PhD thesis

I’ve noticed a that a few people have ended up at my website as a result of a search for ‘Nick Brooks phd’ (of course they may be searching for the work of another Nick Brooks – there are quite a few of us). So I’ve uploaded the chapters of the PhD thesis, which I completed in 1999 and which was formally awarded in 2000, by the University of East Anglia. The thesis was completed at the Climatic Research Unit (CRU), under the supervision of Mike Hulme and Mick Kelly, and was funded by CRU’s (as far as I know first and only) Hubert Lamb Studentship. Old news now, and the field has moved on, but as I didn’t quite get around to publishing much from it, perhaps it will be of interest to some. Note that I’m no longer active in this area of research (dust and climate), so there are better people to approach if you’re interested in this topic.

The thesis looked at recent historical changes in atmospheric dust loadings and the activity of source regions over the Sahara and Sahel, and examined the impacts of dust on the vertical temperature structure of the atmosphere over northern Africa, as well potential reasons for observed changes in atmospheric dustiness. The aim was to examine the potential for atmospheric dust to act as a positive feedback that reinforced drought in the Sahel. This was all done using climate reanalysis data (from NCEP/NCAR) and observations of outgoing longwave (infra-red) radiation from METEOSAT. The latter was converted into the Infra-Red Difference Dust Index (IDDI) by Michel Legrand at the University of Lille. Please contact Michel (michel . legrand [at] univ-lille1 . fr>) if you want to use the IDDI data. For more information on the IDDI see this article (2010, free pdf) or this one (2012, subscription required).

The main conclusions that I drew from this work were that (i) IDDI and reanalysis data indicate that dust tends to stabilise the atmosphere in a way likely to inhibit the convection necessary for rainfall, and therefore is a good candidate for a feedback mechanism reinforcing drought, and (ii) the dustiest years (in an admittedly short 10-year period) are those in which reanalysis data indicate the greatest number of convective disturbances, suggesting that observed increases in dustiness may be due to a shift to weaker convective disturbances that are strong enough to mobilise dust, but that do not generate the rainfall that would otherwise remove the dust from the atmosphere.

This provides an alternative explanation for increases in dustiness to the oft-invoked culprits of overgrazing and land degradatation, which many studies now indicate have been greatly overstated. The apparent desertification of the northern Sahel from the late 1960s to the early 1990s was not the result of overgrazing (people love to blame nomadic pastoralists for this sort of thing, regardless of the evidence), but instead was a manifestation of ‘natural’ variations in the strength and position of the African Monsoon. The word ‘natural’ is in inverted commas as there is growing evidence that these changes in monsoon behaviour were influenced by industrial pollution over the North Atlantic originating in Europe and North America. If we can attribute the drought to human activity at all, we should be thinking about rich-country aerosol emissions, not naughty nomads.

You can download front matter (including table of contents), individual chapters and appendices, and the bibliography from the thesis below, in pdf format.

Dust-climate interactions in the Sahel-Sahara zone of northern Africa, with particular reference to late twentieth century Sahelian drought

Nick Brooks

Front Matter

Chapter 1 – Introduction and overview

Chapter 2 – Climate change in the Sahel: the scientific context

Chapter 3 – Data and general methodology

Chapter 4 – Observed and modelled Sahel rainfall variability on long timescales

Chapter 5 – Dust variability over northern Africa

Chapter 6 – Climatic influences on dust production

Chapter 7 – The impact of atmospheric dust on climate

Chapter 8 – Conclusions

Bibliography

Appendix 1 – IDDI and SLP

Appendix 2 – IDDI and wind

Appendix 3 – IDDI-temperature correlations

Appendix 4 – IDDI-vertical velocity correlations

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