The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 156

This is because of the aforementioned complexity of the Milankovitch curve, which
incorporates three orbital variables (see Chapter 1). At times (allowing for climate-
system inertia) of low Milankovitch forcing we see a glacial, and conversely with high
forcing we see an interglacial. There are a number of ways in which the three orbital
components to Milankovitch can come together to form these peaks and troughs.
Unlike the previous Eemian interglacial, the fourth past interglacial (following what
geologists call Termination V) was borne out of Milankovitch circumstances similar to
those of the current Holocene interglacial. That the Milankovitch circumstances were
different for the more recent Eemian compared to our Holocene is the likely explan-
ation for the differences in ocean circulation (the continental tectonic arrangement
in the last interglacial being barely different from today's). It also affected natural
systems. Pollen records from marine sediment off south-west Greenland indicate
important changes of the vegetation in Greenland over the past million years. Abund-
ant spruce pollen ( Picea spp.) indicates that boreal coniferous forest developed some
400 000 years ago during the height of that interglacial (de Vernal and Hillaire-Marcel,
2008). Indeed, at that time far more of Greenland was ice-free than today. There are
two reasons for this. First, four glacials ago there were fewer previous glacials dur-
ing which ice built up, and also the further back we go with Quaternary glacials
(past 2 million years) the general trend (with the odd exception) is that the glacials
were less severe. Secondly, as mentioned, the interglacial 400 000 years ago was far
longer than the intervening three up to our present Holocene interglacial.
The aforementioned Milankovitch complexities in turn possibly explain why high-
latitude palaeorecords indicate a different temperature profile for the Eemian inter-
glacial compared with the present Holocene, while lower-latitude European palaeo-
records suggest a similar climatic stability.
Importantly, the previous (Eemian) interglacial was short, of the order of 10 000
years or so. Were the Holocene interglacial truly similar, then this would suggest (in
the absence of present-day anthropogenic global warming) that we are due to enter
another glacial in the geologically very near future, as the Holocene is already 11 700
years old. (Indeed, this worried climatologists in the 1970s.) However, it transpires
that the fourth interglacial ago was longer, lasting about 25 000 years. It also differed
in its structure compared to the Eemian. The Eemian began with an extremely warm
period of a thousand years, a time far warmer than our Holocene has been to date. Our
Holocene's initial millennium was also warmer, but only marginally, compared to the
rest of the interglacial. The fourth (Termination V) interglacial ago similarly lacked
the Eemian's notably warmer initial spell and in that sense was similar to the Holocene.
However, whereas our Holocene is comparatively stable to within a degree or so, the
temperature of the fourth interglacial ago seems to have risen to a peak and then
fallen more gently. Superimposing Antarctic deuterium data from both interglacials,
one over the other, shows that the deuterium figures between the Holocene and the
fourth interglacial ago (see Figure 4.11) are closer than that between the Eemian and
the Holocene, but still are not the same. The way temperature changes within the
current interglacial is different from how it changed in the fourth interglacial ago.
Could this be due to other factors such as the gradual evolution of carbon reservoirs
over successive glacial-interglacial cycles? Or perhaps solar forcing, changes in the
Sun's output, affected the climate differently between the two interglacials? After all,
approximately 10% of changes in positive radiative forcing of our current climate