Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. A recent article from the Past Interglacials Working Group of PAGES in Reviews of Geophysics, Interglacials of the last 800,000 years, identifies 11 interglacials, summarises the common features and differences between them, and highlights how particular interglacials can enlighten us about the climate system. They also describe the likely extent of the present interglacial both in the absence and presence of human influence on the climate system. AGU asked the authors of the article to highlight the important results that have emerged from their research and some of the important questions that remain.
Why is this topic timely? What recent advances in particular are leading to a new understanding or synthesis?
We live in an interglacial: Interglacials represent the warm end of the conditions Earth has experienced in the 800,000 year period we considered. Because of human actions, we expect that the world is going, by the end of the century, to be considerably warmer than it has been in the last few thousand years. While the interglacials don’t provide any perfect analogues for that, they do offer a range of examples, providing insights into the impacts of warmer than present conditions in certain regions of Earth. Learning from them has become a practical possibility because huge progress has been made in the last few years in producing well-resolved quantitative datasets covering multiple interglacials in marine sediment cores, ice cores, and terrestrial sequences. The ice core record provides crucial quantification of the climate forcing (from greenhouse gases in particular), while there are now sufficient records of sea surface temperatures that we can, at least semi-quantitatively, recognise the spatial pattern of warmth in each interglacial. Finally, it is only in the last few years that it has become possible to run various classes of climate models with boundary conditions representing a range of interglacial conditions.
Thus we now have a strong motivation to understand the properties of interglacials as a whole and individually and, crucially, the modeling tools and validating data that enable us to do so quantitatively. On this basis, we felt it was timely to summarise what is known, both as a stimulus to further data collection and as a resource for deriving the principles and processes determining the diversity of interglacials.
What are the societal implications of this new synthesis or implications for understanding our current interglacial and anthropogenic warming?
Firstly it’s important to emphasise that there is no analogue for the anthropogenic warming we are currently experiencing. In the 800,000 years we studied, the concentration of CO2 never exceeded 300 ppm until the 20th century, while it recently climbed above 400 ppm and is still rising by 2–3 ppm per year. However, while the causes of warmth have no analogue in our record, we do see times when parts of the world were warmer than today.
Our synthesis highlighted two periods of particular interest in this regard. Marine isotope stage 11, around 400,000 years ago, was relatively warm and particularly long (of order 30,000 years); it therefore provides an opportunity to see what happens to sectors such as ice sheets and terrestrial ecosystems subjected to an extended period of warmth. The last interglacial, running from about 130,000 to 115,000 years ago, seems to be the warmest in the last 800,000 years in many parts of the world. In particular, both the Arctic and Antarctic appear to have experienced periods a few degrees warmer than present; the best evidence is that sea level was 6–9 m higher than present, so this suggests that such conditions, if they last for a few thousand years, take a significant bite out of one or both of the Greenland and Antarctic ice sheets. The polar temperatures experienced in the last interglacial are well within the range of those predicted for 2100 under scenarios without strong mitigation of emissions, and that warmth is expected to persist unless CO2 is actively removed from the atmosphere. Thus the last interglacial acts as a rather solid data-based reminder that the commitment to sea level rise from continued warming could be very scary.
What are the major unsolved or unresolved questions, and where are additional data or modeling efforts needed?
Although we have documented the occurrence of interglacials, there is not yet a convincing account of why and when interglacials occur. We know that it is linked to astronomical changes (in Earth’s orbit and axial tilt). However, not every favourable set of astronomical conditions leads to an interglacial, and the process of interglacial onset is complex, apparently involving synergistic changes in greenhouse gas concentrations, ocean circulation, and ice sheets. Understanding this requires, as always, more data: in particular improvements in the chronologies of different archives are needed if we are to reconstruct the sequence of events across the start of any interglacial before the present one.
While good progress has been made in assembling records of sea surface temperatures, there is still a paucity of long terrestrial records. As a result, our knowledge of how continental climate changed and how terrestrial ecosystems responded in each interglacial remains sparse. Constructing a good suite of such records is clearly a priority.
One other unresolved question I would highlight is that of the trend in CO2 during interglacials. In an influential series of papers, including a recent paper in Reviews of Geophysics, Bill Ruddiman has proposed that the slow increase in CO2 that occurred during the last 8000 years was unusual, resulted from an early anthropogenic influence, and perhaps even staved off the end of our interglacial and the descent into a glacial period. This is obviously an intriguing idea. There are however other interglacial periods during which CO2 increased naturally and no compelling argument as to what determines the trend in each interglacial. Carbon cycle models can reproduce both increases and decreases under different assumptions. We concluded that the extent to which human actions have influenced CO2 concentrations over the last few millennia remains open; this needs to be resolved.
—Mark Moldwin, Editor in Chief, Reviews of Geophysics; email: [email protected]