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1. Climate change is rapidly altering thermal environments across the globe. The effects of increased temperatures in already warm environments may be particularly strong because organisms are likely to be near their thermal safety margins, with limited tolerance to additional heat stress.

2. We conduct an in situ field experiment over 2years to investigate the direct effects of temperature change on an early-season solitary bee in a warm, arid region of the Southwestern USA. Our field experiment manipulates the thermal environment of Osmia ribifloris (Megachilidae) from larval development through adult emergence, simulating both previous cooler (c. 1950; nest boxes painted white) and future warmer (2040-2099; nest boxes painted black) climate conditions. In each year, we measure adult emergence phenology, linear body size, body mass, fat content and survival.

3. Bees in the warming treatment exhibit delayed emergence phenology and a substantial increase in phenological variance. Increases in temperature also lead to reductions in body mass and fat content. Whereas bees in the cooling and control treatments experience negligible amounts of mortality, bees in the warming treatment experience 30%-75% mortality.

4. Our findings indicate that temperature changes that have occurred since c. 1950 have likely had relatively weak and non-negative effects, but predicted warmer temperatures create a high stress thermal environment for O.ribifloris. Later and more variable emergence dates under warming likely compromise phenological synchrony with floral resources and the ability of individuals to find mates. The consequences of phenological asynchrony, combined with reductions in body mass and fat content, will likely impose fitness reductions for surviving bees. Combined with high rates of mortality, our results suggest that O.ribifloris may face local extinction in the warmer parts of its range within the century under climate change.

5. Temperature increases in already warm ecosystems can have substantial consequences for key components of life history, physiology and survival. Our study provides an important example of how the responses of ectothermic insects to temperature increases in already warm environments may be insufficient to mitigate the negative consequences of future climate change.