Publications
Publication details
Nyssen, J., Poesen, J., Moeyersons, J., Deckers, J., Mitiku, H. & Lang, A. 2004. ‘Human impact on the environment in the Ethiopian and Eritrean highlands — a state of the art’. Earth-Science Reviews 64: 273-320. Elsevier. (PR).
Article in a scientific Journal / Article in a Journal
This review analyses the environmental evolution of the Ethiopian highlands in the late Quaternary. The late Pleistocene
(20,000–12,000 14C years BP) was cold and dry, with (1) low lake levels in the Rift Valley, (2) large debris fans on the flanks of
Lake Abhe´ basin, and (3) the Blue Nile transporting coarse bedload. Then, a period with abundant and less seasonal rains
existed between 11,500 and 4800 14C years BP, as suggested by increased arboreal pollen, high river and lake levels, low river
turbidities and soil formation. Around 5000–4800 14C years BP, there was a shift to more arid conditions and more soil erosion.
Many phenomena that were previously interpreted as climate-driven might, however, well be of anthropic origin. Thick
sediment deposits on pediments as well as an increase of secondary forest, scrub and ruderal species in pollen diagrams are
witnesses of this human impact.
One important aspect of the late Quaternary palaeoenvironment is unclear: changes in Nile flow discharges and Rift Valley
lake levels have been linked to changes in precipitation depth. Most authors do not take into account changes in land use in the
highlands, nor changes in the seasonality of rain, both of which can lead to a change in runoff coefficients. Tufa and speleothem
deposition around 14,000 years ago tend to indicate that at the end of the Last Glacial Maximum (LGM), conditions might have
been wetter than generally accepted.
The most important present-day geomorphic processes are sheet and rill erosion throughout the country, gullying in the
highlands, and wind erosion in the Rift Valley and the peripheral lowlands. Based on existing sediment yield data for
catchments draining the central and northern Ethiopian highlands, an equation was developed allowing to assess area-specific
sediment yields:
SY ¼ 2595A0:29 ðn ¼ 20; r2 ¼ 0:59Þ
where SY= area-specific sediment yield (t km 2 year 1), and A= drainage area (km2).
With respect to recent environmental changes, temporal rain patterns, apart from the catastrophic impact of dry years on the
degraded environment, cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere. Causes are changing land use and land cover, which are expressions of human impact on the
environment. Deforestation over the last 2000–3000 years was probably not a linear process in Ethiopia. Studies on land use
and land cover change show, however, a tendency over the last decades of increasing removal of remnant vegetation, which is
slowed down or reversed in northern Ethiopia by a set-aside policy.
Ongoing land degradation requires urgent action at different levels of society. Soil and water conservation (SWC) structures
are now widely implemented. Local knowledge and farmer’s initiatives are integrated with introduced SWC techniques at
various degrees. Impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion.
In high rain areas, runoff management requires greater emphasis during the design of soil conservation structures. In such areas,
investment in SWC might not be profitable at farm level, although benefits for society are positive. This pleads in favour of
public support.
The present land degradation in the Ethiopian highlands has a particular origin, which includes poverty and lack of
agricultural intensification. Causes of these are to be found in the nature of past and present regional social relations as well as in
international unequal development. This review strengthens our belief that, under improved socio-economic conditions, land
husbandry can be made sustainable, leading to a reversal of the present desertification and land degradation of the Ethiopian
highlands.