The term and theory of “nuclear winter” was popularized in 1983 by Carl Sagan and associates stemming from a scientific paper, “Global Atmospheric Consequences of Nuclear War”.Written by R. P. Turco, 0. B. Toon, T. P. Ackerman, J. B. Pollack and Carl Sagan, this work is also referred to as “TTAPS” from the initials of the authors’ last names. The premise was that nuclear war would create a global environmental catastrophe, regardless of where it occurred, resulting in the earth being plunged into a “nuclear winter”. While, nuclear winter describes the climatic effects stemming from a nuclear war, it has also been used to describe the aftermath of asteroid impacts and super-volcano eruptions since the mechanisms are similar.
In theory, vast amounts of dust kicked up into the upper atmosphere from impact, and smoke from burning cities would obscure the sun for days, if not weeks after a nuclear war. This effectively cools the climate and damages the ecosystem during a prolonged state of winter-like conditions. The trail of destruction eventually encompasses millions who may not have even been involved in the nuclear exchange yet still succumb to mass starvation brought on by the ensuing crop failures. At the time of the study, there were only 50,000 nuclear weapons, yielding more than 13,000 megatons of destructive power. Twenty-six years later, the world has seen the emergence of new super-powers and their arsenals, as well as increasing capacity in the pre-existing arsenals.
Cooling from smoke and dust isn’t the only mechanism at work here. It was discovered in the 1970’s that high-yield airbursts chemically burn nitrogen and convert it into nitrogen oxides; these then combine with ozone in the upper atmosphere, destroying the protective ozone in the Earth’s stratosphere. Without the ozone shield, deadly solar ultraviolet radiation would take a toll on the biology of the planet. But, where in this “micro-waved popcorn” nightmare scenario does the nuclear winter prospect come in? The other piece of the puzzle came in 1971, when Mariner 9 arrived at Mars in the midst of a global dust storm. Taking temperature readings over the next months, scientists noticed as the fine dust settled, the Martian surface heated up.
Sagan’s study postulated an explosion of 5,000 to 10,000 megatons in a nuclear conflict and calculated the devastating climatic effects. Sagan and associates knew that nuclear explosions, especially ground-bursts, would throw tremendous quantities of soil into the atmosphere. So for a baseline, Sagan and associates assumed a 5000-megaton war with only a 20% fraction of the yield expended on urban targets. They then calculated the dust and smoke spread patterns, suspension time, and the amount of sunlight the mass of dirt would absorb and the ensuing temperature drop.
The results were frightening. Even in the baseline case, the amount of sunlight at the ground was reduced to a few percent of normal daylight and too dark for photosynthesis. In the Northern Hemisphere land temperatures, except for coastline areas where oceans moderate temperature range, dropped to minus 13 degrees Fahrenheit and stayed below freezing for months even in the summer. And this may have been an underestimation! Assuming the particles reach 6-9 miles above the Earth’s surface as modeled, the absorption of sunlight would further heat the smoke, lifting it into the stratosphere where it would become a global solar barrier; with no rain to wash it out, it would theoretically continue blocking out the sun for years. Furthermore, the resulting smoke would be primarily opaque to solar radiation but transparent to infra-red, cooling the earth by blocking sunlight but not causing any Greenhouse Effect warming.
Not all scientists ascribe to the nuclear winter theory. While some have called the theory a “myth”, others have moderated its effects, embracing the mechanism but not the magnitude. Preferring the label “Nuclear Autumn”, these scientists discount some of the effects as exaggerations. The central problem in both camps (and with the outright unbelievers) is the inability to realistically model and analyze all the variables on a large scale. Consequently, the issue is unlikely to be resolved except with the acquisition and analysis of real data from related, smaller scale events, such as volcanic eruptions, or large fires (such as the oil fires in Kuwait in 1991). In fact, the Kuwait oil fires cast doubt into the assumption of smoke plumes rising to high levels of the atmosphere in sufficient opacity as to block out the level of sunlight predicted in the nuclear winter theory. Smoke from the burning Kuwaiti fields became diluted and didn’t gush into the upper atmosphere as expected.
Some proponents of the nuclear winter theory, however, explain away these discrepancies by noting that the Kuwaiti oil fires are not of the same magnitude as a burning city or forest. Therefore, it still remains a viable theory that smoke from massive fires of that type could indeed reach the upper levels of the atmosphere without becoming overly diluted. However, Sagan admitted after predicting regional cooling and agricultural disruption from the oil fire/smoke threat that his conclusion was not correct. Still, it WAS dark at noon and the temperature DID fall 4-6C over a portion of the region. But, the effects did not persist.
In history, there have been accounts of large volcanic eruptions cooling the climate worldwide for months and years. Unfortunately, there is no real-time, empirical data we can analyze and then model. In conclusion, the only way to definitively put this theory to the test is have a nuclear war. In the place of that, most of us are content to simply investigate the outcomein theory.