Many biological processes, including plant leafout and flowering, occur once cumulative temperatures reach a threshold, a relationship known as the thermal-sum model. In this way, temperature is thought to coordinate the timing of biological events. An important implication is that higher temperatures cause thresholds to be reached sooner so that the timing of spring events, for example, should advance earlier in the calendar year as climates warm. But growing evidence has found that, as climates have warmed, the rate of advancement has slowed (a trend known as declining sensitivity), while the variance in the timing of spring events has increased in many cases (a trend we call declining synchrony). These trends raise questions about the resilience of temperature-based coordination to anthropogenic climate change. To answer these questions, researchers have modified the thermal-sum model by introducing additional factors and mechanisms, such as chilling and photoperiod. We show such complexity is not necessary to explain current trends of sensitivity and synchrony. Using experimental and real-world data, we find these trends are exactly as predicted by the thermal-sum model. In particular, the model predicts that as temperatures continue to increase and springtime events shift from the equinox toward the winter solstice, those events will become less synchronized and more variable, a phenomenon we refer to as a scattered spring. By 2100, our results predict much of the American South will experience a scattered spring under a high-warming scenario, with the average time between spring flowering events increasing by several weeks in many locations.

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