(For 400,000 years, the Earth has experienced four cycles of glaciation. )
The climate variations are correlated with those of insolation, Milanković parameters, albedo, solar cycles and atmospheric concentrations of greenhouse gases such as carbon dioxide and aerosols.
The global climate of the Earth undergoes more or less cyclical modifications of reheating, alternating with coolings that differ in their duration (from a few thousand to several million years) and in their amplitude. For 60 million years, the Earth has been in general cold, with the appearance of the Antarctic Ice Sheet 35 million years ago and the ice cap of the Northern Hemisphere 4 million years ago.
For 800,000 years, the Earth’s climate has undergone several cycles of glaciation and warming, some 100,000 years each. Each cycle begins with a brutal warming followed by a warm period of about 10,000 to 20,000 years, called the interglacial period. This period is followed by a gradual cooling and the installation of an ice age. At the end of the glaciation, a brutal warming begins a new cycle. We have been living for more than 10 000 years in an interglacial period (see figure), and the current climatic evolution is in relation to post-glacial natural warming.
Thanks to the study of ice cores and more specifically the analysis of the isotopic composition of oxygen trapped in ice, the atmospheric temperatures of the glacial cycles of the Quaternary era have been reconstituted. The deepest ice core was drilled as part of the Antarctic Epica project, at a depth of more than 3,500 meters, to study the history of the Antarctic climate for 800,000 years. Ice cores contain air bubbles and indications of the gas content of the atmosphere of yesteryear. These cores have shown that the CO2 content in the atmosphere has increased by 30% during the last century when it was about stable before.
Amplitudes of climatic variations
(Changes in global climate since 540 million years. )
During the Quaternary, the thermal amplitude was of the order of 10 °C, but with temperature increases having never exceeded by more than 4 °C the average annual temperature of the end of the 20th century. On the other hand, for older cycles, such as during the Permian period, the average global temperature reached 22 °C, 8 °C higher than the current average, as can be seen in the graph below. During these warm periods that lasted several tens of millions of years, the Earth was devoid of polar ice caps.
The thermal maximum between Paleocene and Eocene, 56 million years ago, is particularly interesting because it seems due to a release of greenhouse gases, but spread over several thousand years. The global warming was 5 °C, at a moderate rate of 0.025 °C per century, much lower than the current rate. Its impact was significant on marine species, some of which disappeared as a result of ocean acidification, with terrestrial animal and plant species largely able to adapt or migrate.
(Detailed explanation of the graph, reconstructions are made from dendrochronology, measurements in glaciers, among others. )
Within major terrestrial climatic fluctuations, there are shorter and more limited variations in intensity.
Thus, in the last millennium, a warm period between the 10th and 13th centuries, called the “medieval climatic optimum”, appeared in Western Europe: it was the time when Viking sailors discovered and christened Greenland (literally Green Country “) and found colonies in the extreme south of the island.
Similarly, the period of modern times (1550-1850) experienced a period of cooling that historians call the “little ice age” characterized by very harsh winters, including the terrible winter 1708-1709.
According to climatologists’ reconstructions of temperatures, the last decade of the twentieth century and the beginning of the twenty-first century represent the hottest period of the last two millennia (see graph). Our time would be a little warmer (a few tenths of degrees) than was the medieval climatic optimum.