WMOD00435

• Title: Limits on Global Warming
• Date: 5/9/1991
• Weather Modification - Doc Type: Report

-

OUTER SPACE

+105 -342 +2r

--'-II

Solar

Infrared

A +68 -237 = -169
T
t.4
0
S
P =0
H
E
R
E +390 -327 +90 +16 = +169
L 5
e
a n
1 t s
Solar Infrared e I
~U b
n Heat I
t e
+169 -390 +327 -90 -16 =0

EARTli - OCEAN SYST'E),C

Figure 4. - Disposition of solar energy reaching the top
of the atmosphere (based on Ramanathan et
al. 1989). Amounts shown are in watts per
square meter and are to be interpreted as
yearly averages over the entire earth.

effect. The effective IR temperature of 255 oK suggests
that IR sensors in space respond, on average, to IR emitted
from the middle troposphere, say around the 4()()-mb level.

Many studies of possible climate change have dealt
with the impacts of a doubled-C~ world. Barring changes
in cloudiness, a doubling of the concentration to 600 ppm
would have a negligible effect on the atmosphere's ability
to absorb solar radiation, but would make it a more
efficient absorber and radiator ofIR. Ramanathan (1981)
calculated that the increase in IR emissivity of the
atmosphere by itself would increase the downward flux of
IR energy at the earth's surface by 1.2 W m-l, even if the
temperature structure of the atmosphere remained as at
present (fig. 5). However, the troposphere would warm up
because it would absorb more of the outgoing IR from the
surface, and this effect would increase the downward flux
at the surface by another 2.3 W m'l, so the total effect of
CO2 doubling would amount to 3.5 W m-2. The warmer
surface would result in additional evaporation from
the ocean. As water vapor is itself a greenhouse gas, the
troposphere would be warmed still further by absorption of
IR by water vapor, as well as by the release of latent heat
when the additional water vapor precipitated. Ramanathan
calculated this positive feedback effect at 12 W m-2,
bringing the total impact of doubling CO2 to 15.5 W m-2
(fig. 5). Without the effects of the anticipated associated
increase in water vapor, the impact of doubling the
atmospheric CO2 concentration would be so small that it
would be very difficult to detect.

The feedback factor for the effect of the increase in
water vapor in this case can be calculated as 15.5/3.5, or

CO:JDIRECT TROPOSPHeRIC
HEATlNG.....J lit m -I

CD

tJR(C01)....../2... m -2

Figure 5. -

Schematic illustration of the
ocean-atmosphere feedback processes by
which CO2 increase warms the surface of the
earth. All of the numbers correspond to
hemispherically averaged conditions and
apply to a doubling of CO2 (after
Ramanathan 1981, courtesy of the American
Meteorological Society).

4.4. [Some previous authors have used different
definitions of feedback factors.] As defined and calculated
here, a feedback factor greater than 1 signifies positive
feedback and a feedback factor less than 1 signifies
negative feedback.

The 15.5 W m-2 calculated by Ramanathan (1981) as
the total radiative impact of doubling CO2 equals about 5
percent of the mean flux of solar radiation at the top of the
atmosphere, so one would anticipate a substantial impact on
the earth's climate. One crude way to estimate the impact
of doubled CO2 on the temperature at the surface is to
apply the Stefan-Boltzmann equation with the outgoing IR
increased from 237 to 252.5 W m'2 in line with
Ramanathan's calculations. The result is about: 259 OK,
which suggests that global warming under doubled CO2
conditions might amount to about 4 DC. It is surprising
how well the 4 DC rise estimated by this simple method
and the 5 to 6 oC calculated by Arrhenius in 1896 (!) agree
with the results obtained by more complex models.
[Arrhenius assumed that the relative humidity would stay
the same as the earth warmed, and so caught the extremely
important feedback due to increased absolute humidity.]

Of course, global warming will not affect the
outgoing IR unless the earth's albedo is changed; an
observer in space would still record 237 W m-l of outgoing
IR. The surface and lower atmosphere would be warmer
than at present, but the IR sensors in space would be
responding, on average, to higher layers of the atmosphere,
leaving the outgoing IR the same. The highest levels of
the atmosphere would actually cool, as the thickened
blanket of CO2 and water vapor in the troposphere would
absorb some of the IR that presently reaches them from the
surface and the lower atmosphere.

4. ANALOGS OF GLOBAL WARMING

Numerous scientists have examined climatological
records and paleoclimatic data for clues to possible climate
changes due to enhanced concentrations of greenhouse
gases. According to Gleick's (1989) review, ". . . the
Holocene optimum (about 6000 to 5000 years B.P.) is
considered an analogue for a I DC warming; the last
interglacial (about 125,000 years B.P.) is considered an

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