The earth scale temperature rise estimated in the IPCC reports ranges between +1,4 to 6,4°C, depending on the model and scenario, with a mean increase of 2.8°C (1.7 to 2. 4°C) for the A1B scenario. Bates and others (2008) indicate that since the 1960s, temperatures over Africa have been increasing. And while this is the general trend, there are variations across the Nile basin. For instance, in Ethiopia, minimum temperatures have increased slightly faster than maximum or mean temperatures (Conway and others 2004). A summary of projected temperature trends for the Nile basin is shown in the table 1 The results show a clear trend towards higher temperatures in the Nile basin. There appears to be little difference between the seasons both for the central estimate and for the high and low estimates. The hydrological consequences of this trend is that evaporation in the Nile basin will rise, consequently the losses in the Nile basin will increase.
In the Nile Equatorial Lakes Region, projections indicate a positive evolution of the temperatures but the uncertainty about the intensity of this evolution is high. This uncertainty is mainly due to GHG scenarios The mean annual temperatures have increased by 1.0°C since 1960 in Kenya and Tanzania, 1.3°C in Uganda. Daily temperature observations for Uganda and Kenya show significantly increasing trends in the frequency of hot days, and much larger increasing trends in the frequency of hot nights. Between 1960 and 2003, the average number of ‘hot’ days per year increased by 57 (+15.6%) in Kenya, and by 74 (+20.4%) in Uganda (Source: UNDP Climate Change Countries Profiles). The average number of ‘hot’ nights per year increased by 113 (+31%) in Kenya, and by 136 (+37.4%) in Uganda. The frequencies of cold days and nights also significantly decreased (about -5% for the cold days, and -11% for the cold nights. Rising temperatures have implications for evaporation and evapotranspiration with impacts on water availability.
Climate studies in the NEL region recommend the use of results estimated in the SSEA study, for 2 SRES scenarios: A1B and A1FI and resumed in the table 2 (difference of temperature in °C between 1961-90 and 2100) The trend for precipitation is very difficult to ascertain. Modelling outputs do not converge for the Nile basin and this results in a very wide range of possible trends: a significant fall to a significant rise in precipitation. The geographical variations can be significant and climate change in terms of precipitation must be evaluated at a local scale (ideally the river catchment). The extreme events – droughts, floods – will possibly be more frequent, but this cannot be verified by the present models and does not appear from historical analysis. A summary of projected temperature trends for the Nile basin is shown in table 3.
According to the central estimates, relatively small increases can be expected in annual rainfall. However, both magnitude and even the signal of the trends differ a lot between the seasons. For the winter period the central estimate envisages a large increase in rainfall. This will lead to a large increase in runoff from the equatorial lakes. For the runoff from the Ethiopian mountains the change in the period June, July, August is more important as this is the rainy period. In this period the change is a very small decrease. This decrease however has a relatively large effect on the inflow into Lake Nasser as 80% of the water origins from this area.
Strikingly, however is the huge range between the low and the high estimated changes, where even the signal, wetter or drier, conditions are not consistent. For the winter period, the overall picture is an increase in rainfall. For the summer period, the central estimate suggests a small decrease of the rainfall, the different models, however, vary substantially in their predictions, even in the direction of the signal. This uncertainty will be exacerbated in the resulting runoff changes due to the great sensitivity of the Nile discharge to rainfall variations.
Possible future precipitation in the NEL Region
Through the synthetic results of the different GCMs, the NELSAP SSEA studies note that possible changes in terms of precipitations are less convergent than for temperatures. The studies indicate a range of variation for precipitation going from negative to significantly positive, with a global increasing trend for the annual precipitation. Model simulations show wide disagreements in projected changes in the amplitude of future El Niño events (Christensen et al., 2007). East Africa’s seasonal rainfall can be strongly influenced by ENSO, and this contributes to uncertainty in climate projections, particularly in the future inter-annual variability, for this region. The SSEA study gives a percentage variation for precipitation for quarters. The SSEA models project a global increase in precipitation, both annually and for each season. The largest percentage increase is projected for the dry months of June through August. The range is however relatively important for some quarters, in particular for the dry months (June to August), for which the wet model indicates negative trends.
Impacts of Climate Change on Evaporation
Although great uncertainty in terms of evaporation exists due to the uncertainty mainly regarding precipitation, the impact on evaporation seems to be significant but relatively small. The rise expected is of the order of 70 mm / year (0.2 mm/day), compared to actual evaporation of the order of 1000 mm/year (less than 10% /year variation).