Nuclear December or an Unwarranted Lassitude

Nuclear weapons have a potential for damage and destruction beyond imagining. Concern arose in the 1980s that besides immediate deaths, nuclear exchanges could change the planets albedo resulting in a nuclear winter, a climate that would not support agriculture. In 1990 a study was published, called TTAPS after the authors, Turco, Toon, Ackerman, Pollack, and Sagan, that used climate models to predict multiyear nuclear winters caused by the smoke emitted from burning cities. TTAPS assumed that half of the US/USSR nuclear arsenals would be used. Later, more sophisticated studies implied a nuclear winter autumn. The Wikipedia article is a useful summary of the TTAPS conclusions which were later thought to be overblown.

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 stabilised in atmosphere (NCAR). Smoke stabilised 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.

Toon and Turco have found new collaborators, Robock, Bardeen, Oman, and Stenchikov, and access to better climate models. In addition to the direct effects of a small nuclear exchange

We analyze the likely outcome of a regional nuclear exchange involving 100 15-kt explosions (less than 0.1% of the explosive yield of the current global nuclear arsenal). We find that such an exchange could produce direct fatalities comparable to all of those worldwide in World War II, or to those once estimated for a "counterforce'' nuclear war between the superpowers. Megacities exposed to atmospheric fallout of long-lived radionuclides would likely be abandoned indefinitely, with severe national and international implications

They find that

We use a modern climate model and new estimates of smoke generated by fires in contemporary cities to calculate the response of the climate system to a regional nuclear war between emerging third world nuclear powers using 100 Hiroshima-size bombs (less than 0.03% of the explosive yield of the current global nuclear arsenal) on cities in the subtropics. We find significant cooling and reductions of precipitation lasting years, which would impact the global food supply. The climate changes are large and long-lasting because the fuel loadings in modern cities are quite high and the subtropical solar insolation heats the resulting smoke cloud and lofts it into the high stratosphere, where removal mechanisms are slow. While the climate changes are less dramatic than found in previous "nuclear winter'' simulations of a massive nuclear exchange between the superpowers, because less smoke is emitted, the changes are more long-lasting because the older models did not adequately represent the stratospheric plume rise.

That last sentence is rather misleading. These issues have been dealt with in more detail recently in an article appearing in EOS (pay the damn $20 AGU membership fee already and get access)

Toon et al. [2007b] recently found that a regional war between the smallest current nuclear states involving 100 fifteen-kiloton explosions (the number of weapons likely to exist in the arsenals of new nuclear states; India and Pakistan are estimated to have 110–180 weapons between them) could produce direct fatalities comparable to all of those worldwide in World War II. Robock et al. [2007a] showed that smoke from urban firestorms in such a conflict would produce significant global temperature and precipitation changes, lasting a decade or more, shortening the growing season in the midlatitudes by a month in major agricultural areas, and thus affecting world food supplies. In addition, Robock et al. [2007b] found that although the Cold War and its associated nuclear arms race are over, the remaining American and Russian nuclear arsenals could still produce nuclear winter, threatening the lives of billions of people.

Simulations for this new work were carried out using the latest NASA Goddard Institute for Space Studies climate model, ModelE [Schmidt et al., 2006], the result of decades of NASA investment, and the hard work and dedication of a large number of scientists supported by NASA. Because ModelE is able to simulate the entire troposphere, stratosphere, and mesosphere, from the Earth’s surface up to 80 kilometers, and interactively transports black carbon aerosols in response to solar heating and changing wind circulation, we were able to produce fundamentally new results, showing that the smoke would persist in the atmosphere for more than 10 years, an order of magnitude longer than previously assumed. Robock et al. [2007b] also show that early results suggesting that nuclear autumn instead of nuclear winter would follow a fullscale war [Thompson and Schneider, 1986] were based upon climate models that were not adequate to fully address the problem because they did not have deep enough atmospheres, and could not be run long enough.

If you have the nerve to read the full article, "Nuclear Winter Revisited with a Modern Climate Model and Current Nuclear Arsenals: Still Catastrophic Consequences", it can be found on line and soon to appear in J. Geophys. Res.

UPDATE: Roger Pielke Sr. had a post about this back in December. There is also a powerpoint presentation