Projected temperature increases may require shifts in the growing season of cool-season crops and the growing locations of warm-season crops.

Predicting the effects of climate change on where and when crops can be grown under future conditions is critical for maintaining crop production, particularly in Mediterranean ecosystems. The diverse range of Mediterranean climatic conditions in California supports high crop diversity and production, yet California also faces future increased temperatures and more frequent extreme weather events indicative of a changing climate. Evaluating the effect of temperature increase is a crucial first step in estimating future impacts of warming. We compare the temperature constraints under climate projections for five annual crops. We determine maximum and minimum monthly temperatures of historical and future projections for the mid-21st century based on four climate projections (two climate models × two climate change scenarios). We estimate where temperatures were suitable for each crop historically and in the future at two spatial scales (4 km grid-cell; statewide) and two temporal scales (monthly; for each crop's growing season). We found differences between warm- and cool-season crops: temperature affects when cool-season crops (broccoli; lettuce) could be grown more than where, but temperature affects where warm-season crops (cantaloupe; tomato; carrots) could be grown more than when. More than 99% of land where lettuce and broccoli have been grown historically will have temperatures suitable for each crop by mid-century; the increased winter temperatures will enable spring and fall growing seasons to merge in more than 75% of land where each crop has been grown. Only 34-87% of land historically used for growing tomatoes will have temperatures appropriate for tomatoes due to the increase in summer temperatures. We do not predict cantaloupes and carrots to cross their upper temperature threshold. Integration of our results with other factors that affect crops - including management, water availability and helpful and harmful insects - provides guidance for adapting Mediterranean agriculture to climate change.