Impacts of global climate change on African water

Global climate change and water availability in Africa
The impacts of global climate change on Earth's system are increasingly acknowledged, particularly on the availability of water resources. 

Projection of future climate change and African water

Projecting future climate change usually involves processes like downscaling General Circulation Models (GCMs) into higher resolution Regional Climate Models (RCMs), then estimate the future water flows and stores. It is often validated with observation data, however in some regions particularly in Africa lacks most of its observed data (e.g. precipitation or river discharge), increasing the uncertainty in projection of future climate change. 

      One of the examples for predicting climate change and its impact of water resources can be referred to the project led by the Egyptian Ministry for Water Resources and Irrigation (MWRI), DHI and the UK Met Office Hadley Centre, which targets to conduct “Regional Climate Modelling of the Nile Basin: Preparation of climate scenario outputs for assessment of impact on water resources in the Nile Basin”(Butts and Lørup, 2009) since the impacts of climate change on the Nile river basin are expected to be very critical in managing the water sources for various countries sharing the basin water. Many activities were involved to assess the climate change impacts, and developing a RCM for the Nile Basin was part of the scheme (See figure 1); RCM covering the whole region of Nile river Basin has been developed by UK Met Office Hadley Centre, performed with PRECIS ( a modified RCM from HadRM3 model, designed to run on PCs - Jones et al., 2004).  

Figure 1 - RCM (left) and GCM (right) projections of Nile river Basin (from UK Met Office Hadley Centre in Butts & Lørup, 2009). 

The next phase of the project was to apply such climate projections to the grid-based hydrological model ('the Nile Forecasting System'), to assess more detailed impacts on Nile Basin water resources.

Although RCMs have advantage over GCMs, such as higher resolution and greater detail for climate simulations, it still incorporates high uncertainty in projections of climate change. This can be critical in planning the water management schemes especially in regions like Africa where high variability in hydrology exists inherently. 

Impacts of climate change on African water resources

Global climate change trends
Globally, seasonal precipitation range is expected to increase under warming climate scenarios, as well as the precipitation difference between Northern and Southern hemisphere in the boreal winter and summer. These are illustrated by the climate model simulations and past observation analyses, implying that there is a tendency of wet season getting wetter and dry season getting slightly drier or remains same as now (Chou et al., 2007).  

Impacts on Africa
For the climate change impacts specifically on African region, it can be divided into three major areas: 

1) The warming in Africa will continue, likely to exceed 2°C (mean annual temperature) under SRES (Special Report on Emissions Scenarios) A1B and A2 scenarios by the end of this century (Niang et al., 2014). Moreover, the land temperature over Africa is likely to increase faster than the global land average - especially in the more arid regions. 

2) The reduction in precipitation is likely over Northern Africa and Southwestern parts of South Africa by the end of twenty-first century (under SRES A1B and A2 scenarios) (Niang et al., 2014). 

3) Longer period of droughts especially in southwestern Africa and increased variability in river discharge (Niang et al., 2014). 

4) Intensification of precipitation from global warming in Africa will lead to fewer, lower and medium intensity rainfall events while more very heavy rainfall events - hence more extreme precipitation events. Also, in regions of high or complex topography (e.g. Ethiopian Highlands) will likely to experience increases in rainfall & extreme rainfall by the end of twenty-first century (Niang et al., 2014). 

Intensification of precipitation in Africa
Intensification of precipitation in Africa is partly resulted from global warming and associated impacts of 'Clausius Clapeyron relation'; warming air holds more moisture in the atmosphere, leading to increases in low-level moisture with rising temperature as a consequence of Clasius-Clapeyron relation (Allan et al., 2010). Thus, such increase in moisture contributing to the intensification of (extreme) precipitation. 


Furthermore, variable precipitation may lead to more variable soil moisture, impairing the crop yields in Africa. For instance, a study by Ahmed et al. (2015) illustrates that decreases in productivity of some crops are already observed in regions like West Africa - where vulnerability to climate change is relatively higher than other regions in Africa. Both projected increase in temperature and shift in patterns of precipitation are presented to be the major factors influencing the crop yield change. Also, inter-annual variability of crop yields are predicted to increase by the mid-21st century (Ahmed et al., 2015) in West Africa despite projected rise in precipitation in some areas. This is a crucial issue particularly in Africa since its projected population growth is massive (African population can reach 4.4 billion people by 2100 - mentioned in previous post) and regional food security problem can be exacerbated with such predicted intensification in precipitation (Challinor et al., 2007). 

Interestingly, some studies suggest that such precipitation intensity change can affect groundwater recharge positively in the tropics; Study by Jasechko and Taylor (2015) related currently available long-term records of stable isotope ratios - of Oxygen (O) and Hydrogen (H) of modern groundwater - in 15 different tropical precipitation sites. The results indicated that 14 out of 15 tropical sites showed groundwater recharge biased to intensive precipitation (in monthly data), often exceeding the 70th decile (Jasechko and Taylor, 2015). Yet, the processes associated with transmitting intensive rainfall to groundwater systems and (accordingly) enhancing groundwater replenishment remain unclear. 
     Additionally, such results only signify the tendency of increased groundwater replenishment related with intensive rainfall as a result of global warming, and other factors influencing groundwater systems such as land-use/cover change or human disruption (Jasechko and Taylor, 2015) may have also included in the investigation. Nonetheless, such favour in groundwater recharge from intensive precipitation will place groundwater in a more crucial role particularly in regions like Sub-Saharan Africa where projected population growth in very high and food security problems likely to increase in the future (Taylor et al., 2009).