Louis Agassiz (1807-1873)was a zoologist, paleontologist, geologist who discovered that the Earth had been subject to global ice ages in the past. Yes, he got the timing and sequences wrong, but he was the first to recognize that glacial advance and retreat could be a global phenomenon and the first to realize that the whole Earth's climate could change.
The idea that glaciers had expanded and contracted in the past was not an entirely new idea. Few things are. Jean de Charpentier (1786-1855) observed the breakout of an glacially dammed lake in the Val de Bagnes and subsequent flood destroying Martigny, killing many on June 16, 1818. Amazingly, he was an eyewitness to the breakup and flood, which showed him how suddenly glaciers and the natural environment could change. Fascinated by this, and seeing the landforms left underneath the glacier when it disintigrated, he recognized erratic boulders and moraines all over Switzerland! He did not try to explain how glaciers had expanded and retreated---he freely admitted he had no ideal But Charpentier was convinced that all of Switzerland had been covered by glaciers in the distant past.
The ice age theory was expanded a little more by Ignaz Venetz, the chief engineer who had been attempting to drain the ice lake before it catastrophically failed. He read reports of similar glacial landforms across much of northern Europe and Britain, and concluded by 1821 that there had been an ice age over much of Europe in the distant past. He traveled and researched, and in 1833 he published Mémoire sur les Variations de la température dans les Alpes de la Suisse which concluded that there had been times in the past when glaciers had covered all of Switzerland and northern Europe, and that these had happened at the same time.
Another contributor was Karl Friedrich Schimper. In fact he may have been the originator of the theory of global ice ages. It is known that his private papers are filled with the idea of a global ice age cooling, based on new reports from New England, the Midwest, and Canada. They worked together, along with Charpentier for 3 years in the Swiss Alps in the 1830s. Schimper never published anything, although he complained bitterly that Agassiz had taken his ideas and never gave him any credit. It is as if Wallace and Darwin had corresponded about evolution, and Wallace published and gave Darwin no credit. How much Agassiz got his ideas from Schimper is unknown--there is often symbiosis in creating a new idea when bright minds exchange information. But it is certain that at the least, Schimper contributed a lot of ideas and data for a global ice age, and Agassiz never acknowledged his contribution. Not one word. Agassiz did acknowledge some contributions by Charpentier (whose private papers show much less insight into the topic)
The moral of this is 'publish or lose'
Agassiz made his big splash in 1840 with his monumental two-volume work Etudes sur les glaciers (Study of glaciers) that described Switzerland as having been another Greenland in the past, that glaciers had covered much of North America, and that the climate across the entire Earth had been much colder.
Agassiz was just a bit vague about what drove the ice age and how Earth got out of it. He didn't know about greenhouse gases, he didn't know about orbital and axial shifts. He didn't attach himself to a specific idea, although he thought the sun was probably responsible. He wondered if the fall of a large comet or planet into the sun had caused it to brighten and melt the earth out of the ice age.
Agassiz then relocated to the United States, where he became a scientific star. He had some serious foibles late in life. In 1865 he traveled to Brazil, where he was convinced he saw glacial landforms (there have never been large glaciers in Brazil during any of the recent ice ages)
Agassiz also fought against Darwin's theory of evolution for the rest of his life after he published On the Origin of Species Upon encountering African-Americans, Agassiz became convinced that different races had different origins and that the white race from temperate climates was superior to all other races, and was one of the architects of the scientific racism school of thought in the 19th century.
The key thing about the discovery of past ice ages is that for the first time, scientists realized that climate could change, and change dramatically. Which raised an obvious question. How did the global climate change? For the first time, scientists began to investigate how climate change can happen.