Scientists have discovered that the Greenland ice sheet has melted to the ground at least once in the last million years despite CO2 levels far lower than today. This ice sheet holds enough frozen water to swamp coastal cities worldwide.
Based on the discovery of plant fossils in soil samples extracted in the 1960s by US army engineers from beneath two kilometres of ice is proof that Greenland was covered with lichen, moss and perhaps trees in the not so distant past.
In 2019, Greenland lost more than half a trillion tonnes of ice and meltwater. If the ice sheet were to disappear, it would lift global oceans by nearly seven metres.
The study results suggest the ice sheet disappeared about one million years ago and very possibly again about 400,000 years ago.
Global warming driven by the accumulation in the atmosphere of CO2 cannot account for these past events. CO2 ranged naturally over the last million years from about 180 ppm to 290 ppm which is barely above the pre-industrial mark. In contrast, the present concentration is about 415 ppm. Scientists cannot explain how the ice sheet could have disappeared.
https://phys.org/news/2021-03-greenland-ice-million-years.html