(Image: Cody Whitelaw – Ilulissat, Greenland)
‘Ice Age’ or ‘Glacial’; what’s the difference and why do we care?
Summary
- Ice Ages are long events (millions of years) in geological time called Periods, when there’s at least one major ice sheet. An ice sheet is defined as an area 50,0002 km or more. As Greenland and Antarctica still have much larger ice sheets than this, we are still in an Ice Age.
- Glacials are shorter events called Epochs when the ice sheets extend out over continents (Fig.1).
- While the terms ‘Glacials’ and ‘Ice Ages’ are often used interchangeably—even by geologists and climate scientists when speaking in general terms—understanding the difference is crucial to understanding the circumstances that caused ice sheets to melt, and how quickly they melted. That helps give us a better idea how quickly impacts such as rising sea levels might rise might affect us if certain critical tipping points are reached as Earth rapidly warms.
Fig 1. What the Arctic looks like during the different Geological Periods and Epochs. We are currently in an Ice Age and Interglacial (centre figure).
Ice Ages, Glacials and Interglacials
- An Ice Age is a geological Period that lasts millions of years. Today, we’re living in the Quaternary Period, an ice age that began 2.58 million years ago when ice sheets formed over the Arctic and Antarctica and mountain chains around the world including New Zealand’s Southern Alps.
- A Glacial is a very cold stretch of time (a geological Epoch) during an Ice Age, when the ice sheets spread out from the poles and down from (and in many places over the top) of the mountains. It can last thousands or millions of years. The last Glacial was the Pleistocene Epoch (2.58 million to 11,700 years ago).
- An Interglacial is a geological Epoch when most ice sheets retreat back to the poles (today, that’s Greenland and Antarctica) and some high mountain areas. We are currently living in the Holocene Epoch, an interglacial (11,700 years ago to present day).
- Ice sheets vs glaciers: ice sheets form when glaciers join together and cover large areas of land, sometimes entire continents. An ice sheet is generally defined as an area of ice covering at least 50,0002 km.
- Because human activities are causing global climate change, which is melting the Greenland and Antarctic ice caps and triggering mass species extinctions, the term Anthropocene is being proposed as a new geological epoch.
Interstadials and Stadials including the last Glacial Maximum
Smaller geological units of time describe in more detail differences in global temperatures and the extent of ice sheets and glaciers during a Glacial (Fig. 2).
- Stadials: during the last glacial epoch, the Pleistocene, there were shorter periods when it was extremely cold and the ice sheets spread much further. The last of these stadials, from ~33,000 to 19,000 years ago is often referred to as the Last Glacial Maximum (LGM); it was the most recent, the coldest, and the ice sheets were the most widespread during the Pleistocene (the end of the LGM was ~14,500 years ago when the West Antarctic ice sheets collapsed and sea levels aburptly rose).
- Interstadials: between the super cold Stadials, while there still ice sheets across the continents, it was slightly warmer, so the ice sheets retreated although not nearly as much as during interglacials.
- Summary: as there are ice sheets still over Greenland and Antarctica, we’re living in the interglacial Holocene Epoch during the Quaternary Period, an Ice Age.
Fig. 2: The difference between Periods, Epochs, and Stadials. By convention, the terms ‘Before Present (BP)’ or ‘Years Ago (YA)’ is the year 1950. A new epoch, the Anthropocene, has been proposed to describe how humans have changed the planet so much that it shows up in the geological record. As there still are ice sheets on Earth, this proposed Epoch is still in the Quaternary Period Ice Age.
References and further reading
- IPCC: Working Group 1: What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era?
- Geological Time Scales
- Glossary of important terms in glacial geology
- PAGES: Past Global Changes; international coordination of past climate change research
- Quarternary Stratigraphy: Working Group on the ‘Anthropocene’
- National Geographic library: Ice sheets
- 2018: Buizert et al; Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north Nature, 563, 681–685 (2018)
- 2016: Markle et al; Global atmospheric teleconnections during Dansgaard–Oeschger events Nature Geoscience 10, 36–40 (2017)
- 2009: Clark et al; The Last Glacial Maximum, Science 325/5941 710-714