Sugarcane production supports the livelihoods of millions of small‐scale farmers in developing countries, and the bioenergy needs of millions of consumers. Yet, future sugarcane yields remain uncertain due to differences in climate projections, and because the sensitivity of sugarcane ecophysiology to individual climate drivers (i.e. temperature, precipitation, shortwave radiation, VPD and CO2) and their interactions is largely unresolved. Here we ask: how sensitive is sugarcane yield to future climate change, including climate extremes, and what are its key climate drivers? We combine the Soil‐Plant‐Atmosphere model with detailed time‐series measurements from experimental plots in Guangxi, China, and São Paulo State, Brazil. We first calibrated and validated modelled carbon and water cycling against field flux and biometric data. Second, we simulated sugarcane growth under the historical climate (1980–2018), and six future climate projections (2015–2100). We computed the ‘yield‐effect’ of each climate driver by generating synthetic climate forcings in which the driver time series was alternated to that of the historical median. In Guangxi, median yield and yield lows (i.e. lower decile) were relatively insensitive to forecast climate change. In São Paulo, median yield and yield lows decreased under all future climates projections (x¯ = −4% and −12% respectively). At Guangxi, where moisture stress was low, radiation was the principal driver of yield variability (yield‐effect x¯ = −1.2%). Conversely, high moisture stress at São Paulo raised yield sensitivity to temperature (yield‐effect x¯ = −7.9%). In contrast, a number of other modelling studies report a positive effect of increased temperatures on sugarcane yield. We ascribe the disparity between model predictions to the representation of key phenological processes, including the link between leaf ageing and thermal time, and the role of ageing in driving leaf senescence. We highlight climate sensitivity of phenological processes as a key focus for future research efforts. Understanding the sensitivity of sugarcane yields to future climate change is critical to farmer livelihoods and to support future bioenergy needs. We used a detailed ecosystem model to simulate sugarcane yield, under a range of future climate projections, at experimental sites in China and Brazil. At the Chinese site, future yield lows were driven by light limitation. At the Brazilian site, declines in yield were relatively higher, and were instead driven by higher projected temperatures. High temperatures caused an increase in modelled leaf turnover, and consequently canopy density, thus lowering the crops ability to photosynthesize (explaining yield lows).