Last week, after reading the paper on the start of Antarctic deglaciation that Claire wrote about, I got thinking about the Last Glacial Maximum (LGM). The LGM was a period of time extending between about 26.5 and 19 thousand years ago when global climate proved to be ideal for the growth of major ice sheets. The cause of these conditions was largely due to orbital conditions, in which summer temperatures would be at a minimum in the northern hemisphere. Coincident with this period of time were cooler temperatures in the tropical regions of the Pacific, resulting in a higher prevalence of La Nina events. Anyone who remembers the Australian floods a couple of years ago can attest to the increased rainfall during a La Nina.
The Last Glacial Maximum stands as a time when global sea level was at its lowest point. During that time sea level was about 130 m lower than present. When sea level was that low, Papua New Guinea and Australia would have been connected! When you consider the large area that the ocean covers on the Earth, this amounts to a large amount of ice that had to be on land. Researchers here at the ANU estimated that there was 52.5 106 km3 more land-based ice than at present during the LGM.
Despite using the term “global” or “eustatic” sea level, sea level does not rise or fall equally around the world. Earlier, when I said that sea level was about 130 m lower than present, this doesn’t mean that it dropped by that much everywhere! A good example is from a study by Yokoyama et al from 2001. The below figure shows the calculated sea level for three locations in the Bonaparte Gulf, located northwest of Australia. Even though the locations are within 100 km of each other, the estimated sea level at the LGM is upwards of 15 m different!
One of the biggest questions when it comes to the LGM is “where was the ice?” There were large volumes of ice in northern Europe and North America, and additional ice in Antarctica. The volume of ice that was in Europe during the LGM is fairly well constrained, due to a large volume of geological and geophysical data. The amount of data in North America and Antarctica is considerably less. A lot of this has to do with logistical issues – it is very expensive to get to Antarctica and northern Canada. As a result, it is still difficult to know the exact volume of ice in Antarctica and North America. This is especially troubling for those who are trying to figure out ice volume loss that is currently happening in Antarctica. If you want to determine how much ice is melting, you have to take into account the ongoing uplift of the earth due to previous ice sheet retreat.
One thing I can say from my own research is that the ice extent in the western part of the Laurentide Ice Sheet in North America was unprecedented. Aside from the previous glaciation, no other one saw the convergence of ice from the Rocky Mountains and the Laurentide ice sheet. This means climate around northwestern North America did something that had never happened before.
The end of the LGM happened about 18-19 thousand years ago as orbital conditions changed to warm the Earth. At that time, there was a net loss of ice volume, which caused global sea level to rise rapidly. However, reductions in the extent of the major ice sheets didn’t happen simultaneously throughout the world. For example, the Cordilleran Ice Sheet, which covered much of the province of British Columbia in Canada, reached its maximum extent several thousand years after the end of the LGM. This example illustrates the regional variability in the response to a warming Earth. Though warming caused ice sheets to begin retreating in a lot of areas, some places may have got colder, or got more snow! As such, studying the climatic conditions after emerging from the LGM serves as a good analogue to what could happen to the Earth from anthropogenic warming in the future.