2012-02-20: Smoke cloud reveals process of nuclear winter

Soot in clouds of smoke can absorb much sunlight due to their ‘darkness’ and generate so much heat that smoke clouds can rise up to 20 km within a few days. Scientists from the Royal Netherlands Meteorological Institute have revealed the existence of this process by analyzing measurements of the large smoke cloud that emerged after the severe bush fires in Australia on 7 February 2009. This self-lifting effect of smoke clouds is comparable to the process in dust and smoke clouds that emerge after nuclear attacks and on which nuclear winter scenario’s are based.

Nuclear winter scenarios, which first came to attention in the 1980’s, assume that after a nuclear attack large amounts of dust and smoke are released that quickly rise to great heights. The thick layer of dust would remain high in the atmosphere and shield the earth surface from sunlight. This would strongly reduce climate and results in a so-called ‘nuclear winter’. The rise of the dust by absorption of sunlight was only known from model calculations because it has been difficult to prove in reality, if only because there have been no nuclear conflicts. By analyzing satellite measurements of the smoke cloud that emerged on 7 February 2009, KNMI researchers have identified its existence. The smoke cloud – covering an area as large as Western Europe – was observed up to 20 km altitude within a few days after the fires. In the ensuing months the smoke even reached 30 km.
An obvious and logical explanation could have been that due to the large amount of heat produced by the fires, cumulus clouds could have formed that would have transported the smoke swiftly to large heights. This is a well known phenomenon – called pyro-convection leading to the formation of pyro-cumulus (shallow clouds) and pyro-cumulonimbus (deep clouds).
However, weather conditions were unfavorable for the development of large scale pyro-convection. There was limited cloud formation seen in the satellite measurements: during the first 12-24 hours after the fires no smoke and clouds were detected beyond 10 km altitude. The KNMI researchers then focused on the optical properties of the smoke cloud. These consist mainly of soot, are dark of color and absorb lots of sunlight. This generates heat that allows the smoke to continue to rise in the atmosphere, as warm air is lighter and thus rises. The Australian smoke clouds absorbed particularly much sunlight, and calculations showed that sufficient heat could be absorbed to explain the rise to 20 km altitude.
This process in these Australian smoke cloud is comparable to that in the dust and smoke clouds in nuclear winter scenarios. And with that, the existence of this mechanism that forms the basis of these scenarios is confirmed.

A.T.J. de Laat, R. Boers, D. Stein-Zweers, O. Tuinder (2012), A solar escalator: Observational evidence of the self-lifting of smoke and aerosols by absorption of solar radiation in the February 2009 Australian Black Saturday plume, Journal of Geophysical Research, in press.
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Smoke plumes of the ‘Black Saturday’ fires of 7 February 2009 (16:50 local time). White clouds indicate the presence of water, grey clouds of smoke. White spots in the smoke clouds indicate the formation of cloud droplets in or above the smoke, the so-called pyr-cumulus clouds. The outlined red areas indicate fire ares. Melbourne is located in the red circle. Thelength of the large smoke plume is approximately 250 km. Source: MODIS/NASA