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Several integrated studies have looked at feedbacks and complex interactions in African regional systems. In the west African Sahel, land surface-atmosphere interactions have been examined in great detail to explore their role in interannual variability of rainfall since the long drought that started in the late 1960s. Reviews by Nicholson (2000) and Hunt (2000) summarize the state of knoweldge for the physical climate of the Sahel. In general, surface processes modulate rainfall variability, along with SST—but in complex ways. The Sahel is likely to remain a major study topic, and field-based observational studies are on the rise. A field campaign in 1992 called the Hydrologic Atmosphere Pilot Experiment (HAPEX)-Sahel was designed to find ways of improving modeling of land surface properties; a series of HAPEX-Sahel papers were published in a special issue of the Journal of Hydrology (Goutourbe et al., 1997). More recently, a regional land-atmosphere experiment is underway in southern Africa to study fires and emissions, with modeling studies planned to explore land-atmosphere linkages (see <http://safari.gecp.virginia.edu>, accessed October 2000). These studies—and integrated modeling in general—offer comprehensive tools for studying the integrated Earth system. More studies of this kind will assist in our understanding of linkages between land surface processes, regional and global linkages, and human activities.
Biomass burning plays an important role in global atmospheric chemistry (Andreae, 1991), particularly with respect to generation of trace gases that lead to the formation of tropospheric O3, carbon monoxide (CO), nitrogen oxides, CH4, volatile organic carbon (VOC, which also is a GHG), and smoke particles (aerosols), which have an anti-greenhouse effect. Africa is a significant location of biomass burning. In Africa, there are three main types of biomass fires: those associated with land clearing for agriculture, which are mostly located in humid tropical forests and the subhumid tropics; burning of wood for domestic energy (either directly or after first converting it to charcoal); and fires in natural and semi-natural vegetation, which are not associated with changes in land cover or use. Emissions from all three types are of broadly comparable magnitude (Scholes and Scholes, 1998), although the last category has received the most attention.
Fires in natural vegetation are not considered to be a net source or sink in the global carbon cycle because when integrated over large regions and over several years, CO2 (as well as CO, CH4, and VOC, which ultimately converts to CO2) emitted by the fire is taken up again by vegetation regrowth. This is true if the overall fire frequency or intensity is not changing, but if fires become more frequent or consume more fuel over time, the result will be a net CO2 source; conversely, if fire frequency is reduced or the fires burn less fuel, a carbon sink will result, manifested as an increase in woody biomass. There is no evidence that at a contental scale, fires in natural vegetation have increased or decreased in frequency or intensity in the historic period. For some subregions (such as parts of southern and east Africa), where there is clear thickening of the woody vegetation, it is likely that fire regimes have become less frequent and intense during the 20th century.
During years of regional drought, such as those in southern Africa associated with El Niño events, the area burned decreases by about half (Justice et al., 1996). It is believed that this is caused principally by a decrease in fuel availability.
The potential for teleconnections in impacts of land-use change on distant climates further increases the risk in communities that may be at low risk but will be impacted by actions taken in distant areas. For example, deforestation of the central African basin leads to climatic impacts in the savannas to the south in GCM modeling experiments (Semazzi and Song, 2001).
It is clear that rainfall (e.g., intensity) combined with land-use conversion in watershed areas leads to increased soil erosion. Enhanced siltation in rivers and increased use of chemicals also leaching into the runoff interferes with river chemistry (e.g., eutrophication), with major implications for water quality in lakes and coastal systems. Impacts on biodiversity—hence important economic fisheries, and consequent feedback on national economies—is an area of research that needs elucidation.
There is great uncertainty about how climate might change at subregional scales in parts of Africa, especially how this might be influenced by human-driven factors such as deforestation and alternative land uses. Regional climate modeling will help reduce these uncertainties, and there are early results now of such modeling for parts of Africa. For these results to be useful, they will have to incorporate realistic disturbance regimes (e.g., realistic deforestation mechanisms and land-use characterizations).
Climate change will manifest itself through changes in climate variability and hence changes in extreme events. Given its socioeconomic status, Africa is unlikely to respond any better to extreme events than it has in the past. Flooding, droughts, and so forth are increasingly difficult for Africa to cope with given increasing pressures on resources from rapid population growth and dwindling resources. Most African countries remain largely unable to gather adequate data and information in a timely manner to address critical problems and surprises such as droughts and floods. Although progress is being made to design environmental information systems, models for analyzing impacts and policy options are largely nonexistent. Although adequate data probably exist, what is needed is the capacity to access large amounts of data and synthesize it into useful bits of information for decisionmaking. For example, satellite data have been collected over the past 2 to 3 decades, yet their use is largely restricted to mapping and short-term climate predictions. Effective information systems and monitoring have not been achieved.
Evaluation of impacts in monetary or other quantitative terms remains a major obstacle to comprehensive assessment of impacts of climate change for Africa. Regional integrated assessment modeling, such as in Egypt (Yates and Strzepek, 1998), offers a solution, and model development should be accelerated for subregions of Africa where building blocks exist (Desanker et al., 2001).
ENSO-related impacts remain uncertain, given perceived changes in ENSO events in terms of frequency and duration. However, much progress is being reported in prediction of ENSO, and this information should be closely linked with case studies of how different regions and populations respond to specific climate-related events. These studies should document costs and benefits, as well as responses.
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