During the history of Earth there have been five major ice ages, and we are currently in the Quaternary Ice Age at this time, which spans from 2.59 million years ago. Within the ice ages are sub-periods known as glacial and interglacial periods.
Recent measurements of the relative Oxygen isotope ratio in Antarctica and Greenland show the periods of glacial and interglacial periods throughout history over the last few hundred thousand years. This is a measurement of the ratio of the abundance of Oxygen with atomic mass 18 to the abundance of Oxygen with atomic mass 16 present in ice core samples, 18^O/16^O, where 16^O is the most abundant of the naturally occurring isotopes. Ocean water is mostly comprised of H^2-16^O, in addition to smaller amounts of HD-16^O and H^2-18^O. The Oxygen isotope ratio is a measure of the degree to which precipitation due to water vapour condensation during warm to cold air transition, removes H^2-18^O to leave more H^2-16^O rich water vapour. This distillation process leads to any precipitation to have a lower 18^O/16^O ratio during temperature drops. This therefore provides a reliable record of ancient water temperature changes in glacial ice cores, where temperatures much cooler than present corresponds to a period of glaciation and where temperatures much warmer than today represents an interglacial period. The Oxygen isotope ratios are therefore used as a proxy for temperature changes by climate scientists.
The Vienna Standard Mean Ocean Water (SSMOW) has a ratio of 18^O/16^O = 2005.2×10-6, so any changes in ice core samples will be relative to this number. The quantity that is being measured, δ^18O, is a relative ratio and is calculated as follows in the units of % parts per thousand or per mil.
The change in the oxygen ratio is then attributed to changes in temperature alone, assuming that the effects of salinity and ice volume are negligible. An increase of around 0.22% is then defined to be equivalent to a cooing of 1˚C given by
T = 16.5 - 4.3(delta) + 0.14(delta)^2
There are differences in the value of δ between the different ocean temperatures where any moisture had evaporated at the final place of precipitation. As a result the value has to be calibrated such that there are differences between say Greenland and Antarctica. This does result in some differences in the proxy temperature data based on ice core analysis, and Greenland seems to stand out, such as indicating a more dramatic Younger Dryas period (11,600 – 12,900) than other data.
An analysis of this data shows that the climate has varied cyclically throughout its history and is manifest of natural climate change. In particular what emerge out of the data are some interesting lessons about the recent history of planet Earth. Data shows the rapid oscillations of the climate temperature from the average temperature of today, indicative of glacial and interglacial periods. In particular, the data shows that during the Holocene period, beginning approximately 11,700 years before present, the temperature varied between 2-4 ˚C.
It is reasonable to assume that human civilisations under development will do better when the climate is kinder. This means that the warmer it is the better civilisations will do, and the colder it is, the harder the struggles. In particular we can expect that during the conditions of a colder climate that agricultural farming will suffer, and so there will be less food to go around, which will affect both life span and population expansion. To support this it is worth noting that the current epoch, the last 10,000 years has been the longest interglacial period for at least the last quarter of a million years and it is reasonable to therefore assume that this is one of the factors which has allowed human development from the emergence of the Neolithic period coming out of the last ice age.
Temperature proxy data from Greenland ice core samples of Oxygen isotope ratios.
The data also shows that there was a large global warming period known as the Eemian around 115,000 – 130,000 years ago. The average global temperatures were around 22 – 24 ˚C, compared to today where the average is around 14 ˚C. Forests grew as far north as the Arctic circle at 71˚ latitude and North Cape in Norway Oulu in Finland. For comparison North Cape today is now a tundra, where the physical growth of plants is limited to the low temperatures and small growing seasons. Given that Homo sapiens may have been here since around 300,000 years ago, this seems like a major opportunity for the development of human society from a people of hunter gatherers to one of agricultural developers and the development of a civil society.
There have been other interglacial periods that have resulted in global temperatures being either equivalent or above the average today, and the data shows temperature spikes of periods at around 200,000 years, 220,000 years, 240,000 years, 330,000 years and 410,000 years. Each of these interglacial periods will typically last at least 10,000 years.
Is it possible that these earlier periods in history allowed the opportunity for civilization to rise up and become sociologically and technologically advanced towards similar levels of today? The climate certainly seems to have allowed for it. The question is, did it happen?