Technically we are still living in an ice age that began 34 million years ago
Because Earth still has permanent ice caps, geologists say we are inside the Late Cenozoic Ice Age, under way for some 34 million years. The idea of vast ancient glaciers first came not from professors but from Alpine villagers and a chamois hunter, and scientists resisted it for decades.
An ice age is a long stretch of cooler climate in which continental glaciers and polar caps persist or grow. Within it, temperatures swing between colder glacials and milder interglacials. For most of its history Earth has been in a greenhouse state with little or no permanent ice, but at least five major ice ages are recognised, from the Huronian, whose rocks date to about 2.4 to 2.1 billion years ago and are exposed north of Lake Huron, to the current Quaternary glaciation, which began 2.58 million years ago.
Locals understood first. In 1742 the Geneva engineer Pierre Martel heard people in the Chamonix valley explain scattered boulders by glaciers that had once reached much farther. In 1815 Jean-Pierre Perraudin, a carpenter and chamois hunter, pointed the geologist Jean de Charpentier to scratched rock and giant boulders in the Val de Bagnes; Charpentier blamed floods and called the idea absurd. Meanwhile Jens Esmark argued in 1824 for a sequence of worldwide glaciations caused by changes in Earth's orbit, credit for which was later absorbed by others, including Louis Agassiz.
Agassiz did the heavy lifting of persuasion, publishing his Study on Glaciers in 1840 after extensive fieldwork. Even so, the theory won full international acceptance only in the late 1870s, after James Croll's Climate and Time of 1875 offered a convincing explanation of the causes.
Evidence now comes from three directions. Geology supplies scoured rock, moraines, drumlins and stray boulders, although each new glaciation scrambles traces of earlier ones. Chemistry uses isotope ratios in ocean sediments and ice cores, including air trapped in bubbles, to rebuild past temperatures. Fossils reveal cold-loving species spreading toward the equator during glacials. Cores overturned an early assumption: glacials turn out to be long and interglacials short.
Source: Ice age
