WMOD00435

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

cloud-top temperature, thickness, and the physical state and
size distribution of the particles constituting the cloud.
Shallow clouds with warm tops are thought to exert a net
cooling effect because they radiate considerable IR into
space, while clouds with cold tops warm the earth because
they are weak IR radiators. Calculations based on satellite
data of the annual global mean effect of all clouds have
shown that, on balance, clouds cool the earth. Estimates of
the net cooling effect have ranged from near zero to 40 W
m-2 (Fouquart et al. 1990), while recent results generally
fall between 17 and 27 W m'2 (Ramanathan et a!. 1989;
Arking 1991).

Because clouds exert a net cooling effect, it is likely
that any increase in cloudiness associated with a warmer
earth would exert a negative feedback effect, while a
decrease would exert a positive feedback effect. GCMs do
not agree on what the changes in cloud cover would be, so
they can not agree on the feedback effects. A comparison
of 19 different GCMs under perpetual July conditions
found about one-third of them exhibiting a negative
feedback effect and the other two-thirds a positive effect
(Cess et al. 1990; Arking 1991). In summary, there is
currently no way to tell whether the modification of the
earth's clouds by the enhanced greenhouse effect will lead
to a positive or a negative feedback effect upon surface
temperatures.

Multiple runs with the same GCMs have shown that
the results of doubled-C02 runs are also quite sensitive to
the albedos assigned to polar sea ice and seawater at low
sun angles (Meehl and Washington 1990). Varying these
assigned parameters within reasonable limits can lead to an
Arctic Ocean that becomes mostly ice-free in summer or to
one that stays icebound year round. The difference
produces noticeable effects on globally averaged
temperature and has serious implications for the climates of
northern North America and Eurasia.

6. EVIDENCE OF GLOBAL WARMING

The appreciable increases in CO2 that have occurred
already have prompted searches for a corresponding
warming of the earth's surface. The statistical problems
encountered in this approach parallel those involved in
detecting inadvertent weather modification and changes in
local weather produced by operational cloud seeding
programs. Obviously, the special methods developed to
evaluate randomized cloud seeding projects are not
applicable to the global warming issue.

Surface temperature data from different sources
have been combined and examined for evidence of global
warming. There are numerous problems with surface
temperature data, including station relocations, the
heat-island effect of cities, and changes in procedures for
measuring sea-surface temperature. Regional climate
variations over periods of a few decades further confuse the
issue. Still, there is good evidence that a net warming
averaging 0.5 DC over the earth's entire surface (both land
and sea) has occurred over the past century (Jones and
Wigley 1990). The evidence from thermometer readings is
reinforced by other types of evidence, including the
shrinking of glaciers and the warming of some Canadian

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lakes (Houghton and Wood well 1989). The latest satellite
data indicate decreases in the extent of sea ice in the Arctic
regions during the period 1978 to 1987, but show no
significant trend in the extent of ice around Antarctica
(Gloersen and Campbell 1991).

As cloud seeders have learned, finding a trend in
meteorological data is just a starting point for discussions
of possible causes. As climate has been varying for
thousands of years, the observed warming during the
twentieth century is not necessarily due to increased
concentrations of greenhouse gases. Recently Balling
(1991) has pointed out that desertification, which afflicts 30
percent of the land area of the globe to some degree, is
producing a significant warming trend in the affected areas.
Desertification may account for 0.05 DC or more of the
total warming of 0.5 DC observed over the past century.

Skeptics point to the fact that the global warming
was interrupted by a cooling trend in the Northern
Hemisphere from 1940 to about 1965, during which period
the CO2 concentration was going up at an accelerated pace.
Indeed, Gordon (1991) has shown that the observed
warming during the past century may be only a
manifestation of a random walk. Persons who believe that
global warming has begun consider the cooling from 1940
to 1965 a temporary dip due to natural causes. The
existence of the dip is a reminder that global warming due
tb enhanced concentrations of greenhouse gases could be
masked at times by some combination of external driving
forces promoting a cooling of the earth. For example, dust
from the eruption of Mt. Pinatubo in the Phillipines in
1991 will exert a slight cooling effect on the earth's surface
over the next several years. Global warming could also be
masked by climate swings lasting a few decades and due
only to internal feedbacks in the ocean-atmosphere system
itself. Several experiments with GCMs have shown such
swings, as exemplified in two recent 100-year simulations
by Houghton et al. (1991).

Establishing that global warming is underway will
require, for conservative scientists, a demonstration that
changes have occurred that mimic, on a regional scale and
at different levels in the atmosphere, those predicted by
GCMs to accompany global warming. An announcement
from Goddard Space Flight Center in mid-1991 that the
temperature at 55 km altitude had dropped by 2.5 DC over
the previous 10 years is worthy of attention, because it
agrees with some GCM predictions of an enhanced
greenhouse effect. However, proof of global warming
according to the strict standard stated above is impossible
at this time, because the available GCMs do not agree in
their predictions of regional effects (Grotch and
MacCracken 1991).

7. NATURAL LIMITS ON GLOBAL WARMING

There are numerous conceivable feedback effects in
the ocean-atmosphere system, both positive and negative,
that have not been mentioned so far in this paper. For
example, warming of the ocean may release CH4 presently
locked up in sediments on continental shelves, thereby
exacerbating the buildup of greenhouse gases (Revelle
1983). On the other hand, several authors have suggested

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