Exceptional warmth and climate instability occurred in the European Alps during the Last Interglacial period

Warmer temperatures than today, over a period spanning millennia, most recently occurred in the Last Interglacial period, about 129,000 to 116,000 years ago. Yet, the timing and magnitude of warmth during this time interval are uncertain. Here we present a reconstruction of temperatures in the Swiss Alps over the full duration of the Last Interglacial period based on hydrogen isotopes from fluid inclusions in precisely dated speleothems. We find that temperatures were up to 4.3 °C warmer during the Last Interglacial period than in our present-day reference period 1971 to 1990. Climate instability, including an abrupt cooling event about 125,500 years ago, interrupted this thermal optimum but temperatures remained up to 2.0 °C warmer than the present day. We suggest that higher-elevation areas may be more susceptible to warming relative to lowland areas, and that this may hold also for a future climate forced by increasing levels of greenhouse gases.