New insights into a long-standing climate question Earth’s climate has long swung between cold glacial states and warmer interglacial periods. Among these, the so-called lukewarm interglacials—warm phases that occurred roughly between 800,000 and 430,000 years ago—stand out for their relatively cool temperatures and modest atmospheric CO2 levels (about 240–260 ppm). Even the later interglacials, which…