Nuclear winter is a hypothetical global climate condition that is predicted to be a possible outcome of a large-scale
nuclear war. It is thought that severely cold weather can be caused by detonating large numbers of nuclear weapons,
especially over flammable targets such as cities, where large amounts of smoke and soot would be injected into the
Earth's stratosphere. The term has also been applied to one of the after-effects of an asteroid impact or supervolcano
eruption.
Mechanism
The nuclear winter scenario predicts that the huge fires caused by nuclear explosions (particularly from burning
urban areas) would disperse large quantities of aerosol particles into the stratosphere, where they could remain for
months or years, and which would significantly reduce the amount of sunlight that reached the surface. The ash and
dust would be carried by the midlatitude west-to-east winds, forming a uniform belt of particles encircling the
northern hemisphere from 30° to 60° latitude (as the main targets of most nuclear war scenarios are located almost
exclusively in these latitudes). The dust clouds would then block out much of the sun's light, causing surface
temperatures to drop drastically.
History
In 1982 a special issue of the journal Ambio was devoted to the possible environmental consequences of nuclear war;
it included an article by Paul Crutzen and J. Birks on atmospheric effects. They re-assessed and re-affirmed the
consequences for the ozone layer noted in the 1975 National Academies of Science report (up to 70% of the ozone
layer might be destroyed); and drew attention for the first time to the likelihood that large amounts of smoke and dust
would be created.
TTAPS (1983)
In 1983 the "TTAPS" study (from the initials of the last names of its authors, R.P. Turco, O.B. Toon, T.P. Ackerman,
J.B. Pollack, and C. Sagan) undertook a systematic study of the atmospheric consequences; partly inspired to write
the paper both by the suggestions of one Dr. A.M. Salzberg (who, unlike the TTAPS authors, believed that the initial
dust thrown into the air would be primarily responsible for the climate changes) and by cooling effects due to dust
storms on Mars. To carry out a calculation of the effect they used a very simplified two dimensional model of the
Earth's atmosphere that assumed that conditions at a given latitude were constant. The model also assumed a solid,
smooth Earth.
WCRP report (1986)
In 1984 the WMO commissioned G. S. Golitsyn and N. A. Phillips to review the state of the science. They found that
studies generally assumed a scenario that half of the world's nuclear weapons would be used, ~5000 Mt, destroying
approximately 1,000 cities, and creating large quantities of carbonaceous smoke - 1–2 × 1014 grams being mostly
likely, with a range of 0.2 – 6.4 × 1014 grams (NAS; TTAPS assumed 2.25 × 1014). The smoke resulting would be
largely opaque to solar radiation but transparent to infra-red, thus cooling by blocking sunlight but not causing
warming from enhancing the greenhouse effect. The optical depth of the smoke can be much greater than unity.
Forest fires resulting from non-urban targets could increase aerosol production further. Dust from near-surface
explosions against hardened targets also contributes; each Mt-equivalent of explosion could release up to 5 million
tons of dust, but most would quickly fall out; high altitude dust is estimated at 0.1-1 million tons per Mt-equivalent of
explosion. Burning of crude oil could also contribute substantially.
The 1-D radiative-convective models used in these studies produced a range of results, with coolings up to 15-42 °C
between 14 and 35 days after the war, with a "baseline" of about 20 °C. Somewhat more sophisticated calculations
using 3-D GCMs (Alexandrov and Stenchikov (1983); Covey, Schneider and Thompson (1984); which would be
considered primitive by modern standards) produced similar results: temperature drops of between 20 and 40 °C,
though with regional variations.
All calculations show large heating (up to 80 °C) at the top of the smoke layer at about 10 km; this implies a
substantial modification of the circulation there and the possibility of advection of the cloud into low latitudes and
the southern hemisphere.The report made no attempt to compare the likely human impacts of the post-war cooling to
the direct deaths from explosions.
TTAPS (1990)
In 1990, in a paper entitled "Climate and Smoke: An Appraisal of Nuclear Winter," TTAPS give a more detailed
description of the short- and long-term atmospheric effects of a nuclear war using a three-dimensional model:
First 1 to 3 months:
10 to 25 % of soot injected is immediately removed by precipitation, while the rest is transported over the globe in 1
to 2 weeks
SCOPE figures for July smoke injection:
22° C drop in mid-latitudes
10° C drop in humid climates
75 % decrease in rainfall in mid-latitudes
Light level reduction of 0 % in low latitudes to 90 % in high smoke injection areas
SCOPE figures for winter smoke injection:
Temperature drops of 3° to 4° C
Following 1 to 3 years:
25 to 40 % of injected smoke is stabilized in atmosphere (NCAR). Smoke stabilized for approximately 1 year.
Land temperatures of several degrees below normal
Ocean surface temperature drops of 2° to 6° C
Ozone depletion of 50% leading to 200% increase in UV radiation incident on surface.
Scientific Debate
The TTAPS study was widely reported and criticized in the media. Criticisms have not been supported by alternative
model runs. Recent studies (2006) substantiate that smoke from urban firestorms in a regional war would lead to long
lasting global cooling but in a less dramatic manner than the nuclear winter scenario, while a 2007 study of the
effects of global nuclear war supported the conclusion that it would lead to full-scale nuclear winter.
2006 study on consequences of a regional nuclear war
A study presented at the annual meeting of the American Geophysical Union in December 2006 found that even a
small-scale, regional nuclear war could produce as many direct fatalities as all of World War II and disrupt the global
climate for a decade or more. In a regional nuclear conflict scenario where two opposing nations in the subtropics
would each use 50 Hiroshima-sized nuclear weapons (ca. 15 kiloton each) on major populated centers, the
researchers estimated fatalities from 2.6 million to 16.7 million per country. Also, as much as five million tons of
soot would be released, which would produce a cooling of several degrees over large areas of North America and
Eurasia, including most of the grain-growing regions. The cooling would last for years and could be "catastrophic"
according to the researchers.
2007 study on global nuclear war
A study published in the Journal of Geophysical Research in July 2007, Nuclear winter revisited with a modern
climate model and current nuclear arsenals: Still catastrophic consequences, used current climate models to look at
the consequences of a global nuclear war involving most or all of the world's current nuclear arsenals (which the
authors described as being only about a third the size of the world's arsenals twenty years earlier). The authors used a
global circulation model, ModelE from the NASA Goddard Institute for Space Studies, which they noted "has been
tested extensively in global warming experiments and to examine the effects of volcanic eruptions on climate." The
model was used to investigate the effects of a war involving the entire current global nuclear arsenal, projected to
release about 150 Tg of smoke into the atmosphere (1 Tg is equal to 1012 grams), as well as a war involving about
one third of the current nuclear arsenal, projected to release about 50 Tg of smoke. In the 150 Tg case they found
that:
A global average surface cooling of –7°C to –8°C persists for years, and after a decade the cooling is still –4°C (Fig.
2). Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about –5°C, this
would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature
changes are largest over land ... Cooling of more than –20°C occurs over large areas of North America and of more
than –30°C over much of Eurasia, including all agricultural regions.
In addition, they found that this cooling caused a weakening of the global hydrological cycle, reducing global
precipitation by about 45%. As for the 50 Tg case involving 1/3 of current nuclear arsenals, they said that the
simulation "produced climate responses very similar to those for the 150 Tg case, but with about half the amplitude",
but that "the time scale of response is about the same." They did not discuss the implications for agriculture in depth,
but noted that a 1986 study which assumed no food production for a year projected that "most of the people on the
planet would run out of food and starve to death by then" and commented that their own results show that "this
period of no food production needs to be extended by many years, making the impacts of nuclear winter even worse
than previously thought."