The Last Interglacial - An Analogue for the Future?
Posted on 24 June 2011 by Steve Brown
Many of the current predictions for future climate and environmental impacts have been made on the basis of complex computer models of the Earth's systems. Though such models have been successful in many areas, and are improving in accuracy and fidelity with increasing computer power, they still carry large uncertainties and margins of error, especially the further into the future that models are run. One way that such uncertainties might be reduced is by looking at the geological and fossil record for periods in Earth's history that were warmer than today and determine how the environment responded. This may allow us to better estimate the effects of climate change that could face us.
The Last Interglacial, also known as the Eemian in Europe and broadly corresponding to the global interglacial record of Marine Isotope Stage 5e (MIS 5e), was a period with significantly higher temperatures in many parts of the Northern Hemisphere compared to the the present day (IPCC AR4) and similar to some predictions for the next century by current research. The Last Interglacial offers several benefits for comparison with the present day in that continental configuration, flora and fauna are near identical to today, as well as being a recent enough period to provide a range of high-resolution palaeoenvironmental proxy indicators. The Last Interglacial was also a period with higher global sea-level and a corresponding reduction in ice sheet area and volume, which are consistent with IPCC predictions for responses to future global warming. The period also had an atmospheric CO2 level of up to 300 ppm; similar to the pre-industrial era.
Figure 1: Timeline of past 150,000 years. (illustration by jg at brightstarswildomar)
In Europe, the early part of the Eemian, as it was in transition from the preceding glacial stage, was distinguished by treeless, grassy plains that eventually gave way to pine forests and then extensive oak and hazel forests as the climate warmed. Global sea-level was estimated to have been at least 5 metres higher, resulting from a reduction in size of the Greenland and Antarctic Ice Sheets. Coniferous forests were widespread at high-latitudes; deciduous forests were widespread at mid-latitudes, with low-latitudes predominantly savannah with grass plains and sparse tree cover. Northwestern Europe, with its crust being isostatically depressed from the the weight of ice cover from the previous glaciation, was flooded by the Eemian Sea which connected the Barents and North Seas, leaving Scandinavia as an island (Kukla et al., 2002).
Mean temperature peaked in the first 3000 years of the Eemian, while precipitation remained relatively low. The Eemian then progressed from a seasonal continental climate to a more oceanic climate in Europe with higher precipitation. This oceanic climate may have been made more pronounced due to the connection between the Arctic and Atlantic oceans via the Baltic / Eemian Sea that opened up as global sea-level rose in the early Eemian (Zagwijn, 1996).
In Arctic Canada, the mean summer temperature was estimated to be 4 to 5oC higher than today and had vegetation similar to modern southwest Greenland (Frechette et al., 2006); Kienast et al. (2007), produced a pollen based climate reconstruction from the East Siberian Arctic and found the mean temperature of the warmest month during the warmest interval of the Last Interglacial was around 9 to 14.5oC, compared to 2.8oC today. These reconstructions also suggest that mean annual precipitation was up to 50 mm higher than today. In Alaska, summer temperatures were 1 to 2oC higher than today, but winters were 1 to 3oC cooler; in Greenland, summers were up to 5oC warmer than present. Animal fossils identified in a variety of localities around North America indicate a warmer climate than present and provide consistency with Last Interglacial climatic reconstructions from Europe, Japan and Australia (Muhs et al., 2002). Though there were significantly higher temperatures in many regions, it seems unlikely that average global temperatures were higher than 1oC compared to today (Hansen & Sato, 2011)
Overall, the early Eemian climate in the North and Western Europe was characterised as being predominantly oceanic i.e. with relatively cool summers, comparatively warm winters and reliable precipitation, with a modern Mediterranean climate in Southern Europe, while other regions of the Northern Hemisphere maintained a more seasonal, continental climate with warmer summers and colder winters (Brewer et al., 2008; CAPE-Last Interglacial Project Members, 2006).
So here we have a warmer, wetter world than today's, with higher sea-levels, smaller ice-sheets, but a level of atmospheric CO2 much lower than today. Does this mean that increasing levels of CO2 do not cause global warming after all? Climate always changes and we have nothing to worry about, right? We'll explore the answers to these questions and more in this continuing series of posts.
This is Part 1 of an anticipated five part series of posts on the Last Interglacial climate. Next post in the series: The Last Interglacial - Why was it so warm?