Accurately forecasting which species and ecosystems will be most vulnerable to climate change is essential to guide management strategies to minimise extinction. Several key studies have examined global patterns of vulnerability using critical thermal limits: the temperature at which individuals knock-down or die. These assessments assume that these traits accurately predict species’ vulnerability to warming. However, associations between knock-down or lethal temperatures and habitat temperatures are often weak or absent, implying that critical thermal limits may not be an accurate predictor of climate change risk. I will talk about the impact of high temperatures on fertility in insects and discuss whether fertility thermal limits may be better at predicting vulnerability to climate change than critical thermal limits.