Climate change is projected to constitute a significant threat to food security, if no adaptation actions are taken 1,2 . Transformation of agricultural systems, e.g. switching crop types or moving out of agriculture, is projected to be necessary in some cases [3][4][5] .However, little attention has been paid to the timing of these transformations. Here, we develop a temporal uncertainty framework using the CMIP5 ensemble to assess when and where cultivation of key crops in Sub-Saharan African becomes unviable.We report potential transformational changes for all major crops during the 21 st century, as climates shift and areas become unsuitable. For most crops, however, transformation is limited to small pockets (<15 % of area), and only for beans, maize and banana is transformation more widespread (~30 % area for maize and banana, 60 % for beans). We envision three overlapping adaptation phases to enable projected transformational changes: an incremental adaptation phase focused on improvements to crops and management, a preparatory phase that establishes appropriate policies and enabling environments, and a transformational adaptation phase in which farmers substitute crops, explore alternative livelihoods strategies, or relocate. To best align policies with production triggers for no-regret actions, monitoring capacities to track farming systems as well as climate are needed.Agricultural activities are the main means to reduce poverty and improve food security among 850 million undernourished people 2 . Numerous studies have shown that climate change can be a significant threat to food availability and stability by reducing agricultural productivity and increasing inter-annual variations in yields 1,2,6 . Adaptation will be required if food production is to be increased in both quantity and stability in order to meet food security needs during the 21st century. A recent global meta-analysis 1 reported that decreases of ca. 5 % in crop productivity are expected for every degree of warming above historical levels, and that adapted crops yield roughly 7 % greater than non-adapted crops.Yield gains from adaptation through crop management and varietal substitution, however, are highest with moderate or low (< +3 ºC) levels of warming 1,6 , suggesting that more profound systemic and/or transformational changes may be required when and where higher levels of warming occur 5 .Transformational adaptation is defined by the IPCC 7 as a response to the effects of climate change that "changes the fundamental attributes of a system" (see Text S1 for definitions).Transformational change implies shifts in locations for production of specific crops and livestock, or shifting to farming systems new to a region or resource system 3,5 . Here, we consider one type of transformation: switching of staple crop type grown over a large geographic area of 0.3 million ha (the grid cell size of our analysis) or more. We analyze when and where major cropping systems transformations are likely to occur for important crops in Sub-Saharan A...
