M e t h a n e     r e c o r d    a n d     b u d g e t



            Contents   (Version 2, October 1996)

     1.  Methane and CO2 during the past millenium
     2.  Methane during the last glaciation
     3.  Methane budget
         3.1  Methane sources
         3.2  Methane sinks  &  atmospheric increase
         3.3  Totals
         3.4  Recent fates of some methane sources
     4.  References
     5.  Administrivia




1.  Methane and CO2 during the past millenium

Since around 1800, human activities have more than doubled the level
of atmospheric methane, and have increased atmospheric CO2 by over
a quarter.  Records since AD 1000 for methane [IPCC 94, fig. 2.1]
and for CO2 [IPCC 94, fig. 1.5] suggest that before about 1800 the
human influence is hard to detect amongst the natural noise.

Each record is based on about half a dozen studies.  Direct atmosphe-
ric measurements go back to the 1970s for methane and to 1958 for CO2.
Older observations come from ice cores.  For both gases, transition
between ice core and atmospheric measurements is smooth, and measure-
ments from various ice cores agree fairly well.  For methane, there
are two cores each from Antarctica and Greenland.  For CO2, four
Antarctic and no Greenland cores were used.  Greenland ice CO2 data
may be partially compromised by dust swept over from nearby land
areas; the size of the resulting error was not yet clear.

For a given time, individual measurements differ by up to about 10
ppmv for CO2 and roughly 50 ppbv for methane.  The following averages
are eyeballed from the graphs, please don't take them too literally.
1990 methane eyeballed from [IPCC 94, fig. 2.2].
     ppmv   parts per million by volume
     ppbv   parts per billion by volume (American billion = 10**9)


     Year            CO2 (ppmv)       Methane (ppbv)

     1000              277                720
     1100              279                730
     1200              277                750
     1300              282                680

     1400              283                680
     1500              281                730
     1600              280                740
     1700              280                750
     1800              282                770

     1850              288                830
     1900              297                900
     1950              309               1150
     1990              350               1700


Growth rates have not been monotonic.  The CO2 growth rate has kept
fluctuating since the start of direct measurements in 1958.  Many
though not all of these variations appear to be related to El Nino-
Southern Oscillation events [IPCC 95, p 80-82].  The rise of methane
slowed down around the 1920s, then again since the 1970s and, even
more, from 1991 to 1993.  The reasons behind these slowdowns are not
yet known.  Pinatubo's 1991 eruption may have played a role in the
1991-93 anomaly, but the matter is not settled [IPCC 94, p 43, 87-89]
[IPCC 95, p 75-76, 87-88].

Direct radiative forcing due to the rise of atmospheric CO2 and
methane from 1850 to 1990 is currently estimated at 1.56 and 0.47
W/m**2, respectively [IPCC 94, p 194-195].




2.  Methane during the last glaciation

During the last glaciation and deglaciation, atmospheric methane
oscillated between roughly 400 and 700 ppbv.  Methane levels were
correlated with temperature.  The methane variations amplified the
temperature changes, if only slightly.  In this respect, ice-covered
area and CO2 variations were more important [Raynaud] [Jouzel].

In 1996, a methane record for the past 110,000 years from the GISP2
Greenland ice core supplemented former records from Antarctica,
which were less detailed.  It turned out that the atmospheric
methane level tracked most of the rapid climatic shifts during
the last glaciation.  Methane was lower during most of the cold
stadials, it was higher during most of the over 20 interstadial
warming events. [Brook]

Methane presumably slightly amplified these temperature shifts,
but that is a minor point.  The major natural methane source are
wetlands, many of which are in the tropics.  The methane swings
support the notion that the frequent, still enigmatic stadials
and interstadials, also known as Dansgaard-Oeschger events,
were not just North Atlantic, but large-scale regional, perhaps
even global events.

On a longer time scale, orbital forcing also appears to have
a hand in the game: stadial-interstadial methane oscillations
waxed and waned with northern hemisphere summer insolation,
with a period of roughly 20,000 years.  So far, two links are
pondered.  Growth and shrinking of northern ice sheets may cover
and uncover northern wetlands.  Moreover, summer insolation may
affect ice-free wetlands directly: warmer and wetter conditions
tend to enhance methane emissions. [Brook]




3.  Methane budget

The following tables show annual averages for 1980 to 1990 of the
sources and sinks of atmospheric methane [IPCC 94, p 86] [IPCC 95,
p 94].  These numbers aren't the last word.  The best estimates for
total methane sources and for total methane sinks plus atmospheric
increase don't agree yet.  Low and high estimates appear in paren-
theses ().  Yearly methane fluxes are in Tg = teragram = 10**12 g
= million metric tonnes.

Most atmospheric methane (CH4) eventually ends up as CO2 and water.
In the troposphere, the reaction chain starts with oxidation of
methane by hydroxyl (OH).  OH concentration depends on many factors,
including the methane level.  OH is hard to measure, the lifespan
of methane is difficult to determine [IPCC 94, p 79-83].  When some
extra-amount of methane (say, 1 kg) is added to the atmosphere, it is
currently estimated to take 12.2 +- 3 years until about two thirds
(63 %) of this amount are removed [IPCC 95, p 91, 94].



3.1  Methane sources                              Methane (Tg/year)

Natural methane sources                           160  (110-210)
   Wetlands                     115  (55-150)
   Termites                      20  (10- 50)
   Oceans                        10  ( 5- 50)
   Other                         15  (10- 40)

Anthropogenic sources, fossil fuel related        100  ( 70-120)
   Natural gas                   40  (25- 50)
   Coal mines                    30  (15- 45)
   Petroleum industry            15  ( 5- 30)
   Coal combustion                ?  ( 1- 30)

Anthropogenic sources, biospheric carbon          275  (200-350)
   Enteric fermentation [*]      85  (65-100)
   Rice paddies                  60  (20-100)
   Biomass burning               40  (20- 80)
   Landfills                     40  (20- 70)
   Animal waste                  25  (20- 30)
   Domestic sewage               25  (15- 80)

   [*]  mostly belching cattle  :)



3.2  Methane sinks  &  atmospheric increase       Methane (Tg/year)

Methane sinks                                     560  (460-660)
   Troposphere, oxid. by OH     490 (405-575)
   Stratosphere                  40 ( 32- 48)
   Removal by soil microbes      30 ( 15- 45)

Increase of atmospheric methane                    37  ( 35- 40)



3.3  Totals                                       Methane (Tg/year)

Total identified methane sources                  535  (410-660)
   Natural                      160 (110-210)
   Anthropogenic                375 (300-450)

Total methane sinks + atmospheric increase        597  (495-700)



3.4  Recent fates of some methane sources ...

... or, rather, fates of their estimates.  For deep methanologists :)


   Source                  Methane / year  in  Tg  (10**12 g)

                    [IPCC 90, p20]    [IPCC 92, p35]    [IPCC 94, p86]

   Wetlands          115 (100-200)     115 (100-200)     115  (55-150)
   Termites           40  (10-100)      20  (10- 50)      20  (10- 50)
   Oceans             10   (5- 20)      10   (5- 20)      10   (5- 50)

   Natural gas        45  (25- 50)       -     -          40  (25- 50)
   Coal mining        35  (19- 50)       -     -          30  (15- 45)

   Enteric ferment.   80  (65-100)      80  (65-100)      85  (65-100)
   Rice paddies      110  (25-170)      60  (20-150)      60  (20-100)
   Biomass burning    40  (20- 80)      40  (20- 80)      40  (20- 80)
   Landfills          40  (20- 70)      30  (20- 70)      40  (20- 70)
   Domestic sewage     -     -          25  (  ?   )      25  (15- 80)

Since 1994, the tropospheric methane sink was revised upwards and the
lifetime of methane was revised downwards, based on a revised estimate
of mean global OH concentration [IPCC 95, p 91].  The remaining items
of the methane budget stayed unchanged but, as mentioned, current
estimates aren't the last word.




4.  References

[Brook]   Edward J. Brook, Todd Sowers, Joe Orchardo,  Rapid variations
    in atmospheric methane concentration during the past 110,000 years.
    Science 273 (1996), 1087-1091
[IPCC 90]   Climate Change  -  The IPCC Scientific Assessment.
   J.T. Houghton et al., eds,  Cambridge University Press 1990
[IPCC 92]   Climate Change 1992  -  The Supplementary Report
   to the IPCC Scientific Assessment.   J.T. Houghton et al., eds,
   Cambridge University Press 1992
[IPCC 94]   Climate Change 1994:  Radiative Forcing of Climate Change
   and  An Evaluation of the IPCC IS92 Emission Scenarios.
   J.T. Houghton et al., eds,  Cambridge University Press 1995
[IPCC 95]   Climate Change 1995:  The Science of Climate Change.
   J.T. Houghton et al., eds,  Cambridge University Press 1996
[Jouzel]   J. Jouzel, N.I. Barkov, J.M. Barnola, M. Bender, 13 more
    authors,  Extending the Vostok ice-core record of paleoclimate
    to the penultimate glacial period.   Nature 364 (1993), 407-412
[Raynaud]   D. Raynaud, J. Jouzel, J.M. Barnola, J. Chappellaz,
    R.J. Delmas, C. Lorius,  The ice record of greenhouse gases.
    Science 259 (1993), 926-934

In case you are interested in more original research papers,
the four IPCC volumes list heaps of references.




5.  Administrivia

This file wouldn't exist without the curiosity of Eric Stevens.
Typos conceivable, please let me know if you spot one.

This file is archived at:
   http://www.grumbinescience.org/radix/climate/



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