Observed Climate Trends Western Cape
The Western Cape has experienced a substantial rate of temperature increase at approximately 0.2°C per decade at several locations, as recorded over the 1931-2015 period. Daytime maximum temperatures have increased steadily. Extreme warm events have also increased, such that the number of hot days has risen at a rate of about one day per decade. What is equally significant for pome and stone fruit production is that the mean annual minimum temperature has increased by approximately 0.15°C per decade (Midgley et al., 2016a) and the number of cold nights has decreased at approximately 1.5 to 2.5 days per decade.
Trends in annual average temperatures (top) and in annual number of cold nights (bottom) over the period 1931-2015. Filled symbols indicate significance of trend at the 95% confidence level. (Source: SA TNC, 2016; adapted from Kruger and Nxumalo, 2016)
Annual rainfall totals show sta¬tistically significant increases over the central southern interior of South Africa during the period 1921-2015, with the rate of increase as high as 10 mm/decade or more. Associated increases in the number of days with extreme rainfall (daily rainfall above the 95th percentile threshold) have also occurred in this region as well as several other regions of South Africa, at a rate of more than 2 days per decade in some places. Significant decreasing trends in annual rainfall have been recorded over the northern parts of Limpopo.
The analysis also confirmed (results not shown) that the positive trends in annual rainfall totals and extreme rainfall over the southern interior (including the eastern parts of the Western Cape and the western parts of the Eastern Cape) are related mostly to increases in summer rainfall. This type of rainfall is associated with thunderstorm activity, which is projected to increase under climate change.
For the Western Cape, the mean number of rain days per annum has decreased by approximately 2 days per decade, especially along the south coast. The reduction in rain days is most noticeable in late summer and autumn (Midgley et al., 2016a). Where no long term significant changes in rainfall are identifiable in the historical weather records, this does not mean that changes are not occur¬ring, it only means that currently there is insufficient evi¬dence to suggest that any changes identified are not an artefact of natural cycles (10-30-year cycles) and variability, rather than long term steady change. It is thus important to increase rainfall monitoring and analysis in the long term so that emerging trends can be identified.
Trends in total annual precipitation for individual stations (top) and in the 95th percentile of precipitation (bottom) for the period 1921 – 2015. Filled symbols indicate significance of trend at the 95% confidence level. (Source: SA TNC, 2016; adapted from Kruger and Nxumalo, 2016)
The Western Cape has in recent history experienced a high frequency of flooding events associated with intense winter frontal systems and/or cut-off low pressure systems. Ten significant flooding events were experienced in the period 2003-2008 (Holloway et al., 2010) followed by five high impact flooding events between 2011 and 2014 (Pharoah et al., 2016). The region has also experienced several droughts, some extending into multiple years. The devastating drought of 2015-2018 came on the back of droughts in 2002-2003 and 2009-2010 in some regions. In some parts of the Klein Karoo the drought has not yet been broken and has caused significant losses to stone fruit growers. Both flooding and drought events have had large impacts on the region ranging from loss of life and property damage, through to larger scale infrastructure damage, agricultural losses, costly response measures, and economic loss. Other extreme weather risks encountered include storms (strong wind), hail, extreme cold, snow, extreme heat, and severe humidity.
Hailstorms are relatively rare and localised in the Western Cape, but when they occur, they have the potential to inflict substantial physical and economic damages on fruit farms. In November 2006, a hailstorm in the Haarlem area (Langkloof) damaged almost 400 hectares of fruit trees and resulted in loss of employment and income for 354 farmworkers. Hail damage wiped out crops (apple, pears, stone fruit and onions) on scores of farms in the Ceres, Witzenberg and Koue Bokkeveld areas in November 2013 and caused significant damage to much of the remaining crop. The predictability of the occurrence of hail, and modelled projections of future risk, are quite poor because of the dynamic and chaotic nature of the weather systems giving rise to hail.
Scientific analysis cannot yet identify with high accuracy the contribution that climate change may be making to the frequency and severity of climate disasters in South Africa. As mentioned above, one study has shown that the probability of the 2015-2018 drought in the Western Cape was increased three times by human-induced climate change (Otto et al., 2018). Climate models are not yet sophisticated enough to model historical and future patterns of intense localised events such as hail and some storms. This tool thus focuses mainly on gradual climate changes, although extreme patterns of rainfall (dry and wet spells and long duration design of rainfall) and risk of frost (extreme low temperature) and sunburn (extreme high temperature) are included where the modelling can be conducted with sufficient confidence.