The global climate has shifted dramatically throughout Earth’s history, moving between long, warm periods and colder intervals known as Ice Ages. When people refer to “the Ice Age,” they are usually thinking of the most recent major cold period. This event is scientifically known as the Last Glacial Maximum (LGM), which occurred approximately 20,000 years ago, representing the point when massive ice sheets reached their greatest extent. During this time, the planet’s atmospheric and oceanic systems were fundamentally altered.
The Global Average Temperature During the Last Glacial Maximum
The difference between the modern climate and the height of the Ice Age is measured by the global average temperature. Scientific consensus places the global mean surface temperature during the Last Glacial Maximum at about 6 degrees Celsius (11 degrees Fahrenheit) cooler than the present day average. This figure is a global average across all land and ocean surfaces.
A relatively small drop in the planetary average temperature resulted in enormous environmental shifts. Vast ice sheets, sometimes kilometers thick, covered nearly 25% of the land area across North America, Europe, and Asia. The immense volume of water locked up in these ice masses caused the global sea level to drop by approximately 125 meters (410 feet) compared to today.
This cooling fundamentally reorganized global ecosystems and expanded deserts. The total average temperature of the Earth during the LGM has been estimated to be around 8 degrees Celsius (46 degrees Fahrenheit), compared to the 20th-century average of about 14 degrees Celsius (57 degrees Fahrenheit). Analyzing this past climate helps scientists understand how sensitive the Earth’s climate system is to changes in atmospheric carbon dioxide levels.
Temperature Variation Across Different Latitudes
The cooling during the Last Glacial Maximum was not distributed uniformly across the globe; instead, it was heavily concentrated in the higher latitudes. The temperature drop experienced in the polar regions was far more severe than in the tropics, a phenomenon known as polar amplification. In the Arctic, for example, temperatures were estimated to be up to 14 degrees Celsius (25 degrees Fahrenheit) colder than they are now.
Massive temperature reductions in the far north and south sustained the colossal continental ice sheets. This extreme cooling was a direct result of the ice sheets themselves, which reflected significantly more solar energy back into space compared to a dark ocean or land surface. The mid-latitudes also experienced substantial cooling.
In stark contrast, regions near the equator, such as the tropical oceans, experienced a much milder temperature decrease. While earlier estimates suggested a drop of only 1 to 3 degrees Celsius, more recent research using different proxies indicates that low-latitude cooling was likely greater than previously thought. The gradient, where the poles were intensely cold and the tropics were only moderately cooler, led to much steeper temperature differences between the equator and the poles than exist today.
How Scientists Measure Ice Age Temperatures
Since no thermometers existed 20,000 years ago, scientists rely on “proxy” records to reconstruct ancient temperatures. These proxies are natural materials that recorded climate conditions as they formed, acting as paleothermometers. The most detailed and continuous records come from analyzing ice cores extracted from the thick ice sheets of Greenland and Antarctica.
These ice cores contain trapped air bubbles and layers of ice that preserve a record of the atmosphere and local temperature from the time they formed. Scientists analyze the ratio of different oxygen isotopes—specifically the heavier oxygen-18 and the lighter oxygen-16—found in the water molecules of the ice. The ratio of these isotopes changes predictably with temperature because the lighter oxygen-16 evaporates more easily from the ocean and precipitates at lower temperatures.
A lower concentration of the heavier isotope (oxygen-18) in the polar ice layers indicates that the temperatures were colder when the snow fell. Similar isotopic analysis is performed on the calcium carbonate shells of tiny marine organisms found in deep-sea sediment cores. During an Ice Age, the ocean water becomes enriched with the heavier oxygen-18 because the lighter oxygen-16 is locked up in continental ice sheets, providing a record of global ice volume and deep-sea temperature.
Other proxies, such as the distribution of fossil plankton species in ocean sediments or the analysis of noble gases dissolved in ancient groundwater, corroborate the temperature reconstructions from ice cores. These combined data sources allow researchers to build a detailed picture of the global climate, confirming the quantitative temperature estimates for the Last Glacial Maximum.