https://orcid.org/0000-0002-8352-0209
Figures
Abstract
The growing effect of CO2 and other greenhouse gas (GHG) emissions on the extreme climate risks in the Western Cape, South Africa, calls for the need for better climate adaptation and emissions-reduction strategies to protect the region’s long-term social-economic benefits. This paper presents a comprehensive evaluation of changes in the future extreme events associated with drought and heatwave under three different greenhouse gas (GHG) emissions scenarios: Representative Concentration Pathway (RCP) 2.6, RCP 4.5, and RCP 8.5, from moderate to severe, respectively. Various diagnostic indices were used to determine how future heatwaves and drought will respond to each different RCP climate scenario in Western Cape based on Max Planck Institute-Earth System Model/REMO (MPI-ESM/REMO). The projected simulation results revealed that drought and heatwave extreme climate indices suggest strong relationships between future extreme climate risks and GHG emissions for Western Cape, South Africa. Anthropogenic activities and growing GHG emissions will lead to severer extreme climate stress in terms of drought and the duration, frequency, and magnitude of heatwave stresses. As a result, we believe that reducing the GHG emissions to alleviate future extreme climate stress becomes a practical solution to protect the local’s socio-economic system and further maintain the region’s economic prosperity.
Citation: He B, Ding KJ (2023) Global greenhouse gases emissions effect on extreme events under an uncertain future: A case study in Western Cape, South Africa. PLOS Clim 2(1): e0000107. https://doi.org/10.1371/journal.pclm.0000107
Editor: Fredolin Tangang, Universiti Kebangsaan Malaysia, MALAYSIA
Received: September 13, 2021; Accepted: December 15, 2022; Published: January 20, 2023
Copyright: © 2023 He, Ding. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All data and scripts in this study are available at Github (https://github.com/bowenhesteven/extreme_climate_indices_Western_Cape).
Funding: The authors received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Extreme climate events, such as heatwaves and widespread drought, have a strong potential to cause extensive damage and impact cities worldwide. Studies regarding heatwaves revealed that the increasing frequency and intensity of heatwaves were seriously affecting both developed and developing countries [1]. For example, in 2003, an extreme heatwave event occurred in Europe and caused nearly 20,000 deaths [2]. A study regarding droughts based on historical records reported that droughts reduced global crop production by 10% from 1963 to 2007 [3] and global climate models have predicted that drought conditions will intensify in major breadbaskets of wheat and maize globally [4]. Besides, recent research led by Cooperative Institute for Research in Environmental (CIRES) found that in the southern plains and southwest of the U.S., as soil moisture vanishes during severe droughts, cooling by evapotranspiration is more severely curtailed during droughts, suggesting strong interactions exist between heatwaves and drought, and occurrence of one may induce and even amplify the other [5]. Thus, when evaluating a region’s future extreme climate risks, it is necessary to consider both heatwaves and drought events [5]. Furthermore, these climate-related extreme events are increasing in frequency and magnitude due to anthropogenic climate change, and there is increased potential for greater impacts and damage due to the location of urbanization and the ongoing expansion of urban centers and infrastructures [6]. Thus, it is critical that we understand how and when these extreme events might occur and how they respond to different anthropogenic activities.
South Africa has always been one of the most vulnerable regions to climate change-induced extreme weather events. The region’s mean annual temperature has increased by at least 1.5 times the observed global average of 0.65°C during the past five decades [7,8]. Furthermore, other studies have shown that mean temperature across the subtropics, and central tropical Africa are rising at about double the global rate [9]. Changes in the features of extreme weather and climate events have also been observed over recent decades in South Africa [10]. Previous studies have shown the region’s averages of heatwaves frequency, duration, and intensity are increasing in association with the increasing mean temperature [11]. However, that may not necessarily be the case on a regional scale, as temperature varies from place to place depending on factors such as latitude, sea-level elevation, or prevailing weather conditions [11]. For Western Cape Province, the lack of rainfall, decreasing storage levels in major reservoirs, and increasing water demand driven by rapid population growth and urban expansion, combined with problematic and ineffective water management practices, make the region particularly vulnerable to extreme weather events induced by global warming and climate change [12,13].
Many studies that focused on the regional future climate risks such as drought only used simple climate characteristics such as precipitation and maximum surface air temperature to evaluate climate risk, overlooking the role of extreme events’ impact on the region’s social-economic system. For example, He and Ding [14] used a high-resolution GCM-RCM Coordinated Regional Downscaling Experiment Simulations (CORDEX) model chain to highlight the impact of GHG emissions on Western Cape’s local climate system. They stressed that efficient water-management practices and GHG emissions reduction strategies are vital to mitigate more severe droughts such as the “Day-Zero” crisis in 2018, especially for the City of Cape Town and several other coastal regions within the Overberg and Eden district. While their study highlights the projections of a drying and high-heat South Africa region, they only analyzed the region’s future climate risks from a series of basic climate metrics such as precipitation, near-surface air temperature, daily maximum surface air temperature, ignoring the combined effects of these climate signals that can cause extreme events such as drought and heatwaves. Molina et al. [15] employed Euro-CORDEX simulations to fully assess future heatwaves in the Mediterranean region. They highlighted that forcing global models and emissions scenarios play a significant role in future heatwaves in the Mediterranean region. While their study adopted comprehensive extreme events indices such as Warm Spell Duration Index (WSDI) to investigate future heatwaves, they only considered two diagnostic methods, including WSDI and Heatwave Magnitude Index daily (HWM) to study the future heatwave features from limited aspects. Besides, they only considered two emissions scenarios, RCP 4.5 and RCP 8.5. The article could benefit from including more climate scenarios in lower GHG concentrations, such as RCP 2.6, to provide a more comprehensive GHG mitigation strategy that can help alleviate future extreme climate stress. Sillmann and Roeckner [16] calculated comprehensive indices for temperature and precipitation extremes based on the GCM-RCM model chain Coupled Climate model consisting of the Atmospheric general circulation Model and the Max-Planck-Institute Ocean Model (ECHAM5/MPIOM) simulations. They concluded that in the climate projections for the twenty-first century, all considered temperature-based indices show a significant increase worldwide [16]. Although their research delivered integrated knowledge of future extreme events from all possible perspectives, their global-level results can hardly be downscaled to inform regional hotspots of climate change, such as Cape Town, South Africa.
The overarching goal of this study is to quantify the effect of climate change on future extreme events in Western Cape Province at the sub-regional level under three GHG emissions scenarios: RCP 2.6, RCP 4.5, RCP 8.5.
Representative concentration pathway (RCP) 2.6 is a “very stringent” and optimistic pathway that requires that CO2 emissions start to decline by 2020 and go to zero by 2100 [17]. The scenario requires negative CO2 emissions that can be achieved by influential policy approaches and innovative Direct Air Capture (DAC) technologies such as liquid solvent systems [18].
RCP 4.5 is a scenario of long-term, global emissions of greenhouse gases, short-lived species, and land-use-land-cover stabilizing radiative forcing at 4.5 W/m2 up to the year 2100 [19]. RCP 4.5 is a stabilization scenario that assumes that climate policies are invoked to achieve the goal. Like RCP 2.6, RCP 4.5 requires negative CO2 emissions that can be achieved by shifting to lower emissions energy technologies and deploying carbon capture and geologic storage technology [19].
RCP 8.5 is a scenario that does not include any specific climate mitigation strategy. People are conducting business as usual, leading to a radiative forcing of 8.5 W/m2 by the end of this century [20]. Hence, it represents the upper bound of the RCPs’ set system, which can also be called the business-as-usual scenario [20].
By conducting this research, we aim to answer the following research questions: (1) How do different GHG emission scenarios impact future extreme events such as drought and heatwaves in the Western Cape region? (2) How will different districts within Western Cape respond to the 3 GHG emission scenarios in terms of future extreme events such as drought and heatwaves? Various diagnostics methods were applied to project features of these extreme events in the future (2021–2100). Therefore, we can better understand how these extreme weather events will change by the end of the 21st century under different emissions scenarios. The paper is structured as follows: Section 2 describes the data and methods used in the study, Section 3 presents the results and discussions, and Section 4 delivers the conclusions and implications of the study.
Methods
Study region
We chose Western Cape Province in South Africa as our study region, which is in the southernmost section of Southern Africa (Fig 1). It is surrounded by Northern Cape Province and Eastern Cape Province, as well as the Atlantic Ocean in the west and the Indian Ocean in the south. It ranges from 15.0° E and 25.0° E longitudinally and 30.0° S to 35.0° S latitudinally. The Western Cape accounts for 12% of South Africa’s total agricultural area, provides 20% of the nation’s total agricultural production outputs, and nurtures a world-famous wine appellation [21,22]. The climate conditions across the region are the temperate Mediterranean, with warm, dry summers and mild, moist winters, rendering it favorable for cereal farming such as wheat, oats, barley, and viticulture [23–25]. Average summer temperatures range from 5°C to 27°C, while winter temperatures range from 5°C to 22°C [26]. The Western Cape is one of South Africa’s driest regions, with approximately 350 mm of annual precipitation, well below the national yearly average of 500 mm precipitation [27]. Precipitation is also highly heterogeneous and varies greatly, from semi-arid areas to relatively wet areas on the windward slope of mountains [28].
The study domain: (a) Six main districts (City of Cape Town, West Coast, Cape Winelands, Overberg, Cape Winelands, Central Karoo) in the Western Cape region (South Africa). The blue solid line delineates the political boundary of each main district in the Western Cape. (b) The Western Cape locates in the southernmost part of South Africa. Sources: Esri.
Western Cape is the fourth largest of the nine provinces and the third most populous province, with an estimated 7 million inhabitants in 2020 [29]. About two-thirds of these inhabitants live in the metropolitan area of Cape Town, which is also the provincial capital [29]. Western Cape is also the second-largest contributor to the country’s total GDP and one of the fastest-growing economies in the county (Statistics South Africa, 2020).
Data
In this study, we obtained the CORDEX “Phase 1” simulation data from the Earth System Grid Federation (ESGF). The CORDEX models have been proven to correctly capture the spatial distribution of major climate variables over the Western Cape region and reproduce the essential climatic features in the observed temperature and moisture fields [23]. Thus, they are reliable models to predict future extreme events over the Western Cape region. “Phase 1” data were made available at the daily temporal resolution, 0.44-degree spatial resolution, by far the most comprehensive GCM-RCM downscaled data available. We downloaded the data under AFR-44, which indicates the African continent with 0.44-degree downscaling. We selected several variables, including pr (precipitation), tasmax (daily maximum near-surface air temperature), and tasmin (daily minimum near-surface air temperature) as key input variables to calculate a variety of extreme events indices such as WSDI (heatwave), Consecutive Dry Days (CDD, drought) in Western Cape. RCP 2.6, RCP 4.5, and RCP 8.5 were selected as experiment configurations, and daily data were downloaded. We selected MPI-M-MPI-ESM-LR as the driving model because it is currently the only driving model available for all three RCP scenarios. Moreover, its overall performance is better than its predecessor ECHAM5/MPIOM model, based on a modified Reichler-Kim standardized error due to improvements in the extratropical circulation [30]. Furthermore, many previous studies have verified the credibility and advantage regarding the MPI-ESM-LR-REMO (GCM-RCM) chain on the projection of climate change signals over different CORDEX regions [31,32]. Complete information regarding three downscaled GCM-RCMs input simulation data in this study is summarized in Table 1.
Daily spatial climate data of the African continent for 3 RCP scenarios were imported and analyzed in the open-source program RStudio [33]. The aim of using RStudio at this step was to quickly retrieve the climate variable data of interest as input simulation data for further calculation. Specifically, we used the “ncdf4” package in RStudio to retrieve the climate characteristics data such as precipitation, daily maximum near-surface air temperature, and so forth for each RCP scenario for the whole African continent. We used the “ncks” command from NetCDF Operators (NCO) to downscale the data from the African continent to Western Cape, South Africa, to focus on our study area. The downscaled data were then imported as input simulation data into the ClimPACT2 package to be further analyzed and calculated to obtain a series of extreme events indices associated with drought and heatwave. ClimPACT2 is a powerful R software package that calculates the Expert Team on Sector-Specific Climate Indices (ET-SCI) and additional climate extremes events indices from data stored in text or network Common Data Form (netCDF) files.
In this study, we used CDD and Standardized Precipitation Evapotranspiration Index (SPEI) with a scale of 12 months to assess droughts. The CDD index has been extensively used in the climate literature to assess dryness and its impacts on the coupled human-natural systems [34,35]. For instance, Marengo et al. [36] used CDD to review, evaluate, and predict the drought situation in Northeast Brazil for the past, present, and future, respectively. The standardized precipitation-evaporation index (SPEI) is a comprehensive index that measures drought using precipitation and temperature data with the advantage of combining multiscalar characters to include the effects of temperature variability on drought assessment [37]. The index is an extension of the widely used Standardized Precipitation Index (SPI) that takes potential evapotranspiration (PET) into account [37]. A previous study by Nail and Abiodun [23] has confirmed the superiority of SPEI over standardized precipitation index (SPI) that SPI projections may underestimate the influence of global warming on drought because it doesn’t account for the effect of potential evapotranspiration (PET).
Besides, we adopted the Warm Spell Duration Index (WSDI) and five different heatwave indices (HWI) to evaluate heatwaves from several various aspects, including amplitude (HWA), duration (WSDI, HWD), frequency (HWF), magnitude (HWM), and quantities (HWN). WSDI is a popular extreme climate index that measures the duration aspect of heatwaves in many previous studies. For instance, Chen and Sun [38] adopted WSDI to evaluate heatwaves in China. They concluded that the risks of unprecedented heatwaves would be devastating for the country under the background of increasing GHG emissions and global warming. HWA is an index that measures the hottest day (amplitude) of the hottest event, and it has been widely adopted in previous studies [39,40]. HWD is an index that measures the length of the longest heatwave event. HWF is an index that measures the total number of days satisfying the heatwave criteria. HWM is an index that measures the mean heatwave events’ intensity, calculated by averaging the temperature from all participating heatwave days. HWN is an index that quantifies the total number of defined heatwave events. Complete information regarding the ET-SCI extreme events indices investigated in this study is summarized in Table 2.
It should be noted that all HWI in this study are custom percentile-based threshold indices that user needs to specify the threshold on their own. The reference percentiles used in calculating percentile-based indices were computed over a base period of 1961–1990 for long-term climate change assessments as recommended by the World Meteorological Organization [41]. To exhibit the spatial-temporal patterns of these extreme events indices, we averaged each extreme climate index every 20 years for each 0.44 by 0.44-degree inside the study area. The averaged extreme events indices data then were mapped for four two-decade spans from 2021 to 2100 for each of the three climate scenarios.
Results and discussions
Spatial-temporal patterns of drought indices
In this section, each extreme weather index from Table 2 under three different GHG emissions was spatially evaluated using outputs from three downscaled GCMs (Table 1). The results are well aligned with the previous studies by Naik and Abiodun [23], of which the projected changes in drought characteristics over the Western Cape show a robust drying signal under the RCP 8.5 emission scenario. Furthermore, He and Ding [14] also demonstrated that great potential of reducing climate risks and vulnerability exists in lowering GHG emissions for Western Cape region. Besides, by directly investigating extreme weather indices such as drought indices like CDD and heatwave indices like WSDI, this study better illustrates how emission reduction will protect the region’s socio-economic systems by alleviating future extreme climate stress.
The CDD value ranges from 11 to 79 days for the whole Western Cape area, with the inland districts such as West Coast experiencing the longest dry days and coastal districts such as Overberg experiencing the shortest (Fig 2). There is no significant variability for CDD value distribution under the scenario of the RCP 2.6 and the RCP 4.5 since few dry spots can be found from 2081 to 2100 compared to that of 2021–2041. However, the whole region is expected to experience much longer dry days under the RCP 8.5 scenario as opposed to a more heterogenous response in lower emission scenarios with much longer dry days for the inland districts such as West Coast and Central Karoo, and relatively shorter dry days for the coastal regions such as Overberg and Eden. This indicates that GHG emissions scenarios and geospatial characteristics, such as the distance to the ocean, might play an essential role in determining the duration aspect of drought for a region. In addition, the SPEI model projections indicate that the drought intensity increases across the Western Cape for all emissions scenarios, but the drying patterns are not homogeneous across the region (Fig 3). Projections of the SPEI drought intensity under the RCP 2.6 and the RCP 4.5 suggest that a more intense drying signal might occur towards the inland districts such as West Coast and Central Karoo, which aligns well with the previous CDD distribution pattern (Fig 2). Nonetheless, the drought intensity situation will worsen for the whole Western Cape region under the RCP 8.5 emissions scenario, with the whole area experiencing much more severe drought intensity and not many differences between the inland and coastal regions. Combined with the previous CDD spatial distribution, we consider that spatial location, such as the distance to the ocean, might have less significance in determining the intensity aspect of drought compared to drought duration for the region. Thus, the emissions scenario becomes the dominant factor affecting the region’s drought intensity.
Spatial-temporal patterns of CDD in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the CDD projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
Spatial-temporal patterns of 12-month scale SPEI in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the SPEI projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
Spatial-temporal patterns of heatwaves indices
The projections from three climate scenarios reveal that WSDI values in Western Cape range from 1 to 211 days (Fig 4). There is no significant variability for WSDI value distribution under the RCP 2.6 and the RCP 4.5 since few noticeable differences are found from 2081 to 2100 compared to that of 2021–2041 (Fig 4). However, the whole region will experience a much longer period of hot days under the scenario of the RCP 8.5 compared to lower emission scenarios with much longer hot days for the coastal districts such as the City of Cape Town and Overberg, and relatively shorter hot days for the inland regions such as the West Coast and Central Karoo. The strongest increase in WSDI occurs in urban regions such as the City of Cape Town underscores the anthropogenic influence, such as population growth and urban heat island effect on the high-temperature extreme climate events for the Western Cape area.
Spatial-temporal patterns of WSDI in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the WSDI projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
The observed projections of HWA range from 21°C to 49°C (Fig 5). The variabilities between the three emissions scenarios are smaller than drought-related indices such as the CDD and the SPEI, especially for the differences between the RCP 8.5 and the lower emissions scenarios (Fig 5). However, the extremely high temperature is not homogeneous across the region, with the inland areas such as the Cape Winelands and the Central Karoo likely to experience days with higher extreme hot temperatures in the future.
Spatial-temporal patterns of HWA in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the HWA projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
HWD projections range from 3 to 35 days with an approximately homogeneous distribution across the region under the RCP 2.5 and the RCP 4.5 scenarios (Fig 6). Besides, the variabilities between the three emissions scenarios are small, with limited regions near the coast, such as the West Coast and the Overberg, likely to experience an extreme long period of heatwave events during the last 20 years of the 21st century only under RCP 8.5 (Fig 6). Fig 7 displays that HWM ranges from 20°C to 43°C. The mean heatwave events’ intensity exhibits a heterogeneous spatial distribution that the inland regions such as the Cape Winelands and the West Coast will experience more extreme heatwave events. Fig 7 also shows a non-significant variability across the time dimension with similar mean heatwave events’ intensity distribution between 2021–2040 and 2081–2100 for all three emissions scenarios.
Spatial-temporal patterns of HWD in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the HWD projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
Spatial-temporal patterns of HWM in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the HWM projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
HWF ranges from 2 to 105 days (Fig 8). The projections exhibit a homogeneous spatial distribution across the region for the RCP 2.6 and the RCP 4.5 scenarios and a non-significant variability across the time dimension. However, HWF patterns prediction shows that there will be significant increases in the total number of days categorized as heatwaves for the Western Cape by the end of the 21st century under the RCP 8.5 scenario. Besides, Fig 8 displays that some inland regions, such as the Cape Winelands, the northeastern part of the Central Karoo, as well as limited areas close to the coast in the southern part of the Overberg district, will likely experience more hot days by the end of the 21st century under the RCP 8.5 emission scenario. This finding aligns well with the HWD pattern displayed in Fig 6.
Spatial-temporal patterns of HWF in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the HWF projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
HWN is an index that quantifies the total number of defined heatwave events. The HWN projections reveal that inland districts such as the northern part of the Cape Winelands, the northeastern part of the Central Karoo as well as the southern part of the Overberg will likely experience more heatwave events (Fig 9), which aligns well with the HWF patterns shown in Fig 8. There is a strong increasing trend of HWN across the region for the study period under the RCP 8.5, with more heatwave events occurring in the last 2081–2100 compared to the beginning.
Spatial-temporal patterns of HWN in Western Cape, South Africa (for 2021–2100) for 3 emission scenarios: (a-l) exhibit the HWN projection pattern for the each 20-year span from 2021–2100 under the 3 different GHG emission scenarios. Sources: Esri.
Regional trends of heatwave indices
We used the 10-year moving average time series of 5 heatwave indices for each district in Western Cape to assess the region’s overall temporal trend of heatwave conditions. Fig 10 displays the 10-year moving average time series of 5 heatwave indices (HWA, HWD, HWF, HWM, HWN) in 6 districts of Western Cape, South Africa, under three emissions scenarios. All five indices show a distinct pattern of increasing trends for all aspects of heatwaves for each district in the Western Cape region, from lower GHG emissions to higher GHG emissions scenarios. However, the magnitude of the increase for each heatwave index may vary tremendously for a different district. For example, Fig 10 exhibits that the amplitude (HWA) of heatwave events in the City of Cape Town is projected to increase by 9.6%. In comparison, the duration (HWD) of heatwave events in the district is projected to increase by over 300%. This indicates that different districts have various sensitivities to GHG emissions associated with heatwave features such as intensity and duration in Western Cape, South Africa. One possible reason that heatwave events will be severer for the City of Cape Town under the RCP 8.5 scenario than the other regions is that high buildings in urban areas cause urban heat island effect and generation of heat, making the urban center several degrees warmer than its surrounding areas.
10-year moving average time series of heatwave indices in 6 districts of Western Cape, South Africa (for 2021–2100) under 3 emission scenarios: (a-o) exhibit the 10-year moving average time series in 6 districts of Western Cape, Africa for each the heatwave index from 2021–2100 under the 3 different GHG emission scenarios.
Impact of GHG emissions on socio-economic system
Based on the results shown above, it is more likely to expect severer droughts and heatwaves across the Western Cape region when doing business as usual, as opposed to taking serious actions and making policies to reduce GHG emissions. Such impact will be realized on natural and human systems, such as agricultural production, and the wellbeing of the people living in the region. For communities with limited financial, managerial, and technical resources to support climate adaptation, like the Western Cape area, the effect of GHG emissions on the local’s socio-economic system can be further exacerbated [42]. Numerous studies have demonstrated that extreme climate events such as droughts can devastate smallholder farmers, directly impacting local food security [42]. And on top of the drought, heatwaves further accelerate the moisture evaporation of soil and crops, ultimately putting a toll on the crop’s yield [43].
Impact of drought and heatwaves on agriculture production
Previous studies have proven that crop production needs to adapt to the agricultural environment in the background of climate change [44]. As a region to produce over 20% of the nation’s total agricultural production outputs mainly consists of wheat and oats, it is necessary to maintain the sustainability of the region’s agriculture production, including irrigation and soil microbial community. Nonetheless, the business-as-usual emissions scenario can prevent the sustainability of the region’s agriculture production from prolonging. For instance, although wheat is generally a cool season crop and does not require much water, it does need between 12 and 15 inches of rain over a growing season to produce a good crop which cannot be satisfied under the scenario of RCP 8.5 by the end of the 21st century [45]. Oats need more water than most other grains. Previous studies have proved that severe drought spell during the sensitive period of oat production season has a quantifiable negative effect on yield, and the drought is one of the most essential key causes of interannual oat yield variability [46]. In addition, because of dry and heat interactions, extreme heat was much more damaging in arid than in normal conditions for crops like maize, soybeans, and wheat. Based on Matiu et al. [47] study, drought significantly decreased global yields of maize, soybeans, and wheat by 11.6%, 12.4%, and 9.2% respectively combined with the effects of high temperatures.
SPEI is proven to be a sensitive indicator of crop yield. A negative SPEI indicating a mild drought can reduce the crop yield from 20% to 70% based on the location and type [48]. Based on our study results, the West Coast district will suffer extremely dry under the RCP 8.5 scenario based on the SPEI at the end of this century (Fig 3(L)). As one of the main wheat production places in the Western Cape, it is reasonable to predict that the impact of the future drought on the district’s wheat production will be significant. Moreover, combined with Fig 10(I) and 10(O) that predicts over 50% increase in heatwave frequencies and events, the additive effect of extreme heat will make the future crop yield in the West Coast district alarming if we keep the business as usual.
In addition to sufficient water to grow crops, soil function and microbial diversity are critical to maintaining a region’s agricultural efficiency. Previous studies have demonstrated that combined heat-drought climate stresses may induce different microbial responses that may damage the soil function to high yields compared to those observed individual extreme climate events such as heatwaves or drought alone [49]. Bindi [50] also illustrated that to maintain agricultural sustainability in the background of global warming, it is necessary to consider the adaptation options with the multifunctional role of agriculture, and extreme climate stresses. From another perspective, climate change-induced extreme climate stresses are increasingly affecting the global agricultural system from all possible aspects.
Regional extreme climate events including heat waves and prolonged droughts under high carbon emission scenarios are one of the biggest challenges the wine industry faces today [51]. As home to most of the South African wine industry, and the country’s most famous wine regions, Stellenbosch and Paarl, wine production serves as one of the most vital economic activities. Many previous studies found that extreme climate events such as heatwaves and drought caused by high GHG emissions will severely challenge the ability to grow grapes adequately and produce quality wine in the region [24,52]. As Araujo [24] demonstrated that the impact of drought on grape yields highly coincides with the intensity of droughts, and high temperatures and low rainfall during summer and winter can cause additional stress on the grapevines through increased diseases, inadequate chill units, and other physiological stress resulting from inadequate water uptake. The future is not optimistic for the district of Cape Winelands as the most crucial district to produce grapes and quality wine. Fig 3(L) projects that the extreme dry will be a considerable concern for the wine industry. Combined with Fig 10(F), 10(I) and 10(O) which shows over 50% increase in heatwave duration, frequency, and events, the impacts of drought on grapes could be further amplified. Reducing the GHG emissions to alleviate future extreme climate stress could effectively reduce the risk of climate change on the local wine industry and further maintain the prosperity of the local economy.
Impact of heatwaves on human health
Heatwave events can be dangerous to human health. Prolonged exposure to extreme heat can trigger various health conditions, such as heat stroke, heat exhaustion, heat cramps, and even death. Besides, higher temperatures and respiratory problems are also linked since higher temperatures contribute to the build-up and spread of harmful air pollutants [53].
Studies found that heatwaves contribute to the increase in mortality rate across the world. Older people are more vulnerable to the effects of extreme heat through a range of physiological and physical factors. For example, Arbuthnott and Hajat [54] found that the 1995 heatwave in the U.K. was estimated to have caused an increase in mortality of 8.9% over England and Wales, and a 16% increase in the Greater London area. They further informed that in 2013, there was an estimated total of 195 excess deaths across all heatwave days in those older than 65 years, with an excess of 10 deaths per heatwave day [54]. Based on a U.S. Centers for Disease Control and Prevention (CDC) study, extreme heat can be blamed for an average of 688 deaths each year in the U.S. Thus, we are confident to speculate the mortality rate in the Western Cape region will also increase significantly in the RCP 8.5 scenario. Taking the City of Cape Town as an example, Fig 10(F) predicts that both the heatwave duration and frequency in the City of Cape Town will increase by over 300% under the RCP 8.5 scenario compared to the RCP 2.6 scenario. For a major municipal city with over 4 million population and 6.2% elderly people, the impact of heatwaves on the city’s mortality will be significant if we keep business as usual. Thus, we consider lowering the GHG emissions will be a practical solution for alleviating the mortality rate in the City of Cape Town.
Conclusions
In this study, Co-ordinated Regional Climate Downscaling Experiment (CORDEX) data were used to examine the potential impacts of various greenhouse gas emissions (RCP 2.6, RCP 4.5, RCP 8.5) on the future extreme climate stress in six districts within the Western Cape, South Africa. The global simulations have been downscaled with REMO for the future 80 years of the 21st century. The CDD and SPEI were used to predict the future drought condition in the Western Cape region. In addition, the WSDI, HWA, HWD, HWF, HWM, HWN were used to predict the various aspects of future heatwave conditions for the region. Both spatial-temporal analysis and regional-average analysis suggest the following:
- Both drought-related extreme climate indices and heatwave-related extreme climate indices indicated a solid relationship between the future extreme climate risks and the GHG emissions for Western Cape, South Africa. Anthropogenic activities and the growing GHG emissions will lead to severer extreme climate stress in terms of every aspect of drought and heatwave.
- The sensitivities between the different aspects of extreme climate events on GHG emissions may vary. In terms of drought and heatwave, the duration aspect is generally more sensitive to the intensity aspect in Western Cape, South Africa.
- Climate change poses a localized effect on the different regions in Western Cape. The spatial location factor, such as the distance to the ocean, plays a critical role in determining the impact on various aspects of extreme climate risks from GHG emissions. In terms of drought, the inland regions will suffer from longer and severer drought risks. For heatwaves, both the inland regions and the coastal regions will experience more hot days in terms of intensity and duration.
- Under the influence of climate change and global warming, the co-existence of different extreme climate events, such as heatwaves and drought, may exhibit amplifying effects. Efficient water-management practices and greenhouse gas emissions reduction strategies are urgently needed to avoid more severe droughts such as the “Day-Zero” crisis in 2018, especially for the City of Cape Town and several other coastal regions within the Overberg and Eden district. Besides, local government should consider local conditions such as the spatial location and economic development when making decisions to maintain long-term sustainability.
This study highlights the importance of the impact of GHG emissions scenarios (RCP 2.6, RCP 4.5, and RCP 8.5) on the regional extreme climate events simulated by the GCM-RCM model chain MPI-ESM/REMO. The results sufficiently demonstrated the great potential of mitigating extreme climate risks and vulnerability under lower GHG emissions scenarios for the Western Cape region. Since this is the only GCM-RCM model combination available, future studies need more available data products from other downscaled climate models to consider uncertainties of the models, so an integrated comprehensive evaluation of possible extreme climate events signals can be conducted in the region. When more downscaled climate simulation data across different RCPs are available for Africa, it will enable uncertainty analyses and quantification of future extreme climate risks using a large ensemble of simulations.
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