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 (cal bp). During the LGM, continent-wide ice sheets covered high-latitude Europe and North America, and sea levels were between 400–450 feet (120–135 meters) lower than they are today. At the height of the Last Glacial Maximum, all of Antarctica, large parts of Europe, North America, and South America, and small parts of Asia were covered in a steeply domed and thick layer of ice.

Last Glacial Maximum: Key Takeaways

  • The Last Glacial Maximum is the most recent time in earth's history when the glaciers were at their thickest. 
  • That was approximately 24,000-18,000 years ago. 
  • All of Antarctica, large parts of Europe, North and South America, and Asia were covered by ice. 
  • A stable pattern of glacial ice, sea level, and carbon in the atmosphere has been in place from about 6,700 years.
  • That pattern has been destabilized by global warming as a result of the Industrial Revolution. 

Evidence

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 in the vast North American plains, landscapes scraped flat by thousands of years of glacial movement.

In the lead up to the LGM between 29,000 and 21,000 cal bp, our planet saw constant or slowly increasing ice volumes, with the sea level reaching its lowest level (about 450 feet below today's norm) when there was about 52x10(6) cubic kilometers more glacial ice than there is today.

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
  • 2018, +25 centimeters, 408 ppm
  • 1950, 0, 300 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.

The Progress of Global Climate Change

Austria's Pasterze Glacier Reduced to a Lake
Visitors walking on a trail that leads to the melting and rock-covered Pasterze glacier hike past a lake of glacier water in a rocky basin once filled at least 60 meters deep by glacier ice on August 27, 2016 near Heiligenblut am Grossglockner, Austria. The European Environmental Agency predicts the volume of European glaciers will decline by between 22% and 89% by 2100, depending on the future intensity of greenhouse gases.  Sean Gallup/Getty Images

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.

  • 35,000–31,000 cal BP—slow fall in sea level (transitioning out of Ålesund Interstadial)
  • 31,000–30,000 cal BP—rapid fall of 25 meters, with rapid ice growth especially in Scandinavia
  • 29,000–21,000 cal BP—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
  • 21,000–20,000 cal BP—onset of deglaciation,
  • 20,000–18,000 cal BP—short-lived sea level rise of 10-15 meters
  • 18,000–16,500 cal BP—near constant sea level
  • 16,500–14,000 cal BP—major phase of deglaciation, effective sea level change about 120 meters at an average of 12 meters per 1000 years
  • 14,500–14,000 cal BP—(Bølling- Allerød warm period), high rate of se-level rise, average rise in sea level 40 mm annually
  • 14,000–12,500 cal BP—sea level rises ~20 meters in 1500 years
  • 12,500–11,500 cal BP—(Younger Dryas), a much-reduced rate of sea-level rise
  • 11,400–8,200 cal BP—near-uniform global rise, about 15 m/1000 years
  • 8,200–6,700 cal BP—reduced rate of sea-level rise, consistent with the final phase of North American deglaciation at 7ka
  • 6,700 cal BP–1950—progressive decrease in sea level rise
  • 1950–present—first sea rise increase in 8,000 years

Global Warming and Modern Sea Level Rise

By the late 1890s, the industrial revolution had begun throwing enough carbon into the atmosphere to impact the global climate and start the changes that are currently underway. By the 1950s, scientists such as Hans Suess and Charles David Keeling began to recognize the inherent dangers of human-added carbon in the atmosphere. The global mean sea level (GMSL), according to the Environmental Protection Agency, has risen nearly 10 inches since 1880, and by all measures appears to be accelerating. 

Most early measures of current sea level rise have been based on changes in tides at the local level. More recent data comes from satellite altimetry that samples the open oceans, allowing for precise quantitative statements. That measurement began in 1993, and the 25-year record indicates that the global mean sea level has risen at a rate of between 3+/-.4 millimeters per year, or a total of nearly 3 inches (or 7.5 cm) since records began. More and more studies indicate that unless carbon emissions are decreased, an additional 2–5 feet (.65–1.30 m) rise by 2100 is likely. 

Specific Studies and Long-Term Predictions

Climate Change Impacts on the Florida Keys
U.S. Fish and Wildlife ecologist Phillip Hughes inspects dead buttonwood trees which have succumbed to salt water incursion in Big Pine Key, Florida. Since 1963, the Florida Keys upland vegetation is being replaced by salt tolerant vegetation.  Joe Raedle/Getty Images

Areas already impacted by sea level rises include the American east coast, where between 2011 and 2015, sea levels rose up to five inches (13 cm). Myrtle Beach in South Carolina experienced high tides in November 2018 which flooded their streets. In the Florida Everglades (Dessu and colleagues 2018), sea level rise has been measured at 5 in (13 cm) between 2001 and 2015. An additional impact is an increase in salt spikes changing the vegetation, due to an increase in inflow during the dry season. Qu and colleagues (2019) studied 25 tidal stations in China, Japan and Vietnam and tidal data indicate that the 1993–2016 sea level rise was 3.2 mm per year (or 3 inches). 

Long-term data have been collected throughout the world, and estimates are that by 2100, a 3–6 feet (1–2 meter) rise in the Mean Global Sea Level is possible, accompanied by a 1.5–2 degree Celsius in overall warming. Some of the direst suggest a 4.5-degree rise is not impossible if carbon emissions are not reduced.  

The 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 during 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.

Sources