CCAB - Low Risk Scenarios - Biome Delineations 2011

Rutherford et al's (2000) analysis was based on climate data which, although relevant at the time of analysis, is now is extremely dated given advancements in climate science over the last decade. Key differences between the outcomes of the previous study and the current study include that Rutherford et al (2000) indicate an almost complete loss of the Succulent Karoo climate envelope as a result of dramatic predicted reductions in winter rainfall while this report suggests that winter rainfall patterns will change slower than originally predicted and, therefore, contractions in the Succulent Karoo biome will be less pronounced in the short to medium term. Over the last decade, the science of climate change has evolved rapidly. Nevertheless, there is considerable uncertainty about the evolution of climate over 50 or 100 year time-scales, and while confidence in global circulation models is growing there is greater appreciation of the uncertainties involved, especially in ‘downscaling' the global models to produce climate projections at the regional and local scales.

Acknowledging this unavoidable uncertainty, we have developed a statistical approach to incorporate a wide range of possible climate scenarios in impacts models that uses median, and 90th and 10th percentile changes in temperature and rainfall from a number of general circulation models, from which future scenarios are developed. These scenarios were compiled by SANBI (Guy Midgley and Danni Guo). Highest temperature increases and largest rainfall decreases were combined to generate a ‘high-risk' or ‘worst case' scenario, and vice-versa for a ‘low-risk' or ‘best case' scenario. Likewise the median projected changes in rainfall and temperature were combined to generate an intermediate scenario.
Based on outputs from 15 global circulation models that were spatially interpolated, we developed three climate scenarios for South Africa for approximately 2050 as follows:1
- Best case scenario: smallest predicted increases in temperature and changes in rainfall,
- Intermediate scenario: middle of the range (median) predicted increases in temperature increases and changes in rainfall,
- Worst case scenario: greatest predicted increases in temperature and changes in rainfall.
This means that the results presented are not dependent on any particular global circulation model but hold under a very wide range of possible climate futures, enhancing the robustness of conclusions as climate changes and as climate science revises outputs and projections.
Note that the study was based on medium term data (for 2050) as this represented a compromise between the uncertainty associated with the very long time horizon data (2100) and the very small changes predicted by the shorter duration data (e.g. 2020). This 50 year time horizon also represents a reasonable long term planning horizon as it is within the lifetimes of most people currently living. When this study is updated based on new climate data, if possible (we are dependent on work done by a third party), we will include both the medium term and long term predictions. In the interim, as climate change occurs gradually over time, it is useful to conceptualize the worst case scenarios for 2050 as being likely to represent the intermediate case scenarios in 2100.

Available documentation:
Driver A., Sink, K.J., Nel, J.N., Holness, S., Van Niekerk, L., Daniels, F., Madjiet, P.A., Jonas, Z. and Maze, K. 2012. National Biodiversity Assessment 2011: An assessment of South Africa's biodiversity and ecosystems. Synthesis Report. South African National Biodiversity Institute and Department of Environmental Affairs, Pretoria.