Precipitation in Africa – A Warming World

I trust you are well and ready to get stuck into some important issues! In this blog post I’m going to be discussing one of the major impacts of predicted climate change on African water resources – that on the frequency and intensity of rainfall. As we saw last week, there is a strong coupling between rainfall and river discharge in Africa, and as such, changes in the rainfall regime of Africa are going to have serious impacts on river discharge and thus water availability.

The Clausius-Clapeyron Relation

There is one scientific relationship that is particularly important when thinking about the impact of climate change on precipitation in Africa – the ‘Clausius-Clapeyron Relation’ (Figure 1). This dictates the increasing ability of air to retain water vapour with increasing temperature; as such, warm air has a much greater capacity to hold moisture than cold air. A very well determined value is the change in water-holding capacity of the atmosphere, governed by the Clausius-Clapeyron equation, of about 6.5% K^-1 (Trenberth et al., 2003; Owor et al.,2009).

Figure 1. The Clausius-Clapeyron Relation.

The mean annual temperature rise over Africa is likely to exceed 2 ° C relative to the last 20^th century by the end of this one – and that’s just under the medium scenarios (Niang et al., 2014). With this temperature rise, the air is going to be able to hold more moisture before it reaches dew point, condensation forms and precipitation occurs, as dictated by the Clausius-Clapeyron Relation. This leads to heavier, more extreme rainfall – so we move to a situation with a greater number of heavy, extreme precipitation events (i.e. those in the uppermost quartiles of the rainfall distribution) (Trenberth et al., 2003; Owor etal., 2009). Allan et al (2010) identify a 60% increase in the frequency of the wettest 0.2% of rainfall events per K warming. Furthermore, because the heaviest rainfall events usually deplete air of all its moisture, this rise in extremely heavy rainfall events is necessarily joined by a fall in the number of low and medium intensity rainfall events, or an overall decrease in the frequency of rainfall events (Trenberth et al., 2003; Owor et al., 2009). This change in the distribution of rainfall will result in more variable river flows and soil moisture (Owor et al., 2009).

There is already evidence that extreme precipitation changes over eastern Africa, such as droughts and heavy rainfall, were more frequent during the last 30-60 years (e.g. Williams andFunk, 2011; Lyon and DeWitt, 2012). Projected increases in heavy precipitation over the region have been reported with high certainty in the SREX (Seneviratneet al., 2012), and Vizy and Cook (2012) indicate an increase in the number of extreme wet days by the mid-21^st century.

Basically, this means there are fewer rain events, and the ones that do occur are more extreme. But the bad news doesn’t stop here I’m afraid. This relationship is exponential, which means that the higher the starting temperature, the greater the increase in the air’s ability to hold moisture with that 2° C rise in air temperature – NOT good news for the tropics! The colder starting temperatures of the temperate regions – like us here in the UK! – leads to a comparatively smaller increase in the ability of air to hold moisture with global warming. So, despite greater increases in surface temperatures projected for higher latitudes, the exponential relationship with temperature in the Clausius-Clapeyron relation means there is a bigger absolute increase in moisture amount able to be held in the air in lower latitudes compared to higher latitudes (Trenberth et al., 2003; Owor et al., 2009).

Current Evidence

Allan and Soden (2008) use multiple modelling simulation and satellite observations to examine the response of tropical precipitation events to naturally driven changes in surface temperature and atmospheric moisture content. As such, the studies used natural climate variability induced through El Nino and La Nina events to successfully demonstrate a direct link between higher frequencies of very heavy precipitation with warm El Nino events (a statically significant relationship, according to Allan et al., 2010), and lower frequencies with cold La Nina events. Ultimately this supports the argument that temperature rise in Africa will lead to the increased frequency of extremely heavy rainfall at the expense of light rainfall. The studies also found satellite observed amplification of rainfall extremes under this climate change regime to be several times larger than that expected from the Clausius-Clapeyron Relation and at the upper limit of most responses in model simulations, implying that model projections of future changes in rainfall extremes in response to anthropogenic global warming may be currently underestimated (Allan and Soden, 2008; Allan et al., 2010).

Figure 2 describes these trends in the frequency of light- to heavy- precipitation events with time for GPCP data (Allan and Soden, 2008). Taking the straight horizontal line as the mean, it can be seen that there is an increase in frequency above the mean for rainfall events in >60 precipitation percentile (i.e. the heaviest rainfall events), and a decrease in frequency for rainfall events in the smaller precipitation percentiles.

Figure 1. Trends in the frequency of precipitation events with time for GPCP data. Source: Allan et al., 2010.

Few! So there we have - with temperatures rising over Africa, there's going to be an increase in extreme, heavy rainfall events, and a decrease in low- to medium- rainfall events. There is already satellite and model evidence for this. Not only this, but the effects of this are going to be greater in the tropics than in higher latitudes.