cement production      5.5  (5.0-6.0)
      Changes in tropical land use                1.6  (0.6-2.6)

      Total anthropogenic emissions               7.1  (6.0-8.2)

   Partitioning among reservoirs                    GtC / year

      Storage in the atmosphere                   3.3  (3.1-3.5)
      Oceanic uptake                              2.0  (1.2-2.8)
      Uptake by Northern Hemisphere forest
         regrowth                                 0.5  (0.0-1.0)
      Additional terrestrial sinks:  CO2 fer-
         tilization, nitrogen fertilization,
         climatic effects                         1.3 (-0.2-2.8)

These are average annual fluxes for 1980 through 1989.  The ranges
in parentheses are 90 %-confidence intervals, meaning the authors
estimate a 90 % chance that a given range encloses the true value
of the respective flux [Schimel 95, p 79].

CO2 fertilization, N fertilization, northern forest regrowth, and
climatic effects are hard to separate, their relative roles are not
well known [Goulden] [Melillo, p 449, 451-57] [Schimel 95, p 78-82].
For the ocean, a tentative estimate is that roughly 0.4 GtC/year of
anthropogenic carbon stay in the surface layer, while (1-) 1.6 (-2)
GtC/year go to the intermediate and deep ocean  [Denman, p 495-496]
[Schimel 95, p 77] [Siegenthaler].

Today, about 45 % of the anthropogenic CO2 stays aloft.  It is open,
whether and how this may change in future.  A host of processes
in the terrestrial biosphere and in the ocean may eventually affect
the airborne fraction  [Denman] [Melillo] [Schimel 94, p 51-58].



2.3  Carbon reservoirs   (in GtC)

   Atmosphere (1990)        750       Surface ocean             1020
   Terrestrial vegetation   610       Marine biota                 3
   Soils & detritus        1580       Dissolved organic carbon   700
                                      Deep ocean               38100

   Coal, oil       [Butcher, p 256, 259]           ~5000  to  ~10000
   Coal, oil, gas  [IPCC 95/II, p 80, 87]           at least  ~20000

The mass of the marine biota is small, their turnover is huge.
Estimates for oceanic primary production range from 15 to 126
GtC/year, with a best guess of 50 GtC/year.  Phytoplankton carries
out the photosynthesis.  Other organisms recycle most of the resulting
organic carbon compounds on the spot, in the sunlit surface layer
of the ocean.  Detritus and dissolved organic carbon compounds
containing roughly 10 GtC/year go to the intermediate and deep ocean
(the so-called biological carbon pump).  This carbon flux is balanced
mostly by upwelling of deep water enriched in inorganic carbon from
decomposition of the organic remnants.  Current thinking is that the
biological pump was roughly in steady state during the past century.
If so, then the anthropogenic CO2 currently absorbed by the ocean
would mainly take the "inorganic path".  [Butcher, p 252-253] [Denman]
[Siegenthaler]



3.  Fluctuations of the CO2 rise

The average annual increase of CO2 went up from about 0.9 ppmv/year
during the 1960s to about 1.5 ppmv/year during the 1980s.  The annual
CO2 growth rate has kept fluctuating since the start of direct
measurements in 1958.  Many fluctuations appear to be related to
El Nino-Southern Oscillation (ENSO) events.  The drop of the CO2
growth rate between late 1991 and late 1993, however, cannot be
directly linked to an ENSO event.  The rise of atmospheric methane
and of nitrous oxide temporarily slowed down at about the same time.
Mt. Pinatubo's 1991 eruption may have played a role, but the matter
is not settled.  [Heimann] [IPCC 95, p 75-6] [Prather 95, p 87-8]
[Schimel 95, p 80-2]

The oceanic and, presumably even more, the terrestrial net CO2 uptake
appear to vary by a few GtC from year to year, probably in response
to climatic variations [Bender] [C.Keeling] [R.Keeling] [Melillo,
p 456-457].  Elucidating climatic effects on terrestrial CO2 fluxes
requires, as a minimum, long-term monitoring of vegetation in all
major climatic regions.  In a forest, for instance, the net CO2 flux
depends on net photosynthesis (net primary production) and on soil
respiration.  These in turn may depend, among others, on the length
of the growing season and on the timing and amount of rain, snow,
drought and cloud cover.  [Goulden] [Melillo, p 452-454]

The terrestrial biosphere was probably roughly in balance during
the late 1970s and the 1980s.  Over this period, CO2 release
from tropical land-use changes and the average CO2 uptake by
the terrestrial biosphere seem to have almost cancelled, in spite
of year-to-year variations.  From 1991 to 1993, the terrestrial
biosphere probably was a net CO2 sink, in 1994 the CO2 rise was
back to its usual pace.  [Battle] [Bender] [C.Keeling] [R.Keeling]
[Schimel 95, figure 2.2]



4.  References

[Battle]   M. Battle, M. Bender, T. Sowers, P.P. Tans, 7 more authors,
   Atmospheric gas concentrations over the past century measured in air
   from firn at the South Pole.   Nature 383 (1996), 231-235
[Bender]   Michael Bender,  A quickening on the uptake ?
   Nature 381 (1996), 195-196
[Butcher]   Samuel S. Butcher,  Robert J. Charlson et al.  (eds),
   Global Biogeochemical Cycles.   San Diego, CA, Academic Press 1992
[Denman]   K. Denman,  E. Hofmann,  H. Marchant,   Marine biotic
   responses to environmental change and feedbacks to climate.
   Pages 483-516 in [IPCC 95]
[Figge]   Regina A. Figge  and  James W.C. White,
   High-resolution Holocene and late glacial atmospheric CO2 record:
   variability tied to changes in thermohaline circulation.
   Global Biogeochemical Cycles 9 (1995), 391-403
[Goulden]   Michael L. Goulden,  J.William Munger,  Song-Miao Fan,
   Bruce C. Daube,  Steven C. Wofsy,   Exchange of carbon dioxide
   by a deciduous forest: Response to interannual climate variability.
   Science 271 (1996), 1576-1578
[Heimann]   Martin Heimann,  Dynamics of the carbon cycle.
   Nature 375 (1995), 629-630
[IPCC 94]   Climate Change 1994:  Radiative Forcing of Climate Change
   and  An Evaluation of the IPCC IS92 Emission Scenarios.
   J.T. Houghton, L.G. Meira Filho, J. Bruce, Hoesung Lee,
   B.A. Callander, E. Haites, N. Harris and K. Maskell (eds),
   Cambridge University Press 1995
[IPCC 95]   Climate Change 1995:  The Science of Climate Change.
   J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris,
   A. Kattenberg and K. Maskell (eds),  Cambridge University Press 1996
[IPCC 95/II]   Climate Change 1995:  Impacts, Adaptations and
   Mitigation of Climate Change: Scientific-Technical Analyses.
   Robert T. Watson et al. (eds),   Cambridge University Press 1996
[C.Keeling]   C.D. Keeling, T.P. Whorf, M. Wahlen & J. van der Plicht,
   Interannual extremes in the rate of rise of atmospheric carbon
   dioxide since 1980.   Nature 375 (1995), 666-670
[R.Keeling]   Ralph F. Keeling,  Stephen C. Piper  &  Martin Heimann,
   Global and hemispheric CO2 sinks deduced from changes in atmospheric
   O2 concentration.    Nature 381 (1996), 218-221
[Melillo]   J.M. Melillo, I.C. Prentice, G.D. Farquhar, E.-D. Schulze,
   O.E. Sala,   Terrestrial biotic responses to environmental change
   and feedbacks to climate.   Pages 445-481 in [IPCC 95]
[Novelli]   Paul C. Novelli,  Ken A. Masarie,  Pieter P. Tans,
   Patricia M. Lang,   Recent changes in atmospheric carbon monoxide.
   Science 263 (1994), 1587-1590
[Prather 94]   M. Prather, R. Derwent, D. Ehhalt, P. Fraser,
   E. Sanhueza, X. Zhou,  Other trace gases and atmospheric chemistry.
   Pages 73-126 in [IPCC 94]
[Prather 95]   M. Prather, R. Derwent, D. Ehhalt, P. Fraser,
   E. Sanhueza, X. Zhou.  Other trace gases and atmospheric chemistry.
   Pages 86-103 in [IPCC 95]
[Quay]   P.D. Quay, B. Tilbrook, C.S. Wong,   Oceanic uptake of fossil
   fuel CO2: carbon-13 evidence.   Science 256 (1992), 74-79
[Schimel 94]   D. Schimel, I.G. Enting, M. Heimann, T.M.L. Wigley,
   D. Raynaud, D. Alves, U. Siegenthaler,   CO2 and the carbon cycle.
   Pages 35-71 in [IPCC 94]
[Schimel 95]   D. Schimel, D. Alves, I. Enting, M. Heimann, F. Joos,
   D. Raynaud, T. Wigley,  CO2 and the carbon cycle.   Pages 76-86
   in [IPCC 95]
[Schneider]   Stephen H. Schneider,  Global Warming:  Are We Entering
   the Greenhouse Century ?   Vintage Books,  New York 1990
[Siegenthaler]   U. Siegenthaler  &  J.L. Sarmiento,   Atmospheric
   carbon dioxide and the ocean.   Nature 365 (1993), 119-125
[Sundquist]   Eric T. Sundquist,   The global carbon dioxide budget.
   Science 259 (1993), 934-941
[White]   J.W.C. White,  P. Ciais,  R.A. Figge,  R. Kenny  &
   V. Markgraf,   A high-resolution record of atmospheric CO2 content
   from carbon isotopes in peat.   Nature 367 (1994), 153-156.
   Discussion:  Nature 371 (1994), 111-112



5.  Acknowledgements.  Administrivia

Acknowledgements:  Many people helped with explanations and comments.
My wife Rosemarie pointed out several confusing points.  Len Evens
spotted a particularly idiosyncratic outburst.

Caveat:  This is not my field.  Corrections and amendments, especially
by professionals, are welcomed.  Students should not use this article
as a reference for school projects.  They should instead use it as a
pointer to some of the published literature.

Copyright (c) 1996 by Jan Schloerer, all rights reserved.  This article
may be posted to any USENET newsgroup, on-line service and BBS, as long
as it is posted in its entirety and includes this caveat and copyright
statement.  However, please inform me, so I know where the article goes.
This article may not be distributed for financial gain, it may not be
included in commercial collections or compilations without the express
written permission of the author.

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Jan Schloerer                      jschloer@rzmain.rz.uni-ulm.de
                  As from 1997: jan.schloerer@medizin.uni-ulm.de
Uni Ulm    Biometrie & Med.Dokumentation    D-89070 Ulm, Germany