ARMS-D-Request@MIT-MC.ARPA (Moderator @MIT-MC.ARPA) (12/07/85)
Arms-Discussion Digest Monday, November 18, 1985 6:57PM
Volume 5, Issue 23
Today's Topics:
developments in Nuclear Winter theory
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Date: Mon, 18 Nov 85 06:41:14 PST
From: ihnp4!utzoo!henry@ucbvax.berkeley.edu
Subject: developments in Nuclear Winter theory
In the course of catching up on recent issues of Science, I ran across two
papers on nuclear winter whose contents might be of interest to ARMS-D.
They influence predictions in opposite directions.
In the 18 Oct 1985 issue, Malone et al describe some work in more
detailed modelling of the atmospheric effects of massive amounts of smoke.
The heating of smoke particles by sunlight turns out to be significant,
and the general effect is that it makes things worse. First, it induces
convection that lifts the smoke to higher altitudes, where its lifetime
is longer. Second, it tends to make the tropopause move downward below
the smoke, effectively isolating the smoke from most forms of weather
and hence prolonging its lifetime further. There are a couple of effects
that operate the other way -- high-altitude smoke shading low-altitude
smoke, and positive feedback in smoke removal as falling smoke densities
reduce heating effects -- but the net effect is greater smoke persistence.
As with other aspects of nuclear winter, the effects are much weaker if
war occurs in January rather than in July.
In the 2 Aug 1985 issue, we have Small and Bush, "Smoke Production from
Multiple Nuclear Explosions in Nonurban Areas". This is a fairly detailed
study of how much smoke is likely to be generated from fires well away from
cities. The original nuclear winter studies credited wildland fires with
about half the smoke production. This study looked in detail at vegetation
types, burnable mass, fire spread, etc. in the neighborhood of probable
wildland targets. The general conclusion is that the amount of smoke from
wildland fires has been overestimated by a factor of 10-100. There are
several major reasons:
1. Probable targets are not uniformly distributed. In particular, most of
them are located in croplands or grasslands rather than forests.
2. Croplands are seldom flammable. Croplands burn well only between ripening
and harvesting, which is a period of circa two weeks. Furthermore,
planting and harvesting times are staggered, so only a fraction of
total cropland acreage is flammable at any time.
3. Fuel must in general be dried before ignition, which reduces the
ignition radius. Weather heavily influences moisture content.
The climate of target areas thus alters smoke production.
4. Ignition areas for strikes against closely-spaced targets, such as silos
in missile fields, often overlap. So will ignition areas for multiple
strikes against the same target.
5. US Forest Service data indicates that conditions favoring major spreading
of fires are actually fairly uncommon. The vision of continent-
spanning forest fires is totally unrealistic. Fire spread is likely
to increase total burned area by only a few percent even in summer.
6. Smoke emission varies with fire intensity, although the details of the
variation can only be roughly estimated right now.
The final estimates are 3e12 g of smoke in July, and about a tenth of that
in January. Worst-case calculations give upper limits of about 1e13 g. The
earlier nuclear-winter studies assumed 1e14 or more. Numbers and sizes of
weapons are similar.
The paper also noted other effects, outside the domain of the detailed
study, that would reduce things still further. Wildland fires seldom lift
their smoke very high. The study assumed complete burning of the biomass
in ignition areas, which is unrealistic. A substantial fraction of smoke
particles are too small to be optically active.
They also note that the distribution of smoke is markedly non-uniform.
For one thing, wildland military facilities tend to be clumped, which
encourages violent local weather patterns that could alter the net effect.
For another thing, the 14% of targets that are in forested areas account
for a disproportionate fraction of the smoke production, and most of
those are in the Soviet Union.
Their overall conclusion was that smoke from wildland-area targets is not
sufficient to produce more than minor cooling, whose impact would be fairly
insignificant compared to the other effects of nuclear war. Nuclear winter
is plausible only as a result of attacks on urban areas.
Oh yes, *both* of these studies were funded by the Defence Nuclear Agency
(i.e. DoD), with DOE participation in the first one as well.
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry
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End of Arms-Discussion Digest
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