Atmos. Chem. Phys., 7, 2003–2012, 2007 www.atmos-chem-phys.net/7/2003/2007/ Atmospheric © Author(s) 2007. This work is licensed Chemistry under a Creative Commons License. and Physics Climatic consequences of regional nuclear conflicts A. Robock1, L. Oman1, G. L. Stenchikov1, O. B. Toon2, C. Bardeen2, and R. P. Turco3 1Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA 2Department of Atmospheric and Oceanic Sciences and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, USA 3Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, USA Received: 15 August 2006 – Published in Atmos. Chem. Phys. Discuss.: 22 November 2006 Revised: 8 March 2007 – Accepted: 2 April 2007 – Published: 19 April 2007 Abstract. We use a modern climate model and new esti- get zones, may have been an important factor in the end of mates of smoke generated by fires in contemporary cities to the arms race between the United States and the Soviet Union calculate the response of the climate system to a regional nu- (Robock, 1989). clear war between emerging third world nuclear powers us- Now the world faces the prospect of many other states de- ing 100 Hiroshima-size bombs (less than 0.03% of the ex- veloping small nuclear arsenals. Toon et al. (2007a) show plosive yield of the current global nuclear arsenal) on cities that the direct effects of even a relatively small number of nu- in the subtropics. We find significant cooling and reduc- clear explosions would be a disaster for the region in which tions of precipitation lasting years, which would impact the they would be used. Here we examine the climatic effects global food supply. The climate changes are large and long- of the smoke produced by the fires that would be ignited by lasting because the fuel loadings in modern cities are quite a regional conflict in the subtropics between two countries, high and the subtropical solar insolation heats the resulting each using 50 Hiroshima-size (15 kt) nuclear weapons to at- smoke cloud and lofts it into the high stratosphere, where re- tack the other’s most populated urban areas. Based on the moval mechanisms are slow. While the climate changes are analysis by Toon et al. (2007a), such a conflict would gener- less dramatic than found in previous “nuclear winter” sim- ate 1–5 Tg of black carbon aerosol particles injected into the ulations of a massive nuclear exchange between the super- upper troposphere, after the initial removal in black rain. powers, because less smoke is emitted, the changes are more long-lasting because the older models did not adequately rep- resent the stratospheric plume rise. 2 Climate model We conducted climate model simulations with a state-of-the- art general circulation model, ModelE from the NASA God- 1 Introduction dard Institute for Space Studies (Schmidt et al., 2006), which The casualties from the direct effects of blast, radioactivity, includes a module to calculate the transport and removal of and fires resulting from the massive use of nuclear weapons aerosol particles (Koch et al., 2006). The atmospheric model by the superpowers would be so catastrophic that we avoided is connected to a full ocean general circulation model with such a tragedy for the first four decades after the invention calculated sea ice, thus allowing the ocean to respond quickly of nuclear weapons. The realization, based on research con- at the surface and on yearly time scales in the deeper ocean. ducted jointly by Western and Soviet scientists (Crutzen and The climate model (with a mixed-layer ocean) does an ex- Birks, 1982; Aleksandrov and Stenchikov, 1983; Turco et cellent job of modeling the climatic response to the 1912 al., 1983, 1990; Robock, 1984; Pittock et al., 1986; Harwell Katmai volcanic eruptions (Oman et al., 2005). We have and Hutchinson, 1986; Sagan and Turco, 1990), that the cli- also used this model to simulate the transport and removal matic consequences, and indirect effects of the collapse of of sulfate aerosols from tropical and high-latitude volcanic society, would be so severe that the ensuing nuclear winter eruptions (Oman et al., 2006), and have shown that it does would produce famine for billions of people far from the tar- a good job of simulating the lifetime and distribution of the volcanic aerosols. In the stratosphere, these aerosols have an Correspondence to: A. Robock e-folding residence time of 12 months in the model, in excel- ([email protected]) lent agreement with observations. The aerosol module (Koch Published by Copernicus GmbH on behalf of the European Geosciences Union. 2004 A. Robock et al.: Climatic consequences of regional nuclear conflicts - 19 - - 20 - 467 473 Figure 2. Zonal mean change in surface shortwave radiation for the 5 Tg standard case. 474 This should be compared to the global average value of +1.5 W/m2 for a doubling of Fig. 1. Horizontal and vertical distributions of smoke for the 5 Tg 2 468 Fig. 1. Horizontal and vertical distributions of smoke for the 5 Tg standard case. A. Zonal 475 Fig.atmospheric 2. Zonal CO2, or mean to the changemaximum invalue surface of –3 W/m shortwave for the 1991 radiation Mt. Pinatubo for volcanic the 469 standardaverage absorption case. (A)opticalZonal depth, as average function absorptionof latitude and time. optical The depth,polewardas spread func- and 476 5eruption Tg standard (Kirchner case.et al., 1999; This Fig. should 3), the largest be compared of the 20th tocentury. the global average 470 subsequent loss of smoke over time is clearly seen. B. Global average vertical distribution of 477 2 471 tionblack of carbon latitude as a function and time. of time, The plotted poleward as mass mixing spread ratio. and The subsequent semiannual lofting loss is value of +1.5 W/m for a doubling of atmospheric CO2, or to the 472 ofdue smoke to heating over during time the solstice is clearly in each seen. summer(B) hemisphereGlobal (Fig. average 2). vertical dis- maximum value of −3 W/m2 for the 1991 Mt. Pinatubo volcanic tribution of black carbon as a function of time, plotted as mass mix- eruption (Kirchner et al., 1999; Fig. 3), the largest of the 20th cen- ing ratio. The semiannual lofting is due to heating during the sol- tury. stice in each summer hemisphere (Fig. 2). much higher than is typical of weakly absorbing volcanic et al., 2006) also accounts for black carbon particles, which sulfate aerosols (Stenchikov et al., 1998). Supplemen- have an effective radius of 0.1 µm. At visible wavelengths tal Fig. 1 (http://www.atmos-chem-phys.net/7/2003/2007/ the black carbon particles have a mass extinction coefficient acp-7-2003-2007-supplement.zip) shows an animation of of 9 m2/g, a single scattering albedo of 0.31, and a mass ab- the horizontal and vertical spreading of the smoke cloud from sorption coefficient of 6.21 m2/g (also see Toon et al., 2007a). one of the ensemble members. As a result, the aerosols have We run the atmospheric portion of the model at 4◦×5◦ a very long residence time and continue to affect surface cli- latitude-longitude resolution, with 23 vertical layers extend- mate for more than a decade. The mass e-folding time for ing to a model top of 80 km. The coupled oceanic general the smoke is 6 yr, as compared to 1 yr for typical volcanic circulation model (Russell et al., 1995) has 13 layers and eruptions (Oman et al., 2006) and 1 week for tropospheric also a 4◦×5◦ latitude-longitude resolution. In our standard aerosols. After 6 yr, the e-folding time is reduced, but is still calculation, we inject 5 Tg of black carbon on 15 May into longer than that of volcanic aerosols. This long aerosol life- one column of grid boxes at 30◦ N, 70◦ E. We place the black time is different from results found in previous nuclear win- carbon in the model layers that correspond to the upper tro- ter simulations, which either fixed the vertical extent of the posphere (300–150 mb). aerosols (Turco et al., 1983) or used older-generation climate We conducted a 30-yr control run with no smoke aerosols models with limited vertical resolution and low model tops and three 10-yr simulations with smoke, starting from arbi- (Aleksandrov and Stenchikov, 1983; Covey et al., 1984; Mal- trary initial conditions. We present the mean of the ensemble one et al., 1986), artificially limiting the particle lifetimes. of the three runs, and compare it to the mean of the control In addition, the subtropical latitude of the smoke injections, run. The differences between ensemble members are small in the case investigated here, results in more solar heating compared to the response, ensuring us that natural, chaotic than in previous nuclear winter scenarios, which considered weather variability is not responsible for the effects we see. smoke from the midlatitude Soviet Union, Europe, and the U.S. The lower latitude also ensures that lofting would take 3 Results place year-round. Therefore the large effects may not be limited to wars that occur in spring and summer, as previ- In the model, the black carbon particles in the aerosol ously found (Robock, 1984; Covey et al., 1984; Schneider layer are heated by absorption of shortwave radiation. and Thompson, 1988). This heating induces vertical motions and the aerosols The maximum change in global-average surface short- are lofted to near the top of the stratosphere (Fig. 1), wave radiation is −15 W m−2 (Fig. 2). This negative forcing Atmos.