Last Glacial Maximum - The Last Major Global Climate Change

What Were the Global Effects of Ice Covering So Much of Our Planet?

Melting Glacier, Greenland
Glacier, terminal moraine, and bodies of water in the fjords of southern Greenland. Doc Searls

The Last Glacial Maximum (LGM) refers to the most recent period in earth's history when the glaciers were at their thickest and the sea levels at their lowest, roughly between 24,000-18,000 calendar years ago. During the LGM, continent-wide ice sheets covered high-latitude Europe and North America, and sea levels were between 120 and 135 meters (400-450 feet) lower than they are today. The overwhelming evidence of this long-gone process is seen in sediments laid down by sea level changes all over the world, in coral reefs and estuaries and oceans; and the vast North American plains, areas scraped flat by thousands of years of glacial movement.

In the lead up to the LGM between 29,000 and 21,000 bp, our planet saw constant or slowly increasing ice volumes, with the sea level reaching its lowest level (-134 meters) when there was about 52x10(6) cubic kilometers more ice than there is today. At the height of the Last Glacial Maximum, the ice sheets that covered parts of the northern and southern hemispheres of our planet were steeply domed and thickest in the middle.

Characteristics of the LGM

Researchers are interested in the Last Glacial Maximum because of when it happened: it was the most recent globally impacting climate change, and it happened and to some degree affected the speed and trajectory of the colonization of the American continents. The characteristics of the LGM that scholars use to help identify the impacts of such a major change include fluctuations in effective sea level, and the decrease and subsequent rise in carbon as parts per million in our atmosphere during that period.

Both of those characteristics are similar--but opposite to--the climate change challenges we are facing today: during the LGM, both the sea level and percentage of carbon in our atmosphere were substantially lower than what we see today. We do not as yet know the entire impact of what that means to our planet, but the effects are currently undeniable. The table below shows the changes in effective sea level in the past 35,000 years (Lambeck and colleagues) and parts per million of atmospheric carbon (Cotton and colleagues).

  • Years BP, Sea Level Difference, PPM Atmospheric Carbon
  • today 0, 335 ppm
  • 1,000 BP, -.21 meters +-.07, 280 ppm
  • 5,000 BP, -2.38 m +/-.07, 270 ppm
  • 10,000 BP, -40.81 m +/-1.51, 255 ppm
  • 15,000 BP, -97.82 m +/-3.24, 210 ppm
  • 20,000 BP, -135.35 m +/-2.02, > 190 ppm
  • 25,000 BP, -131.12 m +/-1.3
  • 30,000 BP, -105.48 m +/-3.6
  • 35,000 BP, -73.41 m +/-5.55

The major cause of sea level drop during the ice ages was the movement of water out of the oceans into ice and the planet's dynamic response to the enormous weight of all that ice atop our continents. In North America during the LGM, all of Canada, the southern coast of Alaska, and the top 1/4 of the United States were covered with ice extending as far south as the states of Iowa and West Virginia. Glacial ice also covered the western coast of South America, and in the Andes extending into Chile and most of Patagonia. In Europe, the ice extended as far south as Germany and Poland; in Asia ice sheets reached Tibet. Although they saw no ice, Australia, New Zealand and Tasmania were a single landmass; and mountains throughout the world held glaciers.

Progress of Global Climate Change

The late Pleistocene period experienced a sawtooth-like cycling between cool glacial and warm interglacial periods when global temperatures and atmospheric CO2 fluctuated up to 80-100 ppm corresponding with temperature variations of 3-4 degrees celsius (5.4-7.2 degrees Fahrenheit): increases in atmospheric CO2 preceded decreases in global ice mass. The ocean stores carbon (called carbon sequestration) when the ice is low, and so the net influx of carbon in our atmosphere which is typically caused by cooling gets stored in our oceans. However, a lower sea level also increases salinity, and that and other physical changes to the large-scale ocean currents and sea ice fields also contribute to carbon sequestration.

The following is the latest understanding of the process of climate change progress during the LGM from Lambeck et al.

  • 35-31 ka BP slow fall in sea level (transitioning out of Ålesund Interstadial)
  • 31-30 ka rapid fall of 25 meters, with rapid ice growth especially in Scandinavia
  • 29-21 ka, constant or slowly growing ice volumes, eastward and southward expansion of the Scandinavian ice sheet and the southward expansion of the Laurentide ice sheet, lowest at 21
  • 21-20 ka onset of deglaciation,
  • 20-18 ka, short-lived sea level rise of 10-15 meters
  • 18-16.5 near constant sea level
  • 16.5-14 ka, major phase of deglaciation, effective sea level change about 120 meters at an average of 12 meters per 1000 years
  • 14.5-14 (Bølling- Allerød warm period), high rate of se-level rise, average rise in sea level 40 mm annually
  • 14-12.5 ka, sea level rises ~20 meters in 1500 years
  • 12.5-11.5 (Younger Dryas), much-reduced rate of sea-level rise
  • 11.4-8.2 ka BO, near-uniform global rise, about 15 m/1000 years
  • 8.2-6.7 reduced rate of sea-level rise, consistent with the final phase of North American deglaciation at 7ka,
  • 6.7-recent, progressive decrease in sea level rise

    Timing of the American Colonization

    According to the most current theories, the LGM impacted the progress of human colonization of the American continents. During the LGM, entry into the Americas was blocked by ice sheets: many scholars now believe that the colonists began entering into the Americas across what was Beringia, perhaps as early as 30,000 years ago.

    According to genetic studies, humans were stranded on the Bering Land Bridge durng the LGM between 18,000-24,000 cal BP, trapped by the ice on the island before they were set free by the retreating ice.