A microscope image of a front on view of the copepod Euchaeta marina, which was one of the 384 species of copepods used in our study. We assembled nearly 100 000 continuous plankton recorder samples from across the globe to test Bergmann’s Rule in marine copepods. We found that copepod body size declined with increasing temperature, providing strong support for Bergmann’s Rule. Food availability strongly modified this relationship, but oxygen and latitude, were of lesser importance.  Based on our model, the effect of warming under a high emissions scenario could equate to a decrease in the global biomass of copepods by ~7% by 2100. Such a decline would negatively impact global fisheries. Smaller copepod size would reduce vertical migration and carbon flux to the deep ocean.

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