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Archive-name: sci/climate-change/basics
Version: 2.02
Last-modified: 05 April 1997
Posting-Frequency: about every two months

Changes April 1997:  Minor patches in sections 6 and 7,
amendment in section 11, some references and web sites added.
Changes Oct 1996:  Many modifications and amendments,
some references replaced by pointers to [IPCC 95].



                   Climate change: some basics



Subject: 1.  Introduction

By outpouring greenhouse gases humankind has launched an experiment
of geologic proportions.  Will this experiment, if countermeasures
worth mentioning are delayed for some more decades, cause serious
consequences during the next century ?   Alas, there is no simple
yes-no-answer to this question.  Climate, its natural vagaries,
and the long-term effects of rising greenhouse gas levels are only
partially understood.  The shortest defensible answer I can think of,
a first approximation so to speak:  it is roughly an even bet,
fifty-fifty.  The longer greenhouse gas emissions go on uncurbed,
the worse the odds.

A nontechnical, by no means comprehensive outline of some of the basic
science behind this answer follows.  Potential impacts and responses
are not addressed.  Please note that this is not my field.  I have
a fair idea of the broad picture, but I don't understand all the
technical niceties.  I have attempted to sketch some basics in a way
which most readers with some interest in our planet's workings might
be able to understand.


Jan Schloerer
jan.schloerer@medizin.uni-ulm.de



Subject: 2.  Contents

    1.  Introduction
    2.  Contents
    3.  The natural greenhouse effect
    4.  Tropospheric lapse rate
    5.  The enhanced greenhouse effect.  Radiative forcing
    6.  Climate sensitivity.  The modern temperature record
    7.  Human-made tropospheric aerosols
    8.  Ocean and response time
    9.  Feedbacks: water vapor, ice and snow, clouds
   10.  The global carbon cycle.  Biological feedbacks
   11.  Natural climatic variability
   12.  Ice record of greenhouse gases and last glaciation
   13.  Conclusion
   14.  Further reading.  References
   15.  Some web sites
   16.  Acknowledgements. Administrivia. How to get this file



Subject: 3.  The natural greenhouse effect

The sun's radiation, much of it in the visible region of the spectrum,
warms our planet.  On average, earth must radiate back to space the
same amount of energy which it gets from the sun.  Being cooler than the
sun, earth radiates in the infrared.  (An object, when getting warmer,
radiates more energy and at shorter wavelengths.  On cooling, it emits
less and at longer wavelenghts.  Lava or heated iron are examples.)
The wavelengths at which the sun and the earth emit are, for energetic
purposes, almost completely distinct.  Often, solar radiation is called
shortwave, whereas terrestrial infrared is called longwave radiation.

Greenhouse gases in earth's atmosphere, while largely transparent to
incoming solar radiation, absorb most of the infrared emitted by earth's
surface.  The air is cooler than the surface, emission declines with
temperature, so the air or, rather, its greenhouse gases emit less
infrared upwards than the surface.  Moreover, while the surface emits
upwards only, the air's greenhouse gases radiate both up- and downwards,
so some infrared comes back down.  Clouds also absorb infrared well.
Again, cloud tops are usually cooler and emit less infrared upwards
than the surface, while cloud bottoms radiate some infrared back down.
All in all, part of the infrared emitted by the surface gets trapped.

Satellites, viewing earth from space, tell us that the amount of
infrared going out to space corresponds to an  `effective radiating
temperature'  of about -18 o C.  At -18 o C, about 240 watts per square
metre (W/m**2) of infrared are emitted.  This is just enough to balance
the absorbed solar radiation.  Yet earth's surface currently has a mean
temperature near 15 o C and sends an average of roughly 390 W/m**2 of
infrared upwards.  After the absorption and emission processes just
outlined, 240 W/m**2 eventually escape to space; the rest is captured
by greenhouse gases and clouds.  The `natural greenhouse effect' can
be defined as the 150 or so W/m**2 of outgoing terrestrial infrared
trapped by earth's preindustrial atmosphere.  It warms earth's surface
by about 33 o C.

As an aside, note that garden glasshouses retain heat mainly by lack
of convection and advection [Jones].  The atmospheric `greenhouse'
effect, being caused by absorption and re-emission of infrared
radiation, is a misnomer.  We won't get rid of it, though ;-)

Under clear sky, roughly 60-70 % of the natural greenhouse effect is
due to water vapor, which is the dominant greenhouse gas in earth's
atmosphere.  Next important is carbon dioxide, followed by methane,
ozone, and nitrous oxide [IPCC 90, p 47-48].

Clouds are another big player in the game.  Beginners please don't
confuse clouds with water vapor: clouds consist of water droplets or
ice particles or both.  Under cloudy sky the greenhouse effect is
stronger than under clear sky.  At the same time, cloud tops in the
sunshine look brilliantly white: they reflect sunlight.  Globally and
seasonally averaged, clouds currently exert the following effects:

   Outgoing terrestrial infrared trapped (warming)     about 30 W/m**2
   Solar radiation reflected back to space (cooling)  nearly 50 W/m**2
   Net cloud effect (cooling)                        roughly 20 W/m**2

Earth's present reflectivity or albedo (whiteness) is near 0.3.  This
means that about 30 % or slightly over 100 W/m**2 of the sun's incoming
radiation is reflected back to space, while roughly 240 W/m**2 or about
70 % is absorbed.  Almost half of earth's current albedo and perhaps
20 % of the natural greenhouse effect is caused by clouds.  Quantities
involving clouds are hard to measure and may vary by a few W/m**2,
depending on whom you listen to.

Globally averaged, the surface constantly gains radiative energy,
whereas the atmosphere scores a loss.  Sending up about 390 W/m**2,
the surface absorbs roughly 170 W/m**2 solar radiation and over 300
W/m**2 infrared back radiation from greenhouse gases and clouds.
The atmosphere, clouds included, radiates both up- and downwards,
altogether over 500 W/m**2.  It absorbs roughly 70 W/m**2 solar
radiation and 350 W/m**2 terrestrial infrared.

The surface's radiative heating and the atmosphere's radiative
cooling are balanced by convection and by evaporation followed by
condensation.  When evaporating, water takes up latent heat; when
water vapor condenses, as happens in cloud formation, latent heat is
released to the atmosphere.  Information in this section comes from
[Berger] and [Hartmann, chapters 2-4], unless indicated otherwise.



Subject: 4.  Tropospheric lapse rate

At any given location, the temperature profile of the air column varies
between day and night, from winter to summer.  At times and places the
air may get warmer higher up (an inversion).  Globally averaged, the
troposphere, the lower about 10 to 15 km of our atmosphere, gets cooler
with height.  A typical value cited is 6.5 o C cooling / km of altitude.
This is the so-called global mean tropospheric lapse rate.  Some people
attach a plus, others attach a minus sign to this rate [Hartmann, p 3,
69] [Sinha].  In any case, it indicates the average rate of cooling
with height.  For illustration, if the amount of the mean tropospheric
lapse rate should increase by 1 o C / km, then the mean air temperature
at 5 km altitude would drop by 5 o C.

Basically, earth's surface temperature and the greenhouse effect tend
to go up and down with the amount of the tropospheric lapse rate.  To
see why, recall that infrared emitted from the surface rarely reaches
space directly: greenhouse gases and clouds absorb most of it.  Earth's
effective radiating temperature of -18 o C corresponds to an apparent
radiating altitude of 5 or so km.  The bulk of the infrared escaping
to space comes from the middle and upper troposphere.  On its way up,
little of this radiation gets caught: still higher up the air is thin,
there are few greenhouse gases and clouds [Hartmann, p 28, 59-60].

Now imagine that the amount of the global mean tropospheric lapse rate
goes up, while anything else remains equal (a wild simplification, but
never mind).  Then the middle and upper troposphere get cooler and emit
less infrared to space.  The sun keeps shining, so earth's radiation
budget gets out of balance.  The surface (and troposphere) must warm
until they emit enough infrared to restore the balance under the enhan-
ced lapse rate.  The difference between surface emission and emission
to space, that is: the greenhouse effect, increases.  Vice versa, if
the magnitude of the global mean tropospheric lapse rate drops, then
the middle and upper troposphere warm and emit more infrared to space.
To regain the balance, the greenhouse effect must decline.

Once again, this is simplified in order to convey the basic idea.
The mean tropospheric lapse rate is a balance between many processes
of energy transfer, like radiation, convection, evaporation, cloud
formation, and large scale air motions.  Data from the midlatitudes
and tropics suggest that local lapse rate changes currently tend to
amplify local variations of surface temperature and of the greenhouse
effect.  It is unclear whether and how the global mean tropospheric
lapse may change with a changing global climate [Sinha] [Soden].

Finally, note that if the surface warms, while the lapse rate remains
unchanged, then the troposphere will warm by the same amount as the
surface.  Infrared emission to space will rise accordingly.



Subject: 5.  The enhanced greenhouse effect.  Radiative forcing

Since around 1800 and especially during the past few decades, human
activities have increased the atmospheric levels of several greenhouse
gases.  To name a few:  Carbon dioxide (CO2) went up from about 280 ppmv
(parts per million by volume) in the year 1800 via 315 ppmv in 1958
to about 358 ppmv in 1994 [IPCC 95, p 16, 78] [Keeling].  Methane (CH4)
increased from roughly 0.8 ppmv in 1800 to more than 1.7 ppmv in 1992.
Nitrous oxide (N2O) rose from a preindustrial level of about 0.275 ppmv
to 0.310 or so ppmv in 1992 [IPCC 94, p 87-8, 91-2].

The resulting enhanced greenhouse effect is often expressed in terms of
`radiative forcing'.  To get a feeling for this notion, suppose that
greenhouse gas levels go up, while anything else, including temperature,
is kept fixed.  Adding greenhouse gases renders the atmosphere more
opaque to outgoing infrared radiation.  Thus the mean altitude from
which infrared emitted upwards makes it to space (5 or so km) rises.
As mentioned, the troposphere gets cooler with height.  With rising
emission altitude, both earth's effective radiating temperature and,
consequently, the amount of infrared emitted to space decline.  The
influx of solar radiation, to which greenhouse gases are almost trans-
parent, changes little.  So the net influx (the difference between
what goes in and out) is now positive instead of being zero.

Radiative forcing means a  _change_  in the net downward flux of radia-
tion, in W/m**2, at the tropopause, the borderline between troposphere
and stratosphere.  Eventually the climate system must respond and re-
adjust the net flux to zero, but temporarily this flux may get positive
or negative.  Given some perturbation like a change in greenhouse gas
or aerosol levels, radiative forcing is estimated with tropospheric and
surface temperatures (the response of which takes decades) _kept fixed_
at their unperturbed values [IPCC 94, p 169-71].  Rising greenhouse gas
levels cause positive radiative forcing.  Aerosols, to be described
later, can cause negative radiative forcing.

Radiative forcing due to human-made greenhouse gases is currently
estimated at about 2.5 W/m**2.  CO2 causes roughly 1.6 W/m**2 of this,
while methane contributes about 0.5 W/m**2.  Doubling the CO2 level
from its preindustrial 280 to 560 ppmv amounts to a radiative forcing
of a bit over 4 W/m**2.  If business goes on as usual, the combined
effect of the rising greenhouse gas levels is likely to reach the
equivalent of a CO2 doubling around the year 2050 and will hardly
stop there [IPCC 90, p 52] [IPCC 95, p 108-18, 321].

An enhanced greenhouse effect disturbs earth's radiation balance:
less infrared gets out, while the sun keeps shining.  This cannot last,
the balance must be restored.  At least one of the following things
must happen:  earth's surface and troposphere may warm (lapse rate
remaining unchanged), earth's albedo may go up, the amount of the mean
tropospheric lapse rate may drop (the latter, though, might also rise
and thus enhance surface warming), or other changes in earth's climate
system may curb the enhanced greenhouse effect.

In short, something has to give.  Monkeying with earth's radiation
balance will change the climate in some way.  Earth's surface will most
probably warm, although it is uncertain by how much and how swiftly.
In addition, there will probably be a gamut of other changes, some
of which, like changes in the water cycle, are even harder to predict
and may become more troublesome than warming  [IPCC 95] [Morgan].



Subject: 6.  Climate sensitivity.  The modern temperature record

To the best of present knowledge, the so-called equilibrium surface
warming, also known as the `climate sensitivity', is likely to sit
somewhere between  1.5 and 4.5 o C for a CO2 doubling, with a best
estimate of  2.5 o C [IPCC 95, p 34, 48].

Since 1890, average global surface temperature went up by about
0.5 o C with an uncertainty of roughly 0.15 o C both ways: the true
warming is likely to lie somewhere between 0.3 and 0.6 o C.  This
estimate takes into account any known error sources, including urban
heat island bias, relocation of stations, changes in measuring prac-
tices and varying coverage of the globe.  About 0.3 o C warming until
1940 and 0.1 o C cooling until 1975 were followed by renewed warming.
[IPCC 90, chapter 7.4] [IPCC 95, p 26-8, 141-6]

Surface and low to mid-tropospheric temperature are often confused,
but they are not interchangeable.  For tropospheric temperatures, the
radiosonde and satellite record go back to 1958 and 1979, respectively.
Both records are similar since 1979.  On average, both the surface and
lower-to-middle troposphere warmed by about 0.1 o C per decade since
1960.  From 1979 to 1995, however, the surface warmed by 0.13 o C per
decade, while the lower-to-middle troposphere cooled by 0.05 o C per
decade.  Gaps in the southern oceans surface data and errors in the
tropical satellite record may contribute to the difference, but there
are physical reasons as well.  Surface and tropospheric temperatures
responded differently to El Nino-Southern Oscillation, to volcanic
eruptions, and probably also to deep Aleutian (1976-88) and Iceland
(~1980-95) winter lows.  [Hurrell 96/97] [IPCC 95, p 146-8, 165-6]

Since 1960, the lower stratosphere cooled markedly by roughly -0.35 o C
per decade.  Both rising CO2 levels and stratospheric ozone depletion
tend to cool the stratosphere.  Initial model results suggest that,
at the moment, stratospheric ozone loss may play the lead.  It may
also have a hand in the slight cooling of the upper troposphere over
the past decades.  [IPCC 95, p 109-11, 148-9] [Ramaswamy] [Santer]

It is currently hopeless to draw conclusions from the observed tempe-
rature record about the present or future amount of greenhouse gas
induced warming.  (Nonetheless, this is attempted time and again ;-)
Apart from the amount of the eventual warming, its speed is uncertain
as well.  A given rate of warming does not by itself reveal when and
at what level the warming is eventually going to stop.  Moreover, the
effects of several factors cannot yet be disentangled.  Among these,
the presumably most important three are:

      human-made greenhouse gases                 warming
      human-made tropospheric aerosols            cooling
      natural climatic variability     cooling or warming

The geographic and vertical pattern of the temperature changes suggests
an influence from human-made greenhouse gases and aerosols as well as
from stratospheric ozone depletion [IPCC 95, chapter 8] [Ramaswamy]
[Santer] [Tett].  This is a far cry from quantifying the human influen-
ce, let alone the extent of future climate change.  Taking into account
numerous factors that can affect climate, climatologists can only say
that the observed changes are consistent with (though no proof for)
the estimated range of climate sensitivity to greenhouse gases.



Subject: 7.  Human-made tropospheric aerosols

Aerosols are tiny (0.001 to 10 micrometres) airborne particles.  In the
troposphere, the lower about 10 to 15 km of our atmosphere, human-made
aerosols have greatly increased since about 1850.  They present a large
source of uncertainty in assessing human influences on climate.

`Fine' aerosol particles with sizes between about 0.1 and 1 micrometre
can influence climate in two ways.  Under clear sky they scatter and
absorb solar radiation; some of the scattered sunlight goes back to
space (the direct effect).  Acting as cloud condensation nuclei, they
may enhance reflectivity and life-time of clouds (indirect effect).
Sulfur dioxide from fossil fuel burning, yielding sulfate particles
after oxidation, is presently the largest source of fine human-made
aerosols.  Another large source is organic and elemental carbon from
burning of tropical forests and savannahs.  Globally averaged, fine
human-made tropospheric aerosols may currently cancel about 50 % of
the warming effect of human-made greenhouse gases.  So far, though, the
uncertainty range is large, stretching from roughly 10 to 100 %.  [IPCC
94, sections 3, 4.4, 4.7] [IPCC 95, sections 2.3, 2.4.2] [Schwartz]

Moreover, global averages are misleading.  Even if the global averages
of aerosol and greenhouse gas forcing cancel, their different distri-
butions may cause climatic changes.  With life-spans of up to over
100 years, human-made greenhouse gases are fairly evenly distributed.
Most tropospheric aerosols are washed out after about a week, they are
unevenly distributed.  Human-made sulfate aerosols occur mainly down-
wind of northern industrialized areas.  Most biomass smoke rises from
tropical land areas during the dry season.  Cutting back sulfur dioxide
emissions or biomass burning reduces the aerosol load quickly, leaving
over the more longlived greenhouse gases.  [Andreae] [IPCC 94 and 95]

By the way, roughly one third of the tropospheric sulfate load has
natural precursors, mainly oceanic dimethyl sulfide (DMS) and volcanic
sulfur dioxide.  Violent volcanic eruptions, like Pinatubo 1991, give
rise to stratospheric sulfate aerosols which, being more long-lived
than their tropospheric cousins, tend to warm the stratosphere and to
cool the troposphere and surface for a few years. [IPCC 94, p 135-7,
141-4, 186-9] [IPCC 95, p 115-6, 148, 504-6]

'Coarse' aerosols with particle sizes between 1 and 10 micrometres
include mineral dust raised by wind blowing over dry soils.  Human
influences like over-cultivation and soil erosion may have up to
doubled the flux of mineral dust.  Mineral dust is most abundant over
North Africa, the Arabian Sea, and South Asia.  It scatters sunlight
and absorbs outgoing terrestrial infrared.  One study suggests that
these two effects largely cancel at the top of the atmosphere.  If so,
mineral dust has little effect on earth's overall radiation balance,
although it regionally cools the surface and warms the air, which in
turn may affect atmospheric circulation.  However, as with sulfate
aerosols and biomass smoke, there are large uncertainties. [Andreae]
[Sokolik] [Tegen]

Pinning down aerosol effects more precisely will be tough.  Aerosols
are hard to measure.  Size, shape, composition and regional distribu-
tion of the particles vary.  So do their effects on climate.  Aerosols
can cause not just local but also distant responses, because heat or
rather, in the case of many aerosols, coolness is transported by the
atmosphere and ocean.  Assessing the climatic effects of aerosols
involves modeling of regional climates and of clouds, both of which
are not yet very reliable.  [Andreae] [IPCC 94/95] [Peter] [Schwartz]
[Sokolik] [Wielicki, p 2127-29, 2146]



Subject: 8.  Ocean and response time

It is not known whether it will take decades or centuries until
equilibrium is approached for a given enhanced level of greenhouse
gases.  Much of this uncertainty stems from poorly known behavior of
the ocean.  The ocean covers about 70 % of the globe, it transports
large amounts of heat, and it is the major source of atmospheric
water vapor.  The atmosphere and land are affected by variations of
the ocean surface only, which in turn depend on the coupling between
the ocean surface and the deeper ocean.  With its huge heat capacity,
the ocean slows down climate change.  On the other hand, due to the
deep ocean's slow response, temperature may continue to rise for
centuries after stabilization of greenhouse gas levels.

The topmost so-called `mixed layer', being warmer and less dense than
the deeper layers, tends to stay on top.  Cool, particularly salty
(thus dense) surface water sinks and deep water forms in the northern
North Atlantic and near Antarctica.  Subsurface water wells up near
eastern margins of oceans.  For other regions of the ocean, the extent
to which surface and deeper waters are exchanged is less clear.
The replacement time for the deep ocean is many centuries.  In heat
capacity, a water column of about 2.5 m depth matches the atmosphere
lying above it.  Less than 2 m of water match an average land surface.
[Hartmann, p 84-5, chapter 7] [IPCC 95, p 210-4, 290].

How much heat will the ocean's deeper layers store before things get
really going ?   Already in a "stable" climate, ocean circulation is
likely to vary a good deal.  A changing climate may entail major
changes in ocean currents.  For instance, North Atlantic deep water
formation may decline or become more variable, which, in this region,
may inhibit warming or even produce cooling.  Unfortunately, not even
the ocean's present state is fully known.  This should improve over
the next decade, but tracking down natural variations lasting decades
to centuries may be not so easy.  Exchange processes between surface
and deeper layers of the ocean are among the ocean models' weaknesses.
Improving the models is difficult, as the dearth of observational data
hinders judging whether a given model behavior is reasonable [IPCC 95,
p 166-7, 210-4, 266-7, 302-4, 317, 346, 526, 530].  At this point,
the above question is unanswerable.

For illustration, imagine a CO2 rise to 560 ppmv (twice the preindus-
trial level) until about 2050, with CO2 remaining constant thereafter.
Assume that other greenhouse gases and human-made aerosols remain at
their 1990 levels.  For this scenario, 15 out of 16 leading US climate
scientists offered a best guess of between 2 and 4 o C surface warming
by the year 2300, with widely varying time responses.  The sixteenth
expert estimated 0.3 o C and didn't provide a time response.  By 2050,
9 of the 15 respondents expected roughly 50 to 70 % of the eventual
warming, in line with recent estimates from climate models [IPCC 95,
p 297-300].  The remaining 6 divided equally between swifter and
slower warming.  By 2100 most participants expected 80 % or more of
the eventual warming, two suspected a sluggish response of below 25 %
[Morgan, p 469A, 472A, figure 5].

These numbers shouldn't be taken too seriously, yet they highlight
the pickle.  By the way, all 16 researchers estimated some chance,
between 8 and 40 %, that uncertainty about climate sensitivity could
grow by a quarter or more after a 15-year research program [Morgan,
table 1].



Subject: 9.  Feedbacks: water vapor, ice and snow, clouds

If nothing except surface and air temperature changed (and if human-
made aerosols vanished), then a CO2 doubling would eventually warm
earth's surface by 1 to 1.2 o C  [Hartmann, p 231-2] [IPCC 95, p 30].
However, there are feedbacks, including though not confined to:

   water vapor feedback           probably positive
   ice-snow-albedo feedback       presumably positive
   cloud feedback                 poorly understood
   biological feedbacks           see next section

It is widely assumed that warming, which tends to enhance evaporation,
will increase the water vapor content of the troposphere.  This should
amplify the warming, as water vapor is the dominant greenhouse gas
[Hartmann, p 232-4] [IPCC 95, p 197, 200-1, 210].  [Lindzen] proposed
that, in warmer tropics, deep convective clouds might rain out more
thoroughly.  This might dry the tropical upper troposphere and curb the
tropical water vapor feedback.  The available data on spatial patterns
and short-term changes of upper-tropospheric humidity do not support
Lindzen's notion.  However, spatial and short-term variations need not
be reliable surrogates for global climate change.  The same data sug-
gest that some part of the feedback formerly ascribed to water vapor
may instead stem from lapse rate changes, the effects of which were
outlined in section 4 [Sinha] [Soden].

Snow and ice reflect much of the incident sunlight back to space,
thus a reduction of snow and ice cover is likely to enhance warming.
Details remain hazy.  Feedbacks between cloud cover and changes in
sea ice and snow cover are poorly understood.  Another hurdle is
the interplay between atmosphere, surface ocean, and sea ice, in
particular at the always present ice-free patches and near sea ice
margins [IPCC 95, p 156-7, 204, 214, 216, 267, 347].

The cloud feedback may be large, yet not even its sign is known.
Low clouds tend to cool, high clouds tend to warm.  High clouds tend
to have lower albedo and reflect less sunlight back to space than
low clouds.  Clouds are generally good absorbers of infrared, but
high clouds have colder tops than low clouds, so they emit less
infrared spacewards.  To further complicate matters, cloud properties
may change with a changing climate, and human-made aerosols may
confound the effect of greenhouse gas forcing on clouds.  With fixed
clouds and sea ice, models would all report climate sensitivities
between 2 and 3 o C for a CO2 doubling.  Depending on whether and
how cloud cover changes, the cloud feedback could almost halve or
almost double the warming  [Hartmann, p 68, 71-5, 249] [IPCC 94,
p 150-4, 183-5] [IPCC 95, p 34-5, 201-10, 345-6] [Wielicki].
A recent intercomparison of 15 climate models showed mostly small
to modest negative or positive cloud feedbacks.  Sadly, the validity
of this result is doubtful [IPCC 95, p 205-6].



Subject: 10.  The global carbon cycle.  Biological feedbacks

Here, it's tempting to list some numbers  :-)


           Gt    =  gigatonne  =  10**9 metric tonnes,
                 the mass of one cubic kilometre of water
      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

   Carbon reservoirs in GtC

      Atmosphere (1990)        750       Surface ocean            1000
      Terrestrial vegetation   600       Marine biota                3
      Soils & detritus        1600       Dissolved organic carbon  700
                                         Deep ocean              38000

   Natural carbon fluxes in GtC/year,  <-->  denotes a two-way flux

      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  <-->  deep ocean                100

   Human-made CO2 in GtC/year, average fluxes 1980-1989, estimated
   90 % confidence intervals in parentheses  [IPCC 95, p 79]

      Carbon dioxide sources:
         Fossil fuel burning, cement production      5.5  (5.0-6.0)
         Changes in tropical land use                1.6  (0.6-2.6)
         Total emissions                             7.1  (6.0-8.2)

      Partitioning among reservoirs:
         Storage in the atmosphere                   3.3  (3.1-3.5)
         Oceanic uptake                              2.0  (1.2-2.8)
         Northern Hemisphere forest regrowth         0.5  (0.0-1.0)
         Other terrestrial sinks: CO2 fertilization,
            N fertilization, climatic variations     1.3 (-0.2-2.8)


Except for atmospheric CO2, carbon reservoirs and natural fluxes are
hard to measure.  Their estimates vary somewhat across the literature.
Carbon enters and leaves the atmosphere largely as CO2.  Other fluxes
involve various carbon compounds.  The above irreverently lumps land
animals with soils and detritus, and it omits 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. [IPCC 95, chapters 2.1, 9, 10] [Butcher, chapter 11]
[Siegenthaler]

CO2 uptake by land plants through photosynthesis is roughly balanced
by plant and soil respiration.  Depending on whether photosynthesis
exceeds or falls below respiration, the net result is CO2 drawdown
or CO2 release.  Today, photosynthesis is probably slightly ahead.
In future, climatic changes or rising CO2 level may trigger feedbacks
that curb or speed up the rise of atmospheric CO2.  To name a few:
CO2 fertilization should promote photosynthesis and draw down some CO2,
as long as respiration doesn't catch up.  Warming may stimulate or
slow down both photosynthesis or respiration, depending, among others,
on soil moisture.  The mix of species in ecosystems is likely to shift,
which in turn may affect atmospheric CO2.  Dieback of vegetation can
release CO2.  The overall effect of these and other feedbacks is hard
to tell.  Ecosystem models tentatively suggest that carbon storage in
vegetation and soils may eventually win out.  Temporarily, however,
carbon may be released, especially if large and rapid changes should
cause forests to die back.  [IPCC 95, chapters 2.1 and 9]

Turning to the ocean, a sea surface warming of 1 o C may increase
atmospheric CO2 by up to 10 ppmv through degassing [IPCC 94, p 57].
More importantly, marine life, in spite of its low biomass, takes
up and releases about 50 Gt of carbon annually.  Marine biological
production occurs largely in the sunlit surface and is thought to be
limited mostly by nitrogen.  Surface nutrient supplies are replenished
primarily through transport from deeper ocean layers.  (In the open

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