The research, published in Theoretical and Applied Climatology by Eskinder Gidey and Paidamwoyo Mhangara of the University of the Witwatersrand, stitched together two very different strands of climate evidence. For the historical record, spanning 1984 to 2023, the authors drew on NASA POWER data built on the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), a satellite-assimilating reanalysis product that reconstructs consistent daily climate variables across the globe. For the future, they turned to the Coupled Model Intercomparison Project Phase 6 (CMIP6), using the medium-emissions SSP2-4.5 scenario to project conditions in Gauteng from 2024 to 2050.

What makes the analysis distinctive is its statistical machinery. Rather than relying on simple averages, the researchers applied an outlier-resistant, median-based robust approach to tame erratic values in the datasets. They then ran the Modified Mann-Kendall test, a non-parametric trend detector that corrects for the autocorrelation that plagues climate time series, and estimated the magnitude of change with Sen’s slope estimator, which computes the median of all pairwise slopes and is therefore largely immune to extreme outliers. All of the processing was carried out in RStudio, and the combination allowed the team to distinguish genuine long-term signals from the year-to-year noise that dominates semi-arid climates.

The historical results are unambiguous on temperature. Gauteng warmed at a rate of about 0.02 degrees Celsius per year between 1984 and 2023, a statistically significant trend with a Kendall’s tau of 0.28 and a p-value of 0.01. That warming translated directly into more frequent heat extremes: heatwave events climbed by roughly 0.04 events per year, a highly significant increase with tau of 0.30 and p below 0.001. Intriguingly, cold waves also ticked upward slightly, by about 0.01 events per year, but that trend failed the significance test with a p-value of 0.26, suggesting it may be a statistical artefact of natural variability rather than a real climatic shift.

The water-side variables tell a more troubling story. Precipitation across the province declined by 3.63 millimetres per year over the four-decade observation window. Cloud cover fell by 0.33 percent per year, and both soil moisture and relative humidity showed declining trends as well. For a landlocked province perched on continental high ground, far from major rivers, these coupled declines matter enormously: less rain, drier soils, drier air and thinner cloud all conspire to reduce the water that reaches reservoirs, recharges groundwater and sustains the wetlands and streams of the Vaal and Crocodile river systems that Gauteng depends on.

The projections sharpen the warning. Two CMIP6 models, the Australian ACCESS-CM2 and the United Kingdom’s HadGEM3-GC31-LL, both foresee significant warming in Gauteng between 2024 and 2050. ACCESS-CM2 projects a temperature rise of 1.68 degrees Celsius, equivalent to a trend of 0.35 percent per year, while HadGEM3-GC31-LL projects 1.60 degrees Celsius, or 0.33 percent per year. On the precipitation side, the models diverge in magnitude but agree in direction: HadGEM3-GC31-LL projects rainfall declining by 7.26 millimetres per year, while ACCESS-CM2 projects a gentler decline of 1.95 millimetres per year. Either way, the province faces a hotter future with less water falling from the sky.

The implications ripple far beyond climatology. Gauteng is the engine of the South African economy, home to Johannesburg, Pretoria and a population that continues to grow through migration, all of it concentrated on a water supply already stretched by periodic drought. The province’s water security has long depended on inter-basin transfers, most famously the scheme that pipes water from the Lesotho highlands into the Vaal system. A simultaneous increase in evaporative demand, driven by rising temperatures and falling humidity, together with declining rainfall, means that reservoir inflows could shrink even as household and industrial demand climbs. Previous research on urban water demand in Gauteng has already flagged the compounding effect of climate change and population growth on the province’s supply systems.

The study’s authors frame their findings as a contribution to South Africa’s National Development Plan 2030, which calls for climate-resilient water management and robust early warning systems. By combining multi-decadal reanalysis observations with CMIP6 projections and rigorous trend-detection statistics, the work offers water managers a defensible quantitative baseline: not just the assertion that conditions are changing, but measured rates of change for temperature, heatwave frequency, rainfall, soil moisture, humidity and cloud cover, together with model-based expectations for the coming quarter century. That kind of evidence is what allows planners to size new infrastructure, set demand-management targets and design drought triggers with some confidence about the trajectory they are planning against.

There are also broader scientific lessons in the methodology. Reanalysis products like NASA POWER (MERRA-2) provide consistent, freely available daily climate data even in regions with sparse ground-based station networks, a chronic problem across much of Africa. Pairing such observations with the CMIP6 archive, and filtering both through robust, autocorrelation-aware statistics, offers a template that can be replicated for other provinces and catchments. The researchers acknowledge the inherent uncertainties of model projections, and the divergence between the two models’ rainfall estimates is a reminder that scenario-based futures are ranges rather than forecasts. But the direction of travel, warmer, drier and more extreme, is consistent across both the observed record and the projections.

For the residents of Gauteng, the message distilled from forty years of satellite-era data and twenty-six years of model projections is stark but actionable. The province has already lost rainfall at more than three and a half millimetres per year while gaining heatwaves at a statistically robust clip, and the coming decades promise roughly another degree and a half of warming on top of what has already occurred. Whether that future translates into crisis or into a managed transition will depend on how quickly the province’s water institutions, municipalities and households absorb what the data is telling them, and on whether the policies built for the climate of the twentieth century are retooled for the one now arriving.

Subject of Research: Historical and projected climate variability and trends in Gauteng, South Africa, and their implications for water resources

Article Title: Climate variability and its implications for water resources in Gauteng, South Africa (1984–2050) based on NASA POWER (MERRA-2) observations and CMIP6 SSP2-4.5 projections

Article References: Gidey, E., & Mhangara, P. (2026). Climate variability and its implications for water resources in Gauteng, South Africa (1984–2050) based on NASA POWER (MERRA-2) observations and CMIP6 SSP2-4.5 projections. Theoretical and Applied Climatology, 157(10), Article 676. https://doi.org/10.1007/s00704-026-06600-5

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06600-5

Keywords: Gauteng, climate variability, water resources, NASA POWER, MERRA-2, CMIP6, SSP2-4.5, heatwaves, precipitation trends, Sen's slope estimator, Modified Mann-Kendall test, South Africa