The research, conducted by Lindumusa Myeni and Nisa Ayob of North-West University, analysed daily climate records from ten weather stations spanning contrasting agroecological zones in the Free State, Gauteng, KwaZulu-Natal, Mpumalanga and North West provinces, together with district-level maize yield records spanning more than 30 growing seasons from 1993/94 to 2023/24. The five provinces together produce over 85 percent of South Africa’s maize. The authors computed rainfall- and temperature-based extreme climate indices following the Expert Team on Climate Change Detection and Indices framework, then applied Sen’s slope estimator and the Mann-Kendall test to detect trends, Pearson correlation to link extremes with yields, and stepwise multiple regression to identify the dominant climatic drivers of yield variability.

The headline finding is unambiguous warming. Mean seasonal air temperature increased from 0.02 degrees Celsius per annum at the wetter Lydenburg station to 0.06 degrees Celsius per annum at the semi-arid stations of Klerksdorp and Vryburg, while maximum daytime temperatures rose by 0.05 to 0.11 degrees Celsius per annum across all stations. More striking still, the frequency of very hot days increased by 0.19 to 0.50 percent per annum, with the largest increases recorded at Klerksdorp, Newcastle and Carolina. In mirror image, the frequency of extreme cold days declined by 0.10 to 0.29 percent per annum at most stations. Together these trends signal a clear shift toward hotter growing-season conditions, with peak heat intensifying faster than average temperatures, a pattern the authors argue underscores why extremes, not just means, must be monitored.

Rainfall told a subtler story. Total seasonal rainfall showed highly variable and statistically non-significant trends across all stations, ranging between minus 7.08 and plus 5.91 millimetres per annum, consistent with earlier national analyses that found no coherent long-term rainfall signal. But the texture of rainfall is changing. The number of rain days declined significantly at 40 percent of stations, including Bronkhorstspruit, Estcourt, Klerksdorp and Vereeniging, at rates of 0.55 to 1.16 days per annum, implying that rain is becoming less frequent but potentially more intense, with longer dry intervals between events. Consecutive dry days increased significantly only at Vereeniging, at roughly 19 additional days per decade, while heavy rainfall indices rose significantly only at Lydenburg, where more intense downpours raise risks of runoff, erosion and waterlogging on vulnerable soils.

Perhaps most consequential for crops is the trajectory of moisture balance. The Standardized Precipitation Evapotranspiration Index, or SPEI, which captures the competition between water supply and atmospheric demand, declined significantly at 30 percent of stations, including Estcourt, Klerksdorp and Newcastle, at rates of 0.03 to 0.05 per annum. This indicates that evapotranspiration, driven largely by rising air temperatures, is increasingly outpacing precipitation, deepening water stress and drought severity. For rainfed maize, which dominates production in these semi-arid environments, such trends translate directly into greater susceptibility to prolonged dry conditions, reduced soil moisture and unstable yields. The study also noted a significant decrease in minimum nighttime temperatures at Bloemfontein, raising frost risk in that district, a reminder that warming is not spatially uniform.

Maize yields themselves varied enormously across the study municipalities. Mean yields ranged from as low as 2.42 tonnes per hectare at Vereeniging and Bloemfontein to as high as 7.49 tonnes per hectare at Estcourt, with cooler, wetter districts such as Newcastle, Lydenburg and Carolina generally outperforming drier western areas like Vryburg, Klerksdorp and Bloemfontein. Yield stability differed just as sharply: coefficients of variation spanned from 29.24 percent at Vereeniging to 54.28 percent at Vryburg, with high values at Vryburg, Newcastle and Bloemfontein pointing to strong interannual fluctuations likely driven by erratic rainfall, dry spells and temperature extremes during critical growth stages.

The correlation analysis drew a clear line between specific extremes and yield outcomes. Maize yields correlated negatively with heat indices across all municipalities, with the strongest significant relationships in semi-arid regions: at Vryburg, mean seasonal temperature correlated at r equals minus 0.64 and very hot days at r equals minus 0.62, while at Klerksdorp the corresponding values were minus 0.52 and minus 0.48. At Bethlehem, very hot days correlated at minus 0.46. By contrast, cold-related indices showed weak or non-significant correlations at most stations, suggesting cold stress is far less influential than heat. On the moisture side, yields correlated positively with total rainfall, rain days and SPEI, with SPEI reaching r equals 0.76 at Klerksdorp and 0.68 at Vryburg, confirming moisture availability as the primary limiting factor in these water-scarce districts. Consecutive dry days correlated negatively with yields at Klerksdorp, reinforcing the damage inflicted by intra-seasonal drought.

Stepwise multiple regression then quantified how much of the yield variability climate extremes can actually explain. The explanatory power of the models ranged from weak at Lydenburg, where the coefficient of determination was just 0.12, to strong at Klerksdorp and Vryburg, where it reached 0.69, meaning climate variability accounted for up to 69 percent of interannual yield fluctuations in these water-limited regions. SPEI carried large, highly significant positive coefficients at Klerksdorp and Vryburg, leading the authors to propose the drought index as a practical early-warning indicator for maize production forecasting and risk assessment. At Estcourt and Newcastle, with coefficients of determination of 0.49 and 0.67 respectively, both temperature extremes and rainfall characteristics, including amount, frequency and intensity, shaped yields in more complex ways. Where model explanatory power was low, non-climatic factors such as soils, management and technology likely dominated.

The authors stress that these relationships are scale-dependent and that coarser provincial or national analyses can obscure localised impacts, which is why district-level assessment matters for crafting adaptation. Their recommendations diverge by zone: in hot, semi-arid areas, drought- and heat-tolerant seed varieties, adjusted planting dates, conservation tillage, mulching, residue retention, cover cropping, rainwater harvesting and supplementary irrigation offer the most promise, while in wetter regions the priority is managing rainfall distribution variability, mitigating heat stress and optimising planting calendars. Climate information services, seasonal forecasts and agrometeorological advisories, they argue, can help farmers anticipate risks and act proactively rather than reactively.

The study acknowledges limitations, including the coarse resolution of station data, seasonal indices that may miss extremes during critical phenological windows, and district-level yield records that mask local variation in soils, cultivars and management. Future work, the authors suggest, should integrate high-resolution climate projections, finer-scale yield and management data, and machine learning approaches capable of capturing nonlinear climate-yield relationships. With heat extremes intensifying and moisture availability declining across the maize belt, the message for policymakers is that uniform adaptation policies will fall short; resilience must be built district by district, informed by the specific climatic constraints each farming community faces.

Subject of Research: Long-term trends in extreme climate indices and their impacts on district-level maize yields in South Africa's summer rainfall region

Article Title: Long-term changes in the climate extremes and their impacts on maize yields in the summer rainfall region of South Africa

Article References: Long-term changes in the climate extremes and their impacts on maize yields in the summer rainfall region of South Africa. (n.d.). https://doi.org/10.1007/s00704-026-06550-y

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06550-y

Keywords: climate extremes, maize yields, South Africa, heat stress, drought, SPEI, rainfed agriculture, Mann-Kendall trend analysis, semi-arid regions, food security, adaptation strategies, agroclimatology