From Paper to Proof: Revealing Congo Basin Warming Through Rescued Climate Archives

Abstract. The Congo Basin in Central Africa remains one of the few regions globally where the Intergovernmental Panel on Climate Change has neither assessed changes in hot extremes and extreme precipitation since the 1950s nor attributed such changes to anthropogenic influences, primarily due to the sparsity of in situ observational data. Although extensive daily weather records exist, spanning from the 1900s to the early 2000s and covering numerous stations across the basin, the majority of these remain archived on paper, limiting their accessibility for climate analysis. Here we present and analyse over 1 million temperature and precipitation observations from 37 weather stations in the Democratic Republic of the Congo, which have until now been unavailable to the research community. To produce this dataset, we first digitized (imaged) 9,885 paper sheets stored in local archives during two dedicated field campaigns. We subsequently apply an improved version of the MeteoSaver software (version 1.1) to transcribe the imaged sheets, yielding daily time series of maximum, minimum, and average temperatures, precipitation, as well as dry bulb and wet bulb temperatures measured at three times per day (06:00, 15:00, and 18:00 local time). After quality control, analysis of multi-decadal temperature data across the basin reveals a consistent and accelerating warming signal since the 1960s, characterized by a warming shift in the distribution of daily maximum, minimum, and average temperatures with each successive decade. Median trends across 21 of the 37 stations with sufficient data availability are 0.22 °C, 0.10 °C, and 0.15 °C per decade for daily maximum, minimum, and average temperatures, respectively, corresponding to approximately 0.7 °C, 0.3 °C, and 0.5 °C of warming during the 1961–1990 period. We further find an increasing frequency of hot extremes and a decreasing frequency of cold extremes with each successive decade, with the most recent decade exhibiting nearly twice as many hot days per year and fewer cold days compared to the earliest decade. Analysis of precipitation at three stations with sufficient data indicates an increased frequency of heavy precipitation events at two stations and no substantial change at the third; however, limited spatial coverage due to data availability restricts broader conclusions for the DRC. Overall, this analysis of rescued weather data from Central Africa highlights the urgent need to close the knowledge gap on climate trends in the Congo Basin, one of the world’s most data-sparse yet climatically significant regions.