Stefan Hastenrath and his colleagues diligently measured the annual mass change occurring at the surface of the Lewis Glacier on Mt Kenya from 1979-1996, and we restarted this in 2010. The World Glacier Monitoring Service holds the older data from 1981-1990, and 1992-1996. I do not know why 3 years are missing, but I added 1979, 1980 and 1991 to the dataset from Stefan Hastenrath’s book: Glaciological Studies on Mt Kenya 1971-2005, published in 2005 by the University of Wisconsin-Madison. At Lewis Glacier it is conventional to measure the annual mass change from March to March, as March is often the minimum size of the glacier within a given year. the annual mass balance of 1979, refers to the mass change between March 1978 and March 1979.
If you plot the mass changes at the surface as a function of elevation, you get the vertical mass balance profile, and here I plotted it in units of millimeters water equivalent [mm w. e.], that means I am expressing the mass of snow and ice at the surface in terms of their depth equivalent if the snow and ice were melted into water. Glaciologists and snow scientists do this because the density of snow and ice can vary a lot, but water can be considered a constant and so these units facilitate inter-comparison of data.
In this figure I’ve plotted all the years measured by Stefan Hastenrath:You can see it varies quite a lot, but as the glacier was shrinking during this time most of the lines lie in the negative mass balance area.
I was wondering if the years we have measured were very different from those measured between 1979-1996, so here I plotted out stake measurements as dots between the outlier profiles of 1988 (which was the year with greatest mass loss between 1979-1996) and 1989 (which was the only year in which the glacier gained mass in all the years measured). I also plotted the first and last year of Hastenraths data.
If you have any trouble with the terms I’ve used here, you can always look up the Glossary of Mass Balance and Related Terms, which I posted about here: http://lindseynicholson.org/2011/07/glossary-of-glacier-mass-balance/