12.1: Climate Change in Earth’s History

We are currently in the middle of a glacial (cold) period that started about 34 million years ago but became more intense about one million years ago. During that time glaciers have expanded and contracted on a time scale of around 100,000 years. This is not the only period of glaciation in Earth’s history. In general, the Earth has been warm enough to be ice-free for much more of the time than it has been cold enough to be glaciated. Since this is not a Historical Geology class, we will not cover all glaciations, but instead will highlight a few to show variation in timing and causes.

Figure \(\PageIndex{1}\): The Record of Major Glaciations During Earth’s History

The oldest known glacial period is the Huronian and was likely initiated because of the evolution of photosynthetic organisms. This resulted in an increase of free oxygen in the atmosphere and a consequent drop in the levels of the greenhouse gas methane—which triggered cooling. Based on evidence of glacial deposits from the area around Lake Huron in Ontario (and elsewhere), the Huronian glaciation lasted from approximately 2450 to 2400 Ma. Because rocks of that age are rare, we don’t know much about its intensity or global extent.

Evidence for the Andean-Saharan glaciation is found in rocks of the Andean region of South America and in the Sahara region of Africa during the Ordovician and Silurian (Figure \(\PageIndex{2}\)). The glaciation may have lasted for up to 10 million years (from about 435 to 445 Ma) at a time when the supercontinent Gondwana (which was originally part of Pangea) was mostly close to the South Pole. Eyles (2008)^[2] has suggested that the cooling that triggered this glacial episode might have resulted from the formation of a major mountain range in the northern part of what is now Africa, which would have led to enhanced erosion and weathering, and therefore to consumption of atmospheric carbon dioxide.

Figure \(\PageIndex{2}\): The Possible Extent of the Ordovician-Silurian Glaciation (white) on Gondwana at Around 440 Ma

The Earth was warm and essentially unglaciated throughout the Mesozoic (Figure \(\PageIndex{2}\)). Although there may have been some alpine glaciation at this time, there is no longer any record of it; the dinosaurs, which dominated terrestrial habitats during the Mesozoic, did not have to endure icy conditions. Sea levels were very high during this time, as Earth's water was stored in the ocean instead of as ice.

At around 15 Ma subduction-related volcanism between central and South America created the connection between North and South America, preventing water from flowing between the Pacific and Atlantic Oceans. This further restricted the transfer of heat from the tropics to the poles leading to a rejuvenation of the Antarctic glaciation. The expansion of that ice sheet increased the Earth’s reflectivity enough to promote a positive feedback loop of further cooling: more reflective glacial ice, more cooling, more ice, etc. Ice sheets started to grow in Greenland by around 3.5 Ma, and in North America and northern Europe by around 1 Ma (Figure \(\PageIndex{3}\)). The most intense part of the current (“Pleistocene”) glaciation—and the coldest climate—was during the Pleistocene, but if we count Antarctic glaciation, it really extends from the Oligocene to the Holocene (Figure \(\PageIndex{2}\)), and will likely continue into the future.

The Pleistocene has been characterized by significant temperature variations (through a range of approximately 8˚ C) on time scales of 40,000 to 100,000 years, and to corresponding expansion and contraction of ice sheets in the northern hemisphere. These variations are attributed to subtle changes in the orbital parameters of the Earth (Milankovitch Cycles). The angle of the Earth's axis, the shape of Earth's orbit, and the wobble of the Earth's axis all contribute to establish certain cyclical warming and cooling patterns. Over the past million years the glaciation cycles have been approximately 100,000 years, with glacials (cool periods) lasting about 100,000 years and interglacials (warm periods) lasting about 10,000 years.

Figure \(\PageIndex{3}\) shows global mean temperature in the past 500,000 years. The last five glacial periods are marked with snowflakes. The most recent one, which peaked at around 20 ka, is known as the Wisconsin Glaciation. The current interglacial (Holocene) is marked with an H.

Figure \(\PageIndex{3}\): Temperature Fluctuations Over the Past 500,000 Years

At the height of the last glaciation (Wisconsin Glaciation) massive ice sheets covered virtually all of Canada and much of the northern United States (Figure \(\PageIndex{4}\)). The massive Laurentide Ice Sheet covered most of eastern Canada, as far west as the Rockies, and the smaller Cordilleran Ice Sheet covered most of the western region. At various other glacial peaks during the Pleistocene and Pliocene the ice extent was like this, and in some cases even more extensive. The combined Laurentide and Cordilleran Ice Sheets were comparable in volume to the current Antarctic Ice Sheet.

Figure \(\PageIndex{4}\): The Extent of the Cordilleran and Laurentide Ice Sheets Near to the Peak of the Wisconsin Glaciation, around 18.5 ka

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0, after the International Geological Timescale, https://stratigraphy.org/chart
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0, after Eyles, N. (2008)
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0, using data from Lisiecki & Raymo (2005)
• Figure \(\PageIndex{4}\): Steven Earle, CC BY 4.0, based on a public domain NOAA map, Paleo Glaciation, https://www.ncdc.noaa.gov/paleo/glaciation.html

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