Interglacial periods that occurred during Pleistocene times have been recently put under investigation, in order to better understand our present and future climates. In fact, paleoclimatic interpretations often depends on observations drawn from the study of modern/historical processes. In order to better estimate the ”natural range” of climatically important mechanisms, it seems crucial to attempt detailed comparisons of the present interglacial (i.e., the Holocene) with previous warm periods of the Quaternary, such as Marine isotopic stage 11 (hereafter “MIS 11” for the sake of brevity). Similar orbital configurations and comparable atmospheric greenhouse gas concentrations (considering only the period before the beginning of the Industrial Revolution, i.e. 1800 AD ca.) have led to the suggestion that MIS 11 is a suitable, possibly the best, geological analogue for the natural development of Holocene and future climate. Another candidate was MIS 5, but several characteristics do not fit the present conditions. MIS 11 spans from 420 to 360 ka , and represents the longest and warmest interglacial interval of the last 500 kyr. In fact, it shows the highest-amplitude deglacial warming in the last 5 Myr and possibly lasted twice the other interglacial stages. MIS 11 is characterized by overall warm sea-surface temperatures in high latitudes, strong thermohaline circulation, unusual blooms of calcareous plankton in high latitudes, higher sea level than the present, coral reef expansion resulting in enlarged accumulation of neritic carbonates, and overall poor pelagic carbonate preservation and strong dissolution in certain areas.
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As stated above, MIS 11 is considered the warmest interglacial period of the last 500 kyr [1][2]. Nevertheless, some issues concern the lack of evidence for the trends and degree of warming during this interval. Recent isotopic and planktonic faunal data sets from high-accumulation rate marine successions in the North and South Atlantic indicate that MIS 11 was not warmer, but even slightly cooler than the Holocene. According to these data, the warmest interglacial period was likely MIS 5, although it was shorter.
Carbon dioxide concentration during MIS 11 was possibly similar to that documented in the pre-industrial period, but not especially high when compared to other interglacial periods (for example, CO2 concentration was probably higher during MIS 9[3]. In addition, a peculiar feature of MIS 11 is that an early CO2 peak, usually associated to the deglaciation in response to increasing temperatures, is not detected. Seemingly, the long-lasting interglacial conditions that are documented during MIS 11 depend on the peculiar interplay between CO2 concentration and insolation. In fact, during periods of both eccentricity and precession minima, even small variations in total insolation might lead the control of climate to greenhouse gasses, in particular CO2.
In shallow-water environments, the development of several major reef systems (such as the Great Barrier Reef) accompanied increased reef carbonate production. Calcium carbonate production peaked in subpolar and subtropical oceans, reflecting a shift in plankton ecology from diatoms to calcareous plankton due to changes in seawater temperatures, which were apparently warmer at low latitudes. The increased production of carbonate in either continental shelves and mid-latitude open-ocean environments may partly explain the high rates of carbonate sediments dissolution during MIS 11 throughout ocean basins, such as the Indian and Pacific. Indeed, increases in regional carbonate productivity can be only explained by increased carbonate dissolution in other (source) areas. Another explanation for the presence of barrier-reef tracts at low latitudes during MIS 11 is that tropical continental shelves were (at least partly) flooded in response to a major marine transgression (see below).
Beach deposits in Alaska, Bermuda and the Bahamas, as well as uplifted reef terraces in Indonesia, suggest that global sea level reached as much as twenty metres above the present [4][5][6]. During MIS 11, δ18O records show isotopic depletions that are consistent with a sea-level highstand, but temperature effect cannot be confidently disentangled from glacioeustasy. Moreover, the collapse of at least one major ice sheet has to be inferred in order to produce similar high sea-levels, nevertheless, the stability of these ice sheets is one of the main questions in climate-change research: in fact, controversial geologic evidences suggest that present-day polar ice sheets might have been disrupted (or drastically shrunk) during previous Pleistocene interglacials. The increased sea level requires reduction in modern polar ice sheets and is consistent with the interpretation that both the West Antarctica and the Greenland ice sheets were absent, or at least greatly reduced, during MIS 11. However, similar conditions would have led to much lighter δ18O values than the present, in contrast to what observed in oxygen isotope records worldwide.
Unlike most other interglacials of the late Quaternary, MIS 11 cannot be explained and modelled solely within the context of Milankovitch forcing mechanisms. According to various studies, the MIS 11 interglacial period was longer than the other interglacial stages; moreover, the sustained interglacial warmth lasted as long as it did, because the eccentricity was low and the amplitude of the precessional cycle diminished, resulting in several fewer cold substages during this period and also induced an abrupt climate change at MIS 12–11 transition, the most intense of the past 500 kyr. Noteworthy, MIS 11 developed just after one of the most “heavy” Pleistocene δ18O glacials (MIS 12). According to some Authors, MIS 12 is likely to represent a “minimum” within the 400-kyr cyclicity (which is apparently “stretched” into ca. 500-kyr cycles in the Pleistocene), same as the MIS 24/MIS 22 complex (ca. 900 ka; Wang et al., 2004). In support to this inference is the fact that these dramatic glacial intervals are coincident with periods of major climate reorganisation, namely the “Mid-Brunhes Event” (Jansen et al., 1986) and the “Mid-Pleistocene Revolution” (Berger & Jansen, 1994), respectively. Considering the variability in the astronomically-driven insolation, MIS 11 is the interval in which insolation is highly correlated with predicted near future situation. Using the 2-D Northern Hemisphere climate model to simulate climate evolution over MIS 11, MIS 5 and the future, it appears that climatic feature and length of MIS 11 are very similar to the present-future interglacial. This consideration may lead to the conclusion that actual interglacial period (begun 10 kyr) will continue for approximately 20-25 kyr.