Enormous sheets of ice over a mile thick once covering much of North America is a basic fact of Earth science. However, the idea that Earth’s past climate was once drastically different from the climate we have today is a recent concept. This idea is not obvious or intuitive and therefore required an accumulation of evidence to become gradually accepted before becoming an established fact of our planet’s history. Fortunately, these enormous glaciers disappeared without erasing the evidence of their existence.
Clues Left Behind by the Ice
The evidence of these enormous glaciers had been laying around for centuries. The most visible clue was rock – in this case called an erratic boulder. It was a large rock, sometimes as big as a house, sitting on top of something else: say granite perched on top of limestone. These were observed all over Europe but there was no explanation how they got there. The Biblical flood was proposed as an explanation, but in the early 18th century an alternative idea based on actual evidence was suggested by Jean-Pierre Perraudin. He noticed groves in the rock, called striations, which he thought were caused by glaciers that had since retreated. He shared the idea with the geologist Jean de Charpentier in 1815 who immediately dismissed it, only to come around to it years later.
Striations are reliable in showing which direction the ice flowed based on analysis from the rocks texture. Using this information, geologists could track erratic boulders back to their bedrock source. In other words, you could reconstruct the path the glaciers actually flowed. This could be done at enough sites where a pattern emerges of a continent-wide system of ice movement.
Louis Agassiz Makes a Surprising Proposal
In 1836, the Swiss naturalist Louis Agassiz spent some time with Charpentier in Switzerland. Charpentier, along fellow Swiss Ignaz Venetz and the German botanist Karl Schimper likely convinced him of their glacial theories. On July 24, 1837 he presented at the Swiss Society of Natural Sciences his idea of the ice age: that the Earth had undergone a prolonged winter with huge sheets of ice covering much of the Northern Hemisphere.
The idea, as with most new and radical ideas in the history of science, was not initially well received. Additional evidence such as moraines – the piles of debris a glacier pushes ahead of itself, made the case stronger. Agassiz did field work, observing how real glaciers actually move. He personally pushed the idea for years, slowly converting other scientists who in turn convinced yet others. It took another 20 years for the scientific community to come around to what he’d proposed in 1837.
Ice Ages as we Understand them Today
Agassiz suggested that the much of the Earth was once covered in thick sheets of ice, but since then we’ve learned that these ice ages have happened several times in our planet’s history. Earth has experienced several major ice age periods in its history. Within an ice age, the climate varies between a glacial period, where the ice expands across the planet, and an interglacial period, where the ice retreats. We currently live in an interglacial period called the Holocene which began roughly 11,700 years ago. Not long after this period began agriculture was discovered and civilization as we know it began.
One of the last questions about ice ages to be answered was why the occurred at all. As early as the 19th century it was suggested that Earth’s orbit could influence the advance and retreat of ice. It wasn’t until the early 20th century when the Serbian mathematician and astronomer Milutin Milankovic developed the idea in much greater detail. Milankovic calculated that long term variations in Earth’s eccentricity (Earth’s orbit varies between more circular and more elliptical), obliquity (changes in the tilt of the Earth’s axis), and precession (the wobble of its rotational axis) alter the amount of sunlight reaching the Earth. Additionally, various positive and negative feedback loops, such as Earths albedo, either reinforce or diminish the effects of the varied sunlight. His theory unsurprisingly remained controversial until the 1970s when evidence from deep sea sediments showed that major climate changes closely followed his predictions. These orbital variations are now known as Milinkovic cycles and provide a powerful explanation for our planets ice ages.
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