http://www.chem.brown.edu/chem12/globalwarming/greenhousbasics.html#The%20global%20carbon%20cycle.%20Biological%20feedbacks
Subject: Climate change: some basics
All FAQs in Directory: sci/climate-change
All FAQs posted in: sci.environment
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. Contents
1. Contents
2. Introduction
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: 2. 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
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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.
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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.
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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].
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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