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Ice nucleators from vegetation in the Sahel: the impact of overgrazing


The search for ice nuclei with remarkably efficient activity has been a pre-occupation of the atmospheric sciences long before the recent interest in biological ice nucleators propelled this search into the limelight.  Some of the data and observations that constitute the collective knowledge about these ice nucleators are not available in accessible publications.  Russ Schnell has contributed some more of the gems from the collection of observations that he has amassed during his long career.  (For more information about Russ, see the post from 2015/03/04). The following observations concern sources of ice nuclei in the Sahel.

In 1973 at the height of the great Sahel Drought of the early 1970s, Russ noticed a satellite photo that showed a fenced area of some 50,000 hectares in central Niger, Africa, (black & white photo, lower center section) where vegetation was growing much better inside the fenced area than outside.  Outside of the fence the land was heavily overgrazed and even shrubs and trees cut down to feed goats (color photo, taken southeast of the place on satellite photo labeled “photo here”) The fenced property was later identified as the “Ekrafane Ranch”.


Russ suspected that that overgrazing had removed the most active biological ice nuclei and thus reduced precipitation that in turn exacerbated the drought. To test this hypothesis he sought and obtained funding from the Rockefeller Foundation and then for about a month in August-September 1974 traveled alone across the Sahel area of Niger, often on foot, collecting vegetation and soil samples.  He tested them for ice nuclei content using the method described by Schnell and Vali in 1976 (1) (referred to as the Univ. Wyoming report AR111 in Russ’s report to the Rockefeller Foundation).

In his report to the Rockefeller Foundation (2), Russ presented a series of freezing spectra for the vegetation, litter and soil samples illustrating that vegetated areas contained more active immersion freezing ice nuclei than nearby less-vegetated areas.  Interestingly, the most active ice nuclei (active at -7° C) were from vegetation closer to the Sahara Desert where all of the cattle and goats had died the two years before and vegetation was recovering. The results in the report were based on a partial analysis of the samples; a more comprehensive analysis of the data led to the same conclusions.

The drought ended in October 1974 and the Rockefeller Foundation lost interest in the project. It was particularly frustrating for Russ to not be able to assure the Foundation that the local source of active ice nuclei was possibly important in the precipitation process. Without any assurance of this importance, the Foundation was not interested in funding any further work.  Also, soon after Russ moved to Kenya, Africa to work for the UN in a different orientation – but he has kept all of his precious log books that contain the data from this trek in the Sahel.

Almost 40 years later, drought and desertification are rampant and are threatening major world economies as well as the developing world. If, as a scientific community, we needed to provide guidance for the best ways to exploit natural reservoirs of the most active ice nucleators to help fend off disaster, what would we say?  What key observations would we need to assure ourselves and to assure the end users of such guidance?



  1. Schnell R.C. and Vali G. 1976: Biogenic Ice Nuclei: Part I. Terrestrial and Marine Sources. J. Atmos. Sci., 33, 1554–1564. (see the Mendeley data base for the link to this paper).
  2. Schnell R. 1974. Biogenic and inorganic sources for ice nuclei in the drought-stricken areas of the Sahel – 1974.  Interim report to the Directors, Rockefeller Foundation, New York
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