From jan.schloerer@medizin.uni-ulm.de Mon Aug 25 13:30:36 EDT 1997
Article: 147065 of sci.environment
From: jan.schloerer@medizin.uni-ulm.de (Jan Schloerer)
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Subject: Climate change: some basics
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Summary: Nontechnical outline of some basic climate change topics,
         including natural and enhanced greenhouse effect, feedbacks,
         aerosols, natural climatic variability, and ice cores.
Keywords: climate change, greenhouse effect, radiation balance,
         aerosols, carbon cycle, natural variability, past climates
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Archive-name: sci/climate-change/basics
Version: 2.02
Last-modified: 05 April 1997
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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
ocean, iron can be limiting; it enters the ocean mainly in airborne
dust and via rivers.)   The export of organic carbon from the surface
to deeper ocean layers, known as the biological pump, is not or little
affected by CO2 availabilility, but it may be affected by changes in
temperature, cloud cover, ocean currents, nutrients availability,
or ultraviolet radiation.

These and other marine biological processes are complex.  Researchers
cannot yet say how they will respond to disturbances.  It has been
estimated that, with no biological pump, preindustrial atmospheric CO2
would have been 450 instead of 280 ppmv, whereas a marine life seizing
all available surface nutrients could have lowered this to 160 ppmv.
On the other hand, preliminary results suggest that changes in the
biological pump may affect atmospheric CO2 only by 10s rather than
100s of ppmv.  [IPCC 94, p 57-8] [IPCC 95, p 79-80, chapter 10]

Biological feedbacks on climate are not limited to the carbon cycle.
For instance, dimethyl sulfide (DMS) from the ocean is a major natural
source of tropospheric sulfate aerosols.  Shifts in DMS production may
affect marine cloud cover and surface temperature.  DMS production is
hard to predict, because it depends, among many others, on the local
biomass and mix of species.  [IPCC 95, p 488, 504-6]

Back to the land, spreading of boreal forest into tundra may lead to
warmer winters.  Trees protrude above the snow-covered ground, they
reflect less sunlight back to space than snow-covered tundra.  During
and after deglaciation, the expansion of boreal forests amplified the
warming of northern land areas.  The reverse process, displacement
of boreal forest by tundra, probably played a role in the onset of the
last glaciation.    For another example, rising CO2 tends to improve
the water-use efficiency of vegetation.  Plants may then release less
water vapor to the ambient air.  Regionally, this may warm the surface
and affect precipitation and soil moisture.  [Gallimore] [IPCC 95,
p 217-21, 450, 469-71]

These few illustrations should do to show that, for better or for
worse, human land-use changes like de- or reforestation can make
a difference.



Subject: 11.  Natural climatic variability

What course would earth's temperature have taken without human
influences ?   We don't know.  [Burroughs] opens his intriguing book
on weather cycles:   "The history of meteorology is littered with
whitened bones of claims to have demonstrated the existence of
reliable cycles in the weather."   Too little is known about natural
climatic fluctuations lasting decades to centuries.

Some players that may cause climatic variations on this time scale:
atmospheric variability including shifts of the polar front, varia-
tions in the circulation of the North Atlantic and Pacific Ocean,
solar variability, volcanism.  During the Holocene, the past about
10,000 years, these factors, taken together, probably did not cause
global mean surface temperature changes exceeding 1 o C [Rind].
Unraveling climate's natural vagaries may take a long time, because
sufficiently long and detailed climatic records are scarce [IPCC 95,
p 173-4, 180-1, 411, 418-21].

The Little Ice Age, from about 1450 to the 19th century, and the
Medieval Warm Period, from perhaps the 9th to the 14th century, are
cases in point.  The data, including historical, tree ring, coral
and ice core records, are gappy, in particular for the tropics and
southern oceans.  The global patterns of the climatic changes and
the mechanisms behind these changes are not yet known.  Formerly it
was presumed that both the Medieval Warm Period and the Little Ice
Age were globally more or less uniform.  Now the available data begin
to suggest that no major globally synchronous cool or warm period
occurred during the past millennium.  Instead, asynchronous regional
coolings and warmings appear to have been common.  [Bradley] [Crowley
& North, chapter 5] [Hughes] [IPCC 95, p 174-7] [Overpeck] [Rind]

For illustration, summers in northwest Sweden were, by and large,
warmer than their 1860-1959 mean between AD 1000 and 1200 and, again,
between 1400 and 1550.  From 1200 to 1400, summers tended to be cooler.
Year-round sea surface temperatures in the Sargasso Sea appear to have
taken a similar course.  On the other hand, summer temperatures over
the northern Urals show more or less the opposite pattern with cool
summers around AD 1000 and warm summers between 1200 and 1400 [Briffa]
[Keigwin 96].  Over Northern Hemisphere land areas, summers tended to
be cool during the 16th, 17th and 19th century, though with strong
regional differences.  Chinese summers, for instance, were unusually
cool around 1650.  This spell was weaker over the northern Urals and
at other Arctic sites, it is absent or barely noticeable in a central
European and in some North American records [Bradley] [Briffa].

There are not yet enough data to tell whether the so-called Medieval
Warm Period, globally averaged, was warmer than the Little Ice Age,
let alone the present century.  The Little Ice Age, though not a
globally synchronous cooling spell, was probably, on average, cooler
than the last hundred years [Bradley] [Hughes] [IPCC 95, p 174].
The warming since around 1900 appears to be one of the globally
most uniform temperature shifts during, at least, the past several
centuries [Crowley & North, p 99] [Overpeck].

Several clues suggest a decline of solar activity during the Maunder
Minimum (about 1645-1715), amounting to a radiative forcing of
somewhere between -0.5 and -1.5 W/m**2.  Decline and subsequent rise
of solar activity to its present level may have contributed to the
Little Ice Age and to the warming thereafter.  Solar forcing since
1850 has been tentatively estimated at between +0.1 and +0.5 W/m**2.
[IPCC 94, p 189-92, 194] [IPCC 95, p 115-8]

Without knowing natural climatic variations reasonably well, elucida-
ting their causes is difficult.  Even the causes of wellknown events
can be hard to identify.  1976-77 the behavior of El Nino-Southern
Oscillation appears to have changed.  El Nino episodes got more fre-
quent, sea surface temperatures in the tropical Pacific tended to be
high, precipitation over the tropics and subtropics from Africa to
Indonesia declined.  While some model results suggest that greenhouse
gas induced climate change may look similar, it is still open whether
this was incipient climate change or a natural fluctuation [IPCC 95,
p 153-55, 165] [Meehl].



Subject: 12.  Ice record of greenhouse gases and last glaciation

During the past millennium, until about the 19th century, atmospheric
greenhouse gas levels varied little and hence, during that time,
probably contributed little to climatic variations.  On a longer time
scale, changes of greenhouse gas levels probably contributed signifi-
cantly to the coolings and warmings of the last two glacial cycles.

Ice cores from Greenland and Antarctica indicate that there was a close
link between greenhouse gases and temperature [Raynaud].  For instance,
the Vostok ice core from Antarctica exhibits a striking correlation
between temperature and the concentrations of carbon dioxide (CO2)
and methane (CH4) over the past 220,000 years [Jouzel].  The level of
nitrous oxide (N2O) probably also varied more or less in phase with
temperature [Raynaud, p 928].  The variations of these trace gases
may account for up to about 50 % of the estimated temperature changes
[Crowley, p 2364] [Raynaud, p 932].  CO2 was most important, while
methane and nitrous oxide contributed less.

During the onset of the last glaciation, the CO2 decrease markedly
lagged the onset of the cooling.  During the past two deglaciations,
CO2 may have risen in phase with temperature or with an, in geologic
terms, modest lag of up to about 1000 years [Raynaud, p 931].  Whether
greenhouse gases led or lagged the climatic change, that is, whether
they were a primary cause for the change or whether they acted as a
positive feedback (which amplified a climatic shift already under way),
is important for finding out just exactly what happened, but it is not
by itself relevant for estimating the effect of the trace gases on
surface temperature  [Raynaud, p 932].

In spite of this, the effect is hard to quantify.  During the last
deglaciation, roughly 18,000 to 10,000 years ago, the rise of trace
gas levels amounted to a radiative forcing of about 2.5 to 3 W/m**2.
The meltdown of the huge glacial ice shields reduced earth's albedo,
accounting for another perhaps 3 to 3.5 W/m**2.  These figures are
compatible with the IPCC estimate of about 1.5 to 4.5 o C surface
warming for a CO2 doubling.  They do not permit to narrow down the
uncertainty, there remain many unknowns [Crowley, p 2366].

Perhaps most important: How cold was the last ice age ?  This is not
yet clear.  Tropical oceans, for instance, may have been between 1
and 5 o C cooler than they are now [IPCC 95, p 173-4], and Greenland
may have been several degrees colder than previously thought [Cuffey].
Another point to keep in mind: The sensitivity of earth's present
climate and the sensitivity of the last glacial maximum's climate
to a radiative forcing of so or so many W/m**2 need not be equal.
The starting positions differ.

Glaciations and deglaciations are triggered by variations in earth's
orbit.  Tilt of earth's axis, season of the perihelion (closest ap-
proach of earth to sun, now in January), and eccentricity of earth's
elliptical orbit vary.  These variations cause, among others, changes
in high northern latitude summer insolation, which are critical for
the waxing and waning of ice sheets.  Northern summer insolation was
unusually low at the onset of the last glaciation around 115,000 years
ago, it was high during deglaciation.

The direct effect of the "orbital trigger" was too small to cause
glaciation or deglaciation.  Instead, a cascade of feedbacks and
interacting processes with widely varying timescales led to the final
result.  Shifts in atmospheric or oceanic circulation may occur within
decades.  Southward spread of tundra or poleward expansion of boreal
forests can take centuries to millennia.  Over 10,000s of years,
the weight of an ice sheet depresses the underlying bedrock, which
eases melting.  Many twists of the story, like the frequent partial
breakdowns of ice sheets, remain enigmatic, even though the trigger
and the gist of the eventual outcome are known [Crowley & North,
chapters 6-7] [Eddy, chapters 17 & 21] [Gallimore] [IPCC 95, p 177-9]
[Keigwin 95].  In today's climate change gamble even the trigger or,
rather, its aerosol component is poorly known.

As we are at it:  For the next 25,000 years, high northern latitude
summer insolation will not drop anywhere near its minimum of 115,000
years ago [Eddy, p 40-41].



Subject: 13.  Conclusion

                    We need to know just about everything.   ...
           Is climate system modelling the ultimate example of hubris,
           or, by chopping away at areas of ignorance, will we truly
           improve our predictive capability ?

                                 David Rind,  Nature 363 (1993), 312

Current climate models tend to predict gradual climate change.  This
is no guarantee against unpleasant surprises.  Climate models as well
as the knowledge fed into the models are far from perfect [IPCC 95,
p 416-8, chapters 2, 4-6, 9-11] [Morgan] [Wielicki].  Rapid changes in
atmospheric circulation, of ocean currents, in ecosystem functioning,
or in the West Antarctic ice sheet's behavior may not be likely,
yet such risks can, at present, neither be excluded nor quantified.
[IPCC 95, p 45-6, 213, 304, 389, 525, 527-8] [Morgan]

Vice versa, sudden climatic shifts during the last ice age [IPCC 95,
p 177-9] [Keigwin 95] do not imply that similar shifts must necessarily
happen in the near future: during glaciation the ice sheets were much
larger and less stable than they have been for the past 10,000 years.
Past climates help to understand the climate system's workings, but
they do not readily reveal what to expect.  Our climate seems to be
headed for a  "warm atmosphere-cold pole combination"  which may be
unique in earth history.  No completely satisfactory geologic analog is
known [Crowley & North, chapter 14] [Eddy, p 17-27, 39-71] [Overpeck].

Much of the public debate focuses on warming, an admittedly likely
reaction of the climate system.  Disturbing earth's radiation balance,
however, may change the climate in a host of other potentially serious
ways.  Warming need not even be the practically most relevant part
of the response.  This is why many climatologists prefer the term
`climate change' over `global warming'.

For example, spatial and seasonal patterns of precipitation, evapora-
tion, soil moisture and river runoff may shift.  These in turn may
affect agriculture and freshwater availability, which are critical
for many poor countries and a potential source of migrations and
conflicts.  Cloud patterns, ocean currents, atmospheric circulation
or the distribution of extreme weather events may change.  Terrestrial
and marine life will be affected and may in turn affect the climate
via changes, for instance, of carbon storage, evaporation, or albedo
[IPCC 95, chapters 9-10].  The risk of rapid climate change is linked
to many other problems of concern, like population growth, poverty,
loss of biodiversity, or stratospheric ozone depletion.

Building a balanced public perception of the risks posed by climate
change is difficult.  There is an almost irresistible temptation
to view extreme weather events, like droughts or storms, as signs
of climate change, even if they are well within the limits of natural
variability.  At the same time, gradual change tends to go unnoticed.
Natural climatic variability can lead to temporary coolings; these
would be perceived as all-clears by many.  We are up against a long-
distance race and tend towards a sprinter's outlook. [Maunder, p 75]

Human-made greenhouse gases and aerosols will change our climate.
There is no free lunch, we cannot alter earth's radiation balance
for nothing.  It is uncertain by how much, how swiftly and with what
twists the climate will change.  This is dubious comfort, since
uncertainty cuts two ways.  The present best estimates may well
overstate the risk, but they may as well understate it.  Climate
change resembles a gamble with high stakes.

Current knowledge of the carbon cycle suggests that atmospheric CO2
will respond sluggishly to CO2 emissions changes  [IPCC 95, p 82-5,
323].  The response of the climate system to a given CO2 level takes
decades or longer.  Barring surprises, the lag time between changes
in CO2 emissions and their eventual effects on climate is very long.

It is an open question how soon the uncertainties can be narrowed
down, and whether climatologists will be able to predict details
reliably before they start to happen in the real world [IPCC 95]
[Morgan].   There is a natural inclination to wait and see until
we know what we shall have to face.  By then it may be too late.



Subject: 14.  Further reading.  References

Introductory articles, mainly on questions not addressed here:
    [Schelling]   pensive, allround:  science, impacts, responses
    [Ausubel]     potential impacts:  critical, though not complacent
    [Morgan]      presents the scientific pickle in a nutshell
    [Trenberth]   skills and limits of climate models
    [White]       history, some basics, climate debate up to 1990

For details, you might try [Houghton, chapters 1-7] or, if you want
to dig deeply, the reports [IPCC 90/94/95] by Working Group I of the
Intergovernmental Panel on Climate Change (IPCC).  Working Groups II
and III address impacts and responses [IPCC 95 II/III].  For the
physics of climate, [Hartmann] is a moderately technical starter,
while the professionals often turn to the more rigorous [Peixoto].


[Andreae]   Meinrat O. Andreae,  Raising dust in the greenhouse.
    Nature 380 (1996), 389-390
[Ausubel]   Jesse H. Ausubel,  A second look at the impacts of
    climate change.   American Scientist 79 (1991), 210-221
[Berger]   A. Berger  and  Ch. Tricot,  The greenhouse effect.
    Surveys in Geophysics 13 (1992), 523-549
[Bradley]   Raymond S. Bradley  and  Philip D. Jones,  `Little Ice Age'
    summer temperature variations: their nature and relevance to recent
    global warming trends.   The Holocene 3 (1993), 367-376
[Briffa]   Keith R. Briffa,  Philip D. Jones,  Fritz H. Schweingruber,
    Stepan G. Shiyatov  &  Edward R. Cook,  Unusual twentieth-century
    summer warmth in a 1,000-year temperature record from Siberia.
    Nature 376 (1995), 156-159
[Burroughs]  William James Burroughs, Weather Cycles: Real or Imaginary?
    Cambridge University Press 1992
[Butcher]   Samuel S. Butcher, Robert J. Charlson, Gordon H. Orians
    &  Gordon V. Wolfe, eds,  Global Biogeochemical Cycles.
    San Diego, CA, Academic Press 1992
[Crowley]   Thomas J. Crowley,  Geological assessment of the greenhouse
    effect.    Bulletin of the American Meteorological Society 74
    (1993), 2363-2373
[Crowley & North]   Thomas J. Crowley,  Gerald R. North,
    Paleoclimatology.   Oxford University Press 1991
[Cuffey]  Kurt M. Cuffey, Gary D. Clow, Richard B. Alley, Minze Stuiver,
    Edwin D. Waddington, Richard W. Saltus,   Large Arctic temperature
    change at the Wisconsin-Holocene glacial transition.   Science 270
    (1995), 455-458.   Also:  Doug MacAyeal, ibid. 444-445.  Richard
    Kerr, Science 272 (1996), 1584-1585
[Eddy]   J.A. Eddy  and  H. Oeschger (eds),  Global Changes in the
    Perspective of the Past.  Chichester, UK, John Wiley & Sons 1993
[Gallimore]   R.G. Gallimore  &  J.E. Kutzbach,   Role of orbitally
    induced changes in tundra area in the onset of glaciation.
    Nature 381 (1996), 503-505.   Also:  Mark Chandler,
    Trees retreat and ice advances,  ibid. 477-478
[Hartmann]   Dennis L. Hartmann,  Global Physical Climatology.
    San Diego, CA, Academic Press 1994
[Houghton]   John Houghton,  Global Warming: The Complete Briefing.
    Lion Publishing,  Oxford, UK / Elgin, Illinois, US  1994.
    Albatross Books,  Sutherland, Australia  1994
[Hughes]   Malcolm K. Hughes  and  Henry F. Diaz,
    Was there a `Medieval Warm Period', and if so, where and when ?
    Climatic Change 26 (1994), 109-142
[Hurrell 96]   James W. Hurrell  and  Kevin E. Trenberth,
    Satellite versus surface estimates of air temperature since 1979.
    Journal of Climate 9 (1996), 2222-2232.   Also at:
    http://www.cgd.ucar.edu:80/cas/papers/jclim96/
[Hurrell 97]   James W. Hurrell & Kevin E. Trenberth,  Spurious trends
    in satellite MSU temperatures from merging different satellite
    records.   Nature 386 (13 March 1997), 164-167 
[IPCC 90]   Climate Change  -  The IPCC Scientific Assessment
    J.T. Houghton et al., eds,   Cambridge University Press 1990
[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
[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
[IPCC 95 III]   Climate Change 1995:  Economic and Social Dimensions
    of Climate Change.   James P. Bruce et al., eds,   Cambridge
    University Press 1996.
[Jones]   M.D.H. Jones  and  A. Henderson-Sellers,
    History of the greenhouse effect.
    Progress in Physical Geography  14, 1 (1990), 1-18
[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
[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
[Keigwin 95]   Lloyd D. Keigwin,  The North Pacific through
    the millennia.    Nature 377 (1995), 485-486
[Keigwin 96]   Lloyd D. Keigwin,  The Little Ice Age and Medieval Warm
    Period in the Sargasso Sea.   Science 274 (29 Nov 1996), 1504-1508
[Lindzen]   R.S. Lindzen,   Climate dynamics and global change.
    Annual Reviews of Fluid Mechanics 26 (1994), 353-378
[Maunder]   W. John Maunder,  Dictionary of Global Climate Change.
    London, UCL Press / New York, Chapman and Hall 1992
[Meehl]   Gerald A. Meehl  and  Warren M. Washington,  El Nino-like
    climate change in a model with increased atmospheric CO2 concen-
    trations.   Nature 382 (1996), 56-60
[Morgan]   M. Granger Morgan,  David W. Keith,
    Climate change: Subjective judgments by climate experts.
    Environmental Science & Technology 29, 10 (1995), 468A-476A
[Overpeck]   Jonathan T. Overpeck,  Paleoclimatology and climate system
    dynamics.  Reviews of Geophysics 33, Supplement (July 1995), 863-871
[Peixoto]   Jose Peixoto  and  Abraham H. Oort,   Physics of Climate.
    Institute of Physics Publishing, 1992
[Peter]   Thomas Peter,  Airborne particle analysis for climate studies.
    Science 273 (1996), 1352-1353
[Ramaswamy]   V. Ramaswamy, M.D. Schwarzkopf & W.J. Randel,  Fingerprint
    of ozone depletion in the spatial and temporal pattern of recent
    lower-stratospheric cooling.   Nature 382 (1996), 616-618
[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
[Rind]   David Rind & Jonathan Overpeck,  Hypothesized causes of decade-
    to-century-scale climate variability:  climate model results.
    Quaternary Science Reviews  12 (1993), 357-374
[Santer]   B.D. Santer, K.E. Taylor, T.M.L. Wigley, T.C. Johns, P.D.
    Jones, 8 more authors,  A search for human influences on the thermal
    structure of the atmosphere.   Nature 382 (1996), 39-46
[Schelling]   Thomas C. Schelling,  Some economics of global warming.
    The American Economic Review  82 (March 1992),  1-14
[Schwartz]   Stephen E. Schwartz and Meinrat O. Andreae,  Uncertainty
    in climate change caused by aerosols.  Science 272 (1996), 1121-22
[Siegenthaler]   U. Siegenthaler & J.L. Sarmiento,  Atmospheric carbon
    dioxide and the ocean.   Nature 365 (1993), 119-125
[Sinha]   Ashok Sinha,  Relative influence of lapse rate and water
    vapor on the greenhouse effect.  Journal of Geophysical Research
    100 (1995), 5095-5103
[Soden]   Brian J. Soden and Rong Fu,  A satellite analysis of deep
    convection, upper-tropospheric humidity, and the greenhouse effect.
    Journal of Climate 8 (1995), 2333-2351
[Sokolik]  Irina N. Sokolik & Owen B. Toon,  Direct radiative forcing by
    anthropogenic airborne mineral aerosols.  Nature 381 (1996), 681-683
[Tegen]   Ina Tegen,  Andrew A. Lacis  &  Inez Fung,  The influence
    on climate forcing of mineral aerosols from disturbed soils.
    Nature 380 (1996), 419-422
[Tett]   Simon F.B. Tett, John F.B. Mitchell, David E. Parker, Myles
    R. Allen,  Human influence on the atmospheric vertical structure:
    detection and observations.   Science 274 (15 Nov 1996), 1170-1173
[Trenberth]   Kevin E. Trenberth,  The use and abuse of climate models.
    Nature 386 (13 March 1997), 131-133
[White]   Robert M. White,  The great climate debate.
    Scientific American 263, 1 (July 1990), 18-25
[Wielicki]   Bruce A. Wielicki, Robert D. Cess, Michael D. King,
    David A. Randall, and Edwin F. Harrison,  Mission to Planet Earth:
    Role of clouds and radiation in climate.  Bulletin of the American
    Meteorological Society 76 (1995), 2125-2153



Subject: 15.  Some web sites

If this article is too technical for your taste, you might try
the introduction by Granger Morgan, Tom Smuts, and others at
      http://www.gcrio.org/gwcc/toc.html
  
The quarterly _Consequences_, edited by John A. Eddy, has readable
articles by first rate scientists on past climates, climate models,
and more.  Published by Saginaw Valley State University, Michigan.
      http://www.gcrio.org/CONSEQUENCES/introCON.html

For summaries of the 1995 IPCC reports see
      http://www.unep.ch/ipcc/ipcc95.html

UNEP's Information Unit on Climate Change (IUCC) at Geneva offers
concise fact sheets covering science, impacts and responses:
      http://www.unep.ch/iucc/fs-index.html

Some entry points to the myriad of research and other web sites:
      http://www.epa.gov/globalwarming/
      http://climate.gsfc.nasa.gov/
      http://gcmd.gsfc.nasa.gov/            (Global Change)
      http://www.ncdc.noaa.gov/
      http://www.nerc-bas.ac.uk/public/icd/wmc/met.links.html
      http://www-eosdis.ornl.gov/
      http://www.ucar.edu/dss/faq/          (Meteorology FAQ)

Some sites linked to sci.environment 
      http://grumbinescience.org/radix/climate/
Introduction to sea level change and ice sheets by Robert Grumbine,
Robert Parson's Ozone Depletion FAQ, Torsten Brinch's FAQ on ground
level ozone, an article on atmospheric CO2, and more.



Subject: 16.  Acknowledgements. Administrivia. How to get this file

Acknowledgements:  My wife Rosemarie and Dave Halliwell patiently and
friendly endured an inordinate amount of murky drafts.  Michael Tobis,
Robert Grumbine, Paul Farrar, and many others helped with explanations,
comments, and suggestions.

Caveat:  This is not my field.  Those climatologists who told me their
opinion so far found the article reasonable.  Sole responsibility for
errors and misconceptions is mine, though.  Corrections are welcomed,
the more so as time for maintaining this article is scarce.  However,
please note the motto:  "Not overly detailed" ;-)    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) 1997 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.

How to get this file:  Among others, this article is archived at

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Jan Schloerer                   jan.schloerer@medizin.uni-ulm.de
Uni Ulm    Biometrie & Med.Dokumentation    D-89070 Ulm, Germany