The Ice Age
Ice ages are extended periods of cold climate, when ice sheets permanently cover some portion of the Earth. Typically, ice sheets are polar, continental, or alpine (in mountain regions, not necessarily the Alps). Usually, the term "Ice Age" refers to polar and continental ice sheets, since small permanent patches of ice can exist at high altitudes (on mountain tops) even when the Earth as a whole is quite warm.
For most of its history, the Earth has been remarkably ice free even at higher latitudes. There are at least four known ice ages that have occurred in the past:
- Huronian: in the late Archean and early Proterozoic eons, about 2.7 billion to 2.3 billion years ago.
- Cryogenian: the famed "snowball earth" period in the middle Neoproterozoic, about 850 million to 630 million years ago.
- Andean-Saharan: a minor ice age during the late Ordovician and early Silurian, about 460 million to 430 million years ago.
- Karoo Ice Age: at least two glaciations that occurred during the Carboniferous, about 350 million to 300 million years ago.
In addition to these four ice ages that occurred in the past, there is the fifth ice age in which we live now. This article is chiefly about this last ice age.
Before we begin, let's cover some terminology:
As mentioned before, an ice age is a period when there are permanent ice sheets somewhere on Earth, usually at the poles and sometimes on the continents as well. During ice ages, there are colder periods when the ice sheets advance and cover lower latitudes, and other warmer periods when they retreat. The colder periods are more extended, and are referred to as glacials, while the warmer periods are termed interglacials.
On a finer time scale, even within a glacial or interglacial, there are brief periods of warmer or colder climate. These may be as short as a couple thousand years, or even less. A brief cooling period within an interglacial is called a stadial, and a brief warming period during a glacial is called an interstadial.
The current ice age may have begun as early as 13+ million years ago. There is evidence that the Antarctica ice sheet may have started forming as much as 35 million years ago, and then partially re melted and reformed again about 13 million years ago. However, when we typically refer to the current "ice age", we mean the current glaciation, known as the Quaternary or Pleistocene glaciation, which began about 2.58 million years ago.
Note: This terminology is a bit confusing, because most texts typically consider the Pleistocene to be the epoch from about 1.8 million years ago, to about 11,000 years ago (the beginning of the Holocene). The term "Quaternary" is still in dispute and awaiting settlement by the International Union of Geological Scientists (IUGS). The base of the Quaternary is placed at 2.588 million years ago, which is the start of the current glaciation, but this period straddles part of the Pliocene. Geologically, it refers to the start of the Gelasian, which is a stage that correlates with the period covering the deposition of certain geological features in East Anglia, England. The solution to this problem might be to include this period in the Pleistocene (set the base of the Pleistocene to the start of the Gelasian at 2.588 million years, rather than at 1.8 million years where it is now), to better correspond with major geological events such as an ice age. However, this will have to wait for resolution from the IUGS. Meanwhile, geological time charts continue to show this disputed period with dotted lines, as can be seen in the chart from IUGS here,
The Quaternary glaciation is marked by permanent ice sheets established in Antarctica, and probably Greenland. Fluctuating ice sheets occurred elsewhere, such as the Laurentide ice sheet over North America. As with any ice age, major geological changes included:
- major erosion of topographic features and deposition of sediments across continents affected by the ice sheets
- fluctuations in sea level as water from the oceans was locked up in the ice sheets, which periodically advanced and retreated
- the creation of millions of inland lakes as the ice sheets melted
- deformation of the Earth's crust under the weight of the ice sheets
- modifications in the course and drainage of river systems as a result of the changes mentioned above
Ice sheets have advanced and retreated over this period at somewhat regular intervals (Milankovitch Cycles). There is a 41,000 year cycle that can be identified over much of the Quaternary glaciation. In addition, there is also a 100,000 year cycle that appeared somewhat more recently. There have been at least 80 such cycles during the Quaternary glaciation. Currently, we are in the most recent interglacial, known as the Holocene Interglacial, which started about 11,000 years ago.
Climate data reconstruction is a complex task. For older data, benthic cores (sedimentary cores from the sea bottom) as well as fossils are used. Isotope ratios can be used to infer climatic conditions. For more recent data, ice cores can be used. Some of the longest cores have been drilled from sites in the Antarctica.
Reconstruction of Earth's climate over the past 65 million years. Adapted from GWArt, based on data from L. E., and M. E. Raymo (2005). "A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records". Paleoceanography 20: PA1003.
An example of reconstruction based on Oxygen-18 isotopic ratios in benthic foraminifera is show in the figure above. The latter part of the record can be correlated to temperature records derived from the Vostok ice core. Early "Polar Ocean Equivalent" temperatures are calculated based on the assumption that no ice sheets existed over the poles at this time, therefore any variation in Oxygen-18 ratios was purely due to temperature changes.
The Antarctica started freezing about 35 million years ago, but it thawed during a warming period about 25 million years ago. The current freeze started about 13 million years ago, and it has remained permanently frozen since. This can be considered the start of the current ice age.
Reconstruction of Earth's climate over the past 5 million years. Adapted from GWArt, based on data from L. E., and M. E. Raymo (2005). "A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records". Paleoceanography 20: PA1003.
The climate record of the past 5 million years (seen in the figure above) shows an increase in the negative trend of global temperatures, which had already been on the decline since about 13 million years ago. Before the Quaternary, temperatures had declined to about the same levels as during the previous Antarctica glaciation 35 million years ago. At the beginning of the Quaternary, temperatures took a nosedive as ice sheets started expanding from the poles to cover more temperate latitudes. The ensuing period is marked by cycles of large temperature fluctuations, as the ice sheets continually advanced and retreated. The Antarctica (and possibly Greenland) remained frozen throughout. The cycles were initially about 41,000 years in length, but have lately been prolonged to 100,000 year intervals.
Reconstruction of Earth's climate over the past 450,000 years, based on data from the EPIC and Vostok ice cores from the Antarctica. Adapted from GWArt based on data from EPICA ("Eight glacial cycles from an Antarctic ice core". Nature 429 (6992): 623-628) and Vostok ("Climate and Atmospheric History of the Past 420,000 years from the Vostok Ice Core, Antarctica". Nature 399: 429-436) teams.
The climate of the past half million years can be more accurately reconstructed from ice cores taken from the Antarctica. The Vostok Core was drilled at Vostok Station, and is 2917 meters long. Drilling was stopped to avoid contaminating Lake Vostok, which lies underneath. Data in this core goes back 420,000 years. The EPICA (European Project for Ice Coring in Antarctica) was drilled at a site about 560km from Vostok Station. The core is 3190 meters long, and produces data going back 720,000 years. A reconstruction of climate data based on these cores can be seen in the figure above. Some of the major glaciations during this period are shown, the latest of which (the Würm Glaciation) ended recently, about 11,000 years ago.
The retreat of the glaciers at the beginning of the Holocene around 11,000 years ago marked a warming phase in the Earth's climate. This was the first interglacial in which modern humans existed. As the ice sheets retreated across Europe and the Middle East, the warming climate encouraged the development of agriculture.
It is unclear how long the current interglacial will last. Earlier studies indicated that we were at the beginning of a new glacial, but more recent data paints a more complicated picture. The current interglacial may in fact last as much as another 10,000 to 15,000 years. The effects of human induced global warming will surely factor into this, but it is unknown precisely how the climate will be affected over the next few thousand years.