Since the famous work of Arrhenius in 1896 (1),
the possibility of a net warming of the global climate due to increases
in atmospheric carbon dioxide
(CO2 ) produced by the burning of fossil-fuel
has been recognized. The subject matured with the publication in 1967 by
Manabe & Wetherald (2)
of the first fully self-consistent model calculation of this greenhouse
warming effect. They used a simple one-dimensional (altitude only)
model of the global atmosphere. In the three decades since, a tremendous
amount of observational, theoretical, and modeling research has been directed
at the climate system and possible changes in it due to human activity.
This research strongly demonstrates that potential climate changes are
projected to occur that are well worth our collective attention and concern.
This considerably strengthened climate knowledge base has energized proposals
for aggressive international efforts to mitigate the impact of greenhouse
warming by substantially reducing the use of fossil fuels to supply the
world's growing need for energy. However, that same research effort has
shown that, in projecting future climate changes, remaining scientific
uncertainties are significant. These uncertainties are regarded by many
as good reason to be extremely cautious in implementing any policies designed
to reduce CO2 emissions. Others, however,
argue that the risks of inaction are very large and that the scientific
uncertainties include the possibility that the greenhouse warming problem
could well be worse than current best estimates. Thus, serious policy disagreements
can be amplified by differing perspectives on the current state of greenhouse
warming science.
Some Fundamental Aspects of Greenhouse Warming Science
The earth is strongly heated every day by incoming radiation from the
sun. This heating is offset by an equally strong infrared radiation leaving
the planet. Interestingly, if Earth were without any atmosphere, and if
its surface reflectivity did not change, global-mean surface temperature
would be roughly 33°C colder than it is today. This large difference
is due to the strong atmospheric absorption of infrared radiation leaving
the earth's surface. The major atmospheric infrared absorbers are clouds,
water vapor, and CO2. This strong infrared
absorption (and strong reemission) effect is extremely robust: It is readily
measured in the laboratory and is straightforwardly measured from earth-orbiting
satellites.3 Simply put,
adding CO2 to the atmosphere adds another
"blanket" to the planet and, thus, directly changes the heat
balance of the earth's atmosphere.
Individuals skeptical about the reality of global warming have correctly
noted that, in terms of direct trapping of
outgoing infrared radiation, water vapor is by far the dominant
greenhouse gas on earth. Since water vapor dominates the current radiative
balance, how can it be that CO2 is anything
other than a minor contributor to earth's absorption of infrared radiation?
Part of the answer comes from the well-known modeling result from infrared
spectroscopy that net planetary radiative forcing changes roughly linearly
in response to logarithmic changes in CO2
.3 Thus, a quadrupling
of CO2 gives another roughly 1°C direct
warming over the direct 1°C warming for a CO2
doubling, valid for the extreme assumption that water vapor mixing ratios4
and clouds do not change. Interestingly, this approximate relationship
also holds for a large extended range as CO2
is decreased (see footnote 3).
It is thus hard to escape the conclusion that CO2
provides a measurable direct addition to the atmospheric trapping of infrared
radiation leaving the surface of our planet. However, a simple comparison
of the relative greenhouse efficiencies of water vapor and CO2
quickly becomes problematic because water vapor enters the climate system
mostly as a "feedback" gas. All models and observations currently
indicate that as climate warms or cools, to a pretty good approximation,
the observed and calculated global-mean relative humidity of water vapor
remains roughly constant as the climate changes, whereas its mixing ratio
does not.5 Thus, as climate
warms (cools), the holding capacity of atmospheric water vapor increases
(decreases) exponentially. This is a powerful water vapor positive feedback
mechanism- that is, a process that acts to amplify the original warming
caused by increasing CO2 levels. With this
major positive feedback, the modeled "climate sensitivity" 6
increases by about a factor of three, to roughly 3°C. Lindzen (3)
hypothesized that this water vapor feedback effect could actually be negative
in the upper troposphere. If this were the case, then the water vapor positive
feedback amplifying effect would be roughly one third to one half less
than that currently projected. A conceptual difficulty with making this
hypothesis work is that the relative humidity of the upper troposphere
must then get sharply and progressively lower as the lower troposphere
warms up and moistens in response to the added infrared absorbers. Conversely,
the relative humidity of the upper troposphere must get progressively higher
if something were acting to cool the planet. In effect, this hypothesis
states that the dynamical behavior of the atmosphere would change strongly
in response to altered infrared absorbers (see footnote 4).
Currently, observational evidence remains generally consistent with the
modeling results that project a strong positive water vapor mixing ratio
feedback under approximate constancy of relative humidity as the climate
changes (4, 5).
The quality of water vapor data in the upper troposphere, however, is not
particularly good, and none of the current observational tests can definitively
address the issue at hand- how the water vapor feedback might work a century
from now.
The basic story of human-induced greenhouse warming remains simple. Increased
infrared absorptivity due to increasing CO2
and other trace gases produces a net heating effect on the earth's surface,
due mainly to increased downward infrared radiation. The effect is not
dissimilar to the suppression of nighttime cooling when there is cloud
cover or a very humid weather pattern. The positive feedback effect of
water vapor acts to amplify the warming effect, both locally and globally.
An additional, but smaller, positive feedback is the relationship between
ice (or its absence) at the earth's surface and its reflectivity (albedo)
of solar radiation. In essence, if ice or snow cover melts, the surface
left exposed (ground, vegetation, or water) is generally less reflective
of incoming solar radiation. This leads to more absorption of the solar
radiation, thus more warming, less ice, and so on.
Inclusion of this "ice-albedo" feedback process in mathematical
models of the climate amplifies further the calculated warming response
of the climate to increased concentrations of CO2
and infrared absorbing gases; it also amplifies any calculated cooling.
Other kinds of feedbacks, both positive and negative, result from interaction
of land surface properties (e. g. changes of vegetation that lead to albedo
and evaporation changes) with climate warming/ cooling mechanisms or from
changes in CO2 uptake by the biosphere.
The major source of uncertainty in determining climate feedback concerns
the impact of clouds on the radiative balance of the climate system.7
A CO2-induced increase in low clouds mainly
acts to reflect more solar radiation and thus would provide a negative
feedback to global warming. An increase in high clouds mainly adds to the
absorption of infrared radiation trying to escape the planet and would
thus provide a positive feedback. A change in cloud microphysical and optical
properties could go either way. Which of these would dominate in an increasing-CO2
world? We are not sure. Our inability to answer this question with confidence
is the major source of uncertainty in today's projections of how the climate
would respond to increasing infrared-absorbing gases. Furthermore, it is
not likely this cloud-radiation uncertainty will be sharply reduced within
the next 5 years, no matter what promises are offered, expectations are
stated, or claims are made.
Although clouds dominate the climate modeling uncertainty, other key processes
are also in need of improved understanding and modeling capability. An
example is the effect of human-produced airborne particulates (aerosols)
composed mostly of sulfate (from oxidation of the sulfur in fossil fuels)
and carbon (from open fires). Sulfate aerosols are mostly reflective of
solar radiation, producing a cooling effect, whereas carbonaceous aerosols
mostly absorb solar radiation, producing a net heating effect. Efforts
to reduce the current uncertainty are limited by inadequate measurements.
Even more uncertain are the so-called indirect effects of atmospheric aerosols.
By indirect effect we mean the uncertain role the presence of these aerosols
plays in the determination of cloud amounts and their optical properties.
Another key uncertainty lies in modeling the response of the ocean to changed
greenhouse gases. This affects the calculated rate of response of the climate
over, say, the next century, as well as the possibility of changed ocean
circulation, a potential major factor in shaping regional climate changes.
A frequently overlooked aspect of the human-caused greenhouse warming problem
is its fundamentally very long timescales. The current rate of adding to
the CO2 concentrations of the atmosphere
is a bit more than half a percent per year. Thus, the time required for
CO2 amounts to approach twice preindustrial
levels is roughly a century or so, a process well underway (now about 30%
higher). Also, the climate is not expected to respond quickly to the added
CO2 because of the large thermal inertia
of the oceans. This effect can produce delays in the realized warming on
timescales ranging from decades to centuries. Moreover, the deep ocean
carries over a thousand years of thermal "memory." Thus, it will
take a long time for this problem to reach its full potential.
This great inertia in the climate is also a big factor at the other end
of the problem. What if we get a climate we do not like and want our "normal"
one back? Currently, the apparent net atmospheric lifetime of fossil-fuel-produced
CO2 is about three quarters of a century.
Thus, the natural drawdown of the extra CO2
would take a long time. Also, the gradually warmed ocean would take a long
time to give up its accumulated heat in a climate that had been given a
chance to return toward its essentially undisturbed state.
3Scientists
at GFDL recently performed simple one-dimensional radiative/ convective
model calculations of the effects of reducing CO2.
The log-linear relationship has been found to hold down to CO2
concentrations to as low as one sixty-fourth of preindustrial
levels. As CO2 is
decreased, the atmosphere's ability to hold water vapor collapses and the
global temperatures drop sharply.
4
Relative humidity is the ratio (in percentage) of the vapor pressure of
air to its saturation vapor pressure. The saturation vapor pressure of
air, determined from the Clausius-Claperon equation of classical thermodynamics,
is a strong exponential function of temperature, roughly doubling for each
10°C. Water vapor mixing ratio is the mass of water vapor of air divided
by the mass of dry air; it is generally conserved for a few days following
an air parcel when no condensation is present.
5
Relative humidity (see footnote 3) is determined in the troposphere by
the interplay among evaporation at the earth's surface, upward transfer
of water vapor (by small-scale turbulence, thunderstorm-scale moist convection,
large-scale rising motion), and net removal by precipitation. Equally important
is the local lowering of relative humidity in the troposphere due to adiabatic
warming in regions of descending air under approximate conservation of
water vapor mixing ratio. Any appeal to a sharp change in mean relative
humidity thus necessarily hypothesizes a substantial change in the dynamical
behavior of the troposphere, in this case a large change in the motions
of the troposphere in response to a comparatively small perturbation to
the thermodynamics of the climate system.
6 The term
climate sensitivity typically refers to the level of equilibrium global-mean
surface air temperature increase that the climate system would experience
in response to a doubling of CO2 .
Each model has its own climate sensitivity, almost guaranteed to be somewhat
different from the unknown value for the real world.
7 Clouds
are effective absorbers and reflectors of solar (visible plus ultraviolet)
and infrared radiation. Their net effect is to cool the planet, but the
effect is very small relative to the 33°C "atmosphere/ no atmosphere"
difference noted above. However, for predicting smaller human-caused climate
changes, the effect of clouds becomes crucially important.