Climate change science report for the Western Cape

Temperature and rainfall trends

The report provides strong evidence that the climate of the region is shifting and will continue to shift into the future. As a whole, the region has experienced significant increases in temperature across all zones and all seasons over the past century, with more rapid warming over the past 30 years. Furthermore:

• Temperature trends are very consistent across all zones. Historical trends are approximately 0.1°C/decade, which is in line with global rates.

• Daily maximum temperature trends are slightly larger in the inland (north/east) zones, in line with continental patterns of higher inland temperature trends.

• Daily minimum temperature trends are slightly larger closer to the coast and to the north, which likely reflects shifts in humidity and cloudiness.
  Figure 1 shows the absolute trends in seasonal total rainfall (mm/decade) over the SmartAgri zones based on the CHG CHIRPS merged rainfall product over the period 1982 – 2020. Diagonal hashing indicates trends that are not statistically significant with a p-value threshold of 0.05. MAM=autumn; JJA=winter; SON=spring; DJF=summer. Absolute trends are strongest during the autumn season, with drying found across almost all the zones in this season. The study also presents an analysis of the Day Zero drought of 2015 – 2018, and identifies a strong role of climate change in the increasingly frequent and intense droughts in the region. The main findings include:

• Annual and seasonal rainfall trends are mixed and – for many zones – not statistically significant.

• Autumn (March – May) exhibits the most dominant statistically significant trends, with the largest negative trends in some of the interior zones such as the Bokkeveld.

• The core winter season (June – August) exhibits a mix of drying and increasing trends, but only the Bokkeveld, Cederberg and Hardeveld zones stand out as having a significant trend, with the Bokkeveld exhibiting very strong drying trends.

• Spring (September – November) exhibits relatively weak and insignificant trends, except for some interior and north-eastern zones that are more dominated by summer rainfall dynamics. These trends suggest a tendency towards a later start to the summer rainfall dynamics.

• Core summer (December – February) trends are largely weak and insignificant, except for the zones from the Cederberg through to the Sandveld/Hardeveld along the west coast.

• Trends in consecutive wet days largely follow the same pattern as trends in total rainfall.

• Trends in consecutive dry days are largely insignificant, except for a negative trend during the September – November period.

• Trends in potential evapotranspiration (PET) are significantly positive – consistently – driven by consistently positive and significant temperature trends.

• Trends in PET are highest in the most southern and the most western zones, while being strongest in spring (September – November) and summer (December – February).

Projected future climate

Projected changes in temperature show that the strongest warming will occur in the interior summer rainfall regions, further away from the moderating influence of the oceans. Increases in the maximum temperatures in the hot season and minimum temperatures in the cold season show spatial patterns that are similar to those of average temperatures.

There is a broadly consistent message of decreasing rainfall across all regions and hence, across the whole province. Changes in rainfall indices, on the other hand, show some variation across the province so that, while reductions in rainfall are strongly dominant, it is possible that different patterns of change may unfold. Different dynamics are involved in producing changes in summer rainfall over the northern/eastern parts of the province versus the southern and western regions. The clear message is that reductions in rainfall should be anticipated across the region, but some sub-regions may experience much stronger reductions than other sub-regions.

The analysis of projected changes in water balance (Standardised Precipitation Evaporation Index, SPEI) and PET shows very clearly that increases in temperature, involving very little uncertainty, strongly dominate any uncertainty in rainfall changes. This is the key message for agriculture across the province.

The updated climate analysis for the agricultural sector of the Western Cape will now be widely disseminated and used to update the SmartAgri plan where necessary.

The full report can be downloaded here: SmartAgri – Western Cape Department of Agriculture ( elsenburg.com )

Reference:

Jack, C., L. Van Aardenne, P. Wolski, I. Pinto, K. Quagraine, and P. Kloppers. 2022. SmartAgri: Updated climate change trends and projections for the Western Cape. Report for the Western Cape Department of Agriculture. Climate System Analysis Group, University of Cape Town.

For more information, contact
Prof Stephanie Midgley at: Stephanie.Midgley@westerncape.gov.za.