The current biodiversity crisis is driven primarily by human activities, including global warming resulting from human greenhouse-gas emissions, which act as multifaceted environmental stressors. Rising temperatures compromise organismal performance through thermal stress, creating demographic erosion and triggering ecological cascades that propagate across populations and ecosystems. The fossil record offers a unique archive for evaluating these dynamics, documenting biological responses to a wide spectrum of past climate regimes, including warming events far exceeding those experienced in human history and the conditions projected to take place during the coming century. Drawing on events across the Phanerozoic, we emphasize mechanisms that link climate change to extinction in both marine and terrestrial realms. Extreme ancient warming episodes reveal a recurring “deadly trio” of global warming, ocean acidification and deoxygenation, processes that mirror the emerging drivers of the contemporary crisis. Focusing on the Cenozoic and the Pleistocene Ice Ages, we review mammalian responses to rapid climatic fluctuations and highlight the synergistic contribution of Homo to Late Quaternary megafaunal collapse. Our synthesis shows that past crises provide key insights into extinction thresholds, rates of environmental change, and the vulnerability of ecological functions under warming. Critically, comparison with ancient analogs suggests that future extinction risk on land may exceed anything observed during past warming episodes, due to the unprecedented velocity of modern warming combined with direct anthropogenic pressures such as habitat loss and fragmentation. In the oceans, projected warming and deoxygenation point toward extinction intensities comparable to those of the most extreme hyperthermal events. Collectively, the evidence indicates that Earth is approaching temperature conditions historically associated with major extinction episodes.
