Projected rising temperatures and vapour pressure deficit threaten Arabica coffee production Tanzania's burgeoning coffee region: Empirical insight from Mbinga district, Tanzania

Tanzania is emerging as a key coffee producer, but climate change poses a serious threat to coffee production in the country. While extensive literature addresses climatic variables’ trends and variability in coffee-growing regions globally, elevation, a crucial factor in coffee cultivation, is often overlooked. This paper aims to fill this knowledge gap by examining the future trends and variability of climatic variables identified in previous studies being crucial for Arabica coffee (rainfall, temperature, and vapor pressure Deficit (VPD)) during different phenological stages of Arabica coffee growth across three elevation zones (lowland, midland, and highland). We study the Mbinga district, now a major Arabica-producing area and the forefront of hopes for expanding production in Tanzania. We used a Multi-Model Ensemble (MME) of two Global Circulation Models (GCMs) to estimate the future trends and variability of rainfall, temperature, and VPD under SSP2-4.5 and SSP5-8.5 scenarios from 2023 to 2064. We analyzed these data using non-parametric trend analysis (Mann-Kendal and Sen's slope test estimation) and standardized anomalies. Our results suggest that both temperatures (minimum and maximum) and VPD during critical flowering and growing seasons are projected to increase significantly over the next 42 years across all elevation zones under both emission scenarios. The mean annual temperatures across elevation zones in Mbinga are projected to surpass the optimal temperature envelope (23 °C) for producing quality Arabica, challenging the notion that the highlands might serve as a refuge for future production. The mean annual VPD in the lowlands are projected to surpass the optimal envelope (0.82 kPa) for Arabica productivity from 2023 to 2064 under both emission scenarios, while midlands are projected to surpass the optimal VPD envelope from mid-2040s onwards in constrast, the highlands are projected to stay within the optimal VPD envelope for Arabica productivity from 2023 to 2064 under both emission scenarios. Nevertheless, as highlands are projected to get more warmer in the future because of rising temperatures, this hope might not be feasible in the post-2064. To secure Mbinga's future in global coffee value chains, the Tanzanian government and stakeholders must support adaptation measures, enhancing smallholders’ resilience against climate-induced disruptions.