Greater evolutionary divergence of thermal limits within marine than terrestrial species

There is considerable uncertainty regarding which ecosystems are most vulnerable to warming. Current understanding of organismal sensitivity is largely centred on species-level assessments that do not consider variation across populations. Here we used meta-analysis to quantify upper thermal tolerance variation across 305 populations from 61 terrestrial, freshwater, marine and intertidal taxa. We found strong differentiation in heat tolerance across populations in marine and intertidal taxa but not terrestrial or freshwater taxa. This is counter to the expectation that increased connectivity in the ocean should reduce intraspecific variation. Such adaptive differentiation in the ocean suggests there may be standing genetic variation at the species level to buffer climate impacts. Assessments of vulnerability to warming should account for variation in thermal tolerance among populations (or the lack thereof) to improve predictions about climate vulnerability.