The Penultimate Interglacial period occurred about 125,000 years ago, and at that time Northern Hemisphere summers were slightly warmer than today, roughly 1.7-3.3°F (1-2°C) warmer. This interglacial is also referred to as the Eemian interglacial and is attributed to changes in the Earth's orbit.
One interesting thing to note is that because climatologists consider this period to have had similar temperatures to those we are currently facing with global warming, they have used the geologic record to reconstruct environmental conditions in an attempt to understand the types of conditions humans will be facing in the coming years with global warming. What they were able to reconstruct is that at that time, with the increase in temperature of 1.7-3.3°F (1-2°C), sea levels rose about 13-19 feet (4-6 m) above the levels of today, and the majority of southern Greenland's glaciers had fully melted. Because these glaciers melted, they contributed 6.6-9.8 feet (2-3 m) of the rise in sea level, which is a vivid example of why the glaciers that are melting today in Greenland, the Arctic, Antarctica, and other continents are of such concern to climatologists.
Scientists have run paleoclimatic models on this data and have been able to determine that this major melting of glaciers was triggered by temperatures that were only 5-8.3°F (3-5°C) warmer than today. What is also alarming to climatologists is that the Penultimate Interglacial period's physical conditions are the closest to the physical conditions of today.
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