The Atmosphere After a Nuclear War: Twilight at Noon

Abstract

The immediate effects of a global nuclear war are so severe that any additional longterm effects might at first thought be regarded as insignificant in comparison. However, our investigation into the state of the atmosphere following a nuclear exchange suggests that other severely damaging effects to human life and the delicate ecosystems to which we belong will occur during the following weeks and months. Many of these effects have not been evaluated before.

This text was first published as: Crutzen, P.J.; Birks, J.W. (1982). “The atmosphere after a nuclear war: Twilight at noon”. Ambio, 11 (2/3): 114–125. The permission to republish this text was granted on 28 August 2015 by editor-in-chief of Ambio, Prof. Dr. Bo Söderström and by Prof. Dr. Per Hedenqvist, Executive Director of the Royal Swedish Academy of Sciences. The author is especially thankful to the following scientists for critically reading earlier versions of this article and providing us with advice in their various fields of expertise: Robert Charlson, Tony Delany, Jost Heintzenberg, Rupert Jaenicke, Harold Johnston, Chris Junge, Jeffrey Kiehl, Carl Kisslinger, James Lovelock, V. Ramanathan, V. Ramaswami, Henning Rodhe, Steven Schneider, Wolfgang Seiler, Robert Sievers, Darold Ward, Ellen Winchester, Jack Winchester and Pat Zimmerman.

Appendices

Appendix 1: Production and Spatial Distribution of Nitric Oxide from Nuclear Explosions

There have been numerous estimates [49–52, 72] of the yield of nitric oxide per megaton (Mt) of explosion energy, and these have been reviewed by Gilmore [72]. Nitric oxide is produced by heating and subsequent cooling of air in the interior of the fireball and in the shock wave.

The spherical shock wave produces nitric oxide by heating air to temperatures above 2200 K. This air is subsequently cooled by rapid expansion and radiative emission, while the shock front moves out to heat more air. At a particular temperature the cooling rate becomes faster than the characteristic time constant for maintaining equilibrium between NO and air. For cooling times of seconds to milliseconds the NO concentration ‘freezes’ at temperatures between 1700 and 2500 K, corresponding to NO concentrations of 0.3–2 %. Gilmore [72] estimates a yield of 0.8 × 10³² NO molecules per Mt for this mechanism.

The shock wave calculation of NO production does not take into account the fact that air within the fireball center contains approximately one-sixth of the initial explosion energy, having been heated by the radiative growth mechanism described earlier. This air cools on a time scale of several seconds by further radiative emission, entrainment of cold air, and by expansion as it rises to higher altitudes. These mechanisms are sufficiently complex that one can only estimate upper and lower limits to the quantity of NO finally produced.

A lower limit to total amount of NO finally produced may be obtained by assuming that all of the shock-heated air is entrained into the fireball and again heated to a high enough temperature to reach equilibrium. This is possible since the thickness of the shell of shock- heated air containing NO is smaller than the radius of the fireball. To minimize the cooling rate, and thus the temperature at which equilibrium is not re-established rapidly, it is assumed that this air mass cools only by adiabatic expansion as the fireball rises and by using a minimum rise velocity. The resulting lower limit to total NO production is 0.4 × 10³² molecules per Mt [72].

Since the interior of the fireball is much hotter than the surrounding, shock-heated air, it will rise much faster and possibly pierce through the shell of shock-heated air to mix with cold, undisturbed air above it. Thus, an upper limit to NO production may be obtained by assuming that none of the 0.8 × 10³² NO molecules per Mt produced in the shock wave are entrained by the hot fireball interior. Instead, one assumes that the interior is cooled totally by entrainment of cold, undisturbed air to produce additional NO. The upper limit to total NO production is then estimated to be 1.5 × 10³² molecules per Mt [72]. Thus, the range of uncertainty for total NO_x formation is 0.4–1.5 × 10³² molecules per Mt.

For the purposes of this study we assume a nitric oxide yield of l.0 × 10³² molecules per Mt. One can make strong arguments against either of the extreme values. This estimate of NO production applies only to detonations in the lower atmosphere.

In a nuclear war some bombs may be exploded at very high altitudes for the purpose of disrupting radio and radar signals. The ionization of air by gamma rays, X-rays and charged particles creates a phenomenon known as the “electromagnetic pulse” or “EMP” [73]. The partitioning of energy between the locally heated fireball, shock wave, and escaping thermal radiation changes dramatically as the altitude of the explosion increases above 30 km. As the altitude increases, the X-rays are able to penetrate to greater distances in the low density air and thus create very large visible fireballs. For explosions above about 80 km, the interaction of the highly ionized weapon debris becomes the dominant mechanism for producing a fireball, and for such explosions the earth’s magnetic field will influence the distribution of the late-time fireball. Explosions above 100 km produce no local fireball at all. Because of the very low air density, one-half of the X-rays are lost to space, and the one-half directed toward the earth deposits its energy in the so-called “X-ray pancake” region as they are absorbed by air of increasing density. The X-ray pancake is more like the frustum of a cone pointing upward, with a thickness of about 10 km and a mean altitude of 80 km. The mean vertical position is essentially independent of the explosion altitude for bursts well above 80 km [73].

The absorption of X-rays by air results in the formation of pairs of electrons and positively charged ions. One ion pair is formed for each 35 eV of energy absorbed [74], and in the subsequent reactions approximately 1.3 molecules of NO are produced for each ion pair [75]. A 1-Mt explosion corresponds to 2.6 × 10³⁴ eV of total energy. Thus, considering that only half of the X-rays enter the earth’s atmosphere, the yield of NO is calculated to be 4.6 × 10³² molecules per Mt (i.e. this mechanism is about five times more effective at producing NO than the thermal mechanism described above).

In the course of a nuclear war up to one hundred 1-Mt bombs might be detonated in the upper atmosphere for the purpose of creating radio wave disturbances. The injection of NO would therefore be 4.8 × 10³⁴ molecules or 1.1 Tg of nitrogen. Natural production of NO in the thermosphere due to the absorption of EUV radiation depends on solar activity and is in the range 200–400 Tg of nitrogen per year [40]. Thus the amount of NO injected by such high altitude explosions is about equal to the amount of NO produced naturally in 1 day and falls within the daily variability. In addition, the X-ray pancake is positioned at an altitude where nitrogen and oxygen species are maintained in photochemical equilibrium. Excess nitric oxide is rapidly destroyed by a sequence of reactions involving nitrogen and oxygen atoms as follows:

$$ \begin{aligned} & {\text{R22}}\,{\text{NO}} + hv \to {\text{N}} + {\text{O}} \\ & \frac{{{\text{R23}}\,{\text{N}} + {\text{NO}} \to {\text{N}}_{ 2} + {\text{O}}}}{{{\text{Net}}:\quad 2 {\text{NO}} \to {\text{N}}_{ 2} + {\text{O}} + {\text{O}} \to {\text{N}}_{ 2} + {\text{O}}_{ 2} }} \\ \end{aligned} $$

For these reasons, we expect that high altitude explosions of such magnitudes will have no significant global effect on the chemistry of the stratosphere and below.

Results of past tests of nuclear explosions show that nuclear clouds rise in the atmosphere and finally stabilize at altitudes that scale approximately as the 0.2 power of bomb yield. An empirical fit to observed cloud geometries at midlatitudes gives the following expressions for the heights of the cloud tops and cloud bottoms, respectively [50]:

$$ \begin{aligned} & {\text{H}}_{\text{T}} = 2 2\,{\text{Y}}^{0. 2} \\ & {\text{H}}_{\text{B}} = 1 3\,{\text{Y}}^{0. 2} \\ \end{aligned} $$

where H is in kilometers and Y has units of megatons. Thus, bomb clouds from weapons having yields greater than about 1 Mt completely penetrate the tropopause at midlatitudes. For such explosions all of the NO_x produced in the fireball, and perhaps a significant fraction of that produced in the shock wave but not entrained by the bomb cloud, is deposited in the stratosphere. Oxides of nitrogen formed in nuclear explosions having yields less than 1 Mt have little effect on stratospheric ozone since: (1) only a minor fraction of the NO_x formed is deposited above the tropopause, (2) the residence time in the stratosphere increases with altitude of injection, and (3) the NO_x-catalytic cycle for ozone destruction is most effective at higher altitudes. In fact, below about 20 km NO_x additions to the atmosphere tend to result in ozone concentration increases [76, 77].

The stabilized nuclear bomb clouds have diameters ranging from 50–500 km depending on bomb yield. They are sheared by horizontal winds at constant latitude, and within a few weeks may be uniformly distributed around the earth at a constant latitude [78].

Appendix 2: Model Description

The computer model used in this study is a two-dimensional model of coupled photochemistry and dynamics. It treats transport in both the vertical and latitudinal directions by parameterization of these motions by means of eddy diffusion coefficients and mean motions. The model covers altitudes between the ground and 55 km and latitudes between the South Pole and North Pole, and it attempts to simulate the longitudinally averaged, meridional distributions of trace gases. Therefore, the main assumption is that composition variations in the zonal (East–West) directions are much smaller than those in the vertical and latitudinal directions. Although the 2-D model is a step forward from 1-D models, which take into account only variations in the vertical direction, the neglect of longitudinal variations in air composition will clearly introduce substantial deviations from reality, especially at lower altitudes, where the influence of chemical and biological processes at the earth’s surface are large. One should keep these limitations of the 2-D model in mind especially when interpreting the results obtained for the troposphere.

The model photochemistry considers the occurrence of nearly one hundred reactions, which are now thought to be important in global air chemistry. It takes into account the reactions of ozone and atomic oxygen, and the reactive oxides of nitrogen, hydrogen and chlorine, which are derived from the oxidation of nitrous oxide (N₂O), water vapor (H₂O), methane (CH₄) and organic chlorine compounds. In the troposphere, the photochemistry of simple reactions leading to ozone formation in the presence of NO_x, carbon monoxide (CO), methane and ethane (C₂H₆) are taken into account. The influence of industrial processes is an important consideration of the model. A more detailed description of the model may be found elsewhere [77, 78]. Detailed descriptions of atmospheric photochemistry are given in a number of review articles [40, 79–81].