Greenland, the world’s largest island, is currently covered by an ice sheet across approximately 80% of its surface. This ice holds enough frozen water to raise global sea levels by over 20 feet if it melted completely. The island’s name, however, suggests a historical paradox: how could this landmass have once supported green, thriving life? The answer lies buried deep beneath the ice, revealing dramatic shifts in Earth’s climate over millions of years.
Scientific Proof of a Green Past
Scientists confirm Greenland’s former ecology by analyzing materials preserved at the bottom of the ice sheet. Deep ice cores, such as those from the Camp Century and DYE-3 sites, penetrate through miles of ice to retrieve sub-glacial sediment and basal ice. This frozen material holds an archive of the landscape that existed before the ice sheet formed.
Analysis of this sediment has revealed well-preserved fossil plants, biomolecules, and pollen grains, confirming past ecosystems that were completely ice-free. A more recent technique involves sequencing ancient environmental DNA (eDNA) found within the sediment. This eDNA analysis proves the existence of species like birch, poplar, and various insects. The DNA fragments suggest a conifer forest ecosystem thrived in the south of the island hundreds of thousands of years ago. Furthermore, examining cosmogenic isotopes in the sediment confirms the ground was exposed to sunlight, meaning the ice sheet was absent for long periods.
Greenland During the Deep Past Warm Periods
The primary “green” period for Greenland occurred during the Eocene Epoch (56 to 34 million years ago). During this time, the planet experienced a “greenhouse” climate, including the Paleocene-Eocene Thermal Maximum around 56 million years ago. Atmospheric carbon dioxide levels were extremely high, driven by intense volcanism that released greenhouse gases. This high-CO2 atmosphere prevented the formation of continental ice sheets globally, leaving Greenland entirely ice-free.
The island was covered in lush, swampy woodlands and temperate or boreal forests. In southern Greenland, the climate may have been subtropical, supporting broadleaf trees and organisms unlike anything found there today. Sedimentary records from this time show the formation of laterite, a red, iron-rich soil that develops in hot, wet, tropical conditions. This warmth confirms Greenland was a verdant, biologically rich landmass.
Interglacial Warmth and Ice Sheet Survival
After the permanent ice sheet formed, Greenland experienced repeated, cyclical periods of warmth called interglacials, which were much shorter and less intense than the Eocene. The Eemian Interglacial, which occurred about 125,000 years ago, is one of the best-studied examples. During the Eemian, global temperatures were slightly warmer than pre-industrial levels due to natural variations in Earth’s orbit.
Ice core data from the North Greenland Eemian Ice Drilling (NEEM) project suggests that while temperatures peaked up to 8 ± 4 degrees Celsius above the mean of the last millennium, the ice sheet did not vanish entirely. The ice sheet experienced significant thinning and surface melt, especially around the edges, but a substantial core survived. This partial melting caused a substantial rise in global sea levels, but the island did not revert to its forested Eocene state.
In coastal areas that became ice-free, the landscape was likely dominated by tundra and shrubby vegetation. Sediment dating from a different warm period, Marine Isotope Stage 11 (about 416,000 years ago), showed evidence of a shrubby ecosystem with twigs, leaves, and mosses. This evidence confirms that while recent warm periods caused major ice loss and allowed vegetation to grow, they did not restore the massive forest cover of the deep past.
The Onset of Glaciation
The transition from a green world to the icy state we know today began with a long-term global cooling trend starting in the Oligocene Epoch, around 34 million years ago. This shift was primarily driven by a gradual decrease in atmospheric carbon dioxide. As CO2 levels slowly dropped, the planet’s climate eventually crossed a critical threshold, allowing for the stable formation of large, permanent ice masses.
Geological changes also played a supporting role by altering ocean circulation patterns. The opening of the Drake Passage between South America and Antarctica allowed the Antarctic Circumpolar Current to fully develop. This current thermally isolated Antarctica, initiating the formation of the first major ice sheets on that continent and amplifying global cooling. The combination of falling carbon dioxide and reorganized ocean circulation led to a permanent shift toward an “icehouse” climate. This global cooling trend eventually allowed for the sustained accumulation of snow and ice in the Northern Hemisphere, culminating in the growth of the massive Greenland Ice Sheet.