The History of the Earth is some 4.6 billion years old. It is a length of time that is difficult to comprehend. Scientists divide this extensive history into five distinct groups to make it easier to understand. First, there is the Eon, which is the largest segment of time, next is the Era, then the Period, followed by the Epoch, and finally the Age. For illustrative purposes, the dinosaurs lived, thrived, and died during the Mesozoic Era, which was divided up into the Triassic, Jurassic, and Cretaceous Periods. We are living in the Cenozoic Era, which is also divided into three periods, the Paleogene, the Neogene, and the Quaternary, the latter being the period that is continuing today.
There was a significant event known as the Paleocene-Eocene Thermal Maximum (PETM) that began at the end of the Paleocene Epoch, approximately 55 million years ago. Although the warming event was not uniform over the whole planet, it is believed that the average global temperature rose by approximately 6 °C in just a few thousand years. There was no ice at either the north or south poles. Around the same time the continent of India collided with Asia and began the formation of the Himalayan Mountains. The end of the PETM marks the beginning of the Oligocene and the start of another significant climate event.
Towards the end of the Oligocene, the last epoch of the Paleogene Period, the Late Cenozoic Ice Age began, some 33 million years ago. It is still considered to be continuing today. During the Oligocene, plate tectonics, the movement of the continents over the Earth’s mantle, resulted in Australia and South America separating from Antarctica, which continued moving south towards its current position. Antarctica’s ice cap began forming some 16 million years ago. The formation of the Isthmus of Panama, which links North and South America, changed global ocean currents as the Atlantic and the Pacific were separated. Around 3.5 million years ago the Arctic ice cap started to form. A long cycle of glacial periods, when it got colder and glaciers extended, and interglacial periods when it grew warmer and glaciers retreated accordingly, became a feature of the Earth’s climate. Global average temperatures were 5 to 10 °C below what they are today during the coldest periods. When the ice was at its largest extent it reflected the sun’s energy rather than allowing the Earth to absorb it, adding to the cooling process. Glaciers also erode the land beneath them as well as reducing the amount of land area above sea level. Typically, glacial periods last approximately tens of thousands of years, whereas interglacial periods last only a few thousand years. The current one in which we live is believed to have started some 10,000 years ago.
The beginning of another glacial period would be catastrophic for the human race. Large tracts of land would be lost to the advance of glaciers, impacting on agriculture and living space. The amount of rainfall would reduce, perhaps by as much as half. Forests and grasslands would be destroyed, and large numbers of plants and animals would go extinct as a result. Human habitation and access to natural resources, including food and fresh water, would become severely limited. The economic devastation would be catastrophic. It is not clear whether we are facing the onset of another glacial period, however. The geological evidence is that they are cyclic and that at some time in the future another glacial period will begin.
The problem for most people, however, is in comprehending geological time, or deep time as it is also known. 10,000 years is nothing in terms of the age of the planet, but a huge amount of time in human experience. 100,000 years is even harder to grasp, and yet modern humans are thought to have appeared only 200,000 years ago. During that time the climate has changed on numerous occasions and often with dramatic results. The 200,00 years of modern human history is but a blink of the eye in the 33,000,000 years of the Late Cenozoic Ice Age.