Historical and future changes in climate records and extremes in South Africa

Abstract

Extreme climate events, by definition, are rare. However, due to climate change, the frequency and intensity of these events are changing, with many regions across the globe experiencing an increase in the occurrence of these. It is unequivocal that these types of events are expected to continue to increase into the future. Their impacts are expected to affect societies differently depending on various social-economic, political, and environmental factors. However, no matter where a particular society finds itself, there will be a need to respond to these changes to lessen the negative impacts of these types of events. With scientific knowledge about how our climate is changing, formulating mitigation and adaptive interventions becomes more accessible, especially if that knowledge is at a national or local level. Accordingly, the main objective of this research is to determine the historical and projected changes in the likelihood of occurrence of climate records and extremes over South Africa and the possible consequences of these changes on relevant socio-economic sectors. The research focused on using statistical methods to understand the changing landscape of probabilities of weather and climate extremes. Therefore, the magnitude of the values to be investigated was on the scale of record-breaking observations and extreme values associated with multiple-year return periods. The warming trend at individual stations, specifically the positive (right) tails of the distributions, was considered in the study. Consequently, extreme value theory was applied to study temperature and precipitation extremes changes. Various statistical approaches were pursued depending on the nature of the variable under consideration. For example, the Peak-Over-Threshold (POT) method was applied in the precipitation study because extreme can occur multiple times in wet years and not in dry years, and thus, using threshold values rather than the highest annual value becomes applicable. Threshold and critical values were also considered in the projections of temperature extremes and daily rainfall extremes to analyse changes in the distribution of these values. Considering that the research focuses on both changes from a historical and future perspective, both observational data and model projections were analysed, focusing on daily temperature and precipitation extremes. Key findings of the study were an increase in the expected number of record-breaking daily maxima of maximum and minimum temperatures compared to a stationary climate. For example: On average, the highest maximum to lowest minimum records ratio was 1:1 in 1951, increasing to 9:1 in 2019. In addition, different spatial patterns in terms of breaking daily high temperature records were observed, with stations in the interior of South Africa presently experiencing the highest probability of breaking daily maximum temperature records. For daily highest minimum temperature records, the present probabilities are less defined on a regional basis. In contrast, the number of daily lowest maximum and minimum temperature records decreased below the expected in a stationary climate, so much so that in some locations, no records were broken in the last decade. The average warming trend was able to predict the occurrence of records to an extent. When considering the projected occurrence of high-temperature extremes into the far future (2036-2095), it was found that most stations showed a decrease in return periods and, therefore, a consequent increase in return period values (RPVs). The most significant changes were expected to occur under the RCP8.5 scenario compared to RCP4.5, with these changes most evident at the end of the century. Even after bias correction, the models underestimated the extent of the warming in the right tails of the temperature distributions, and thus, the projected changes found in the study are seen as a conservative estimation of potential changes in extreme temperatures in the future. In terms of daily rainfall extremes across South Africa, this study found that most stations experienced an increase in the probability of receiving more than 50mm per day, considered to be significant, over the last century. The same applies to values greater than 75mm, i.e. heavy rainfall, and 115mm, defined as very heavy rainfall. Maximum values expected for relevant return periods have also increased. In summary, the research quantitatively confirms the widely held expectation that under an anthropogenically caused non-stationary climate, the probabilities of climate extremes are changing towards a situation of higher-than-expected probabilities of records and extremes, specifically with regards to daily values of temperature and rainfall. Therefore, the research contributes to the quantification of the changing probabilities in records and extremes and, if used, may contribute to the development of climate change-relevant adaptation measures in climate-sensitive sectors, e.g. agriculture, health, disaster management and the insurance and building industries. In addition, the estimations of changes in extreme events can assist in the spatial allocation of resources to mitigate specific weather and climate hazards.