The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 168

the megafauna extinctions we know about seem to have been a combination of both
climate change and human impact, even if some species were more affected by one
factor than the other and, indeed, that the strength of these factors changed with time
up and into the Holocene, beginning 10 000 years ago. Since then, as previously
mentioned, the climate was comparably stable within a degree or so of present-day
temperatures. Nonetheless, during this time there were periods when the climate was
warmer and some when it was cooler than the Holocene mean.
The time from 8000 to 5000 years ago marks the aforementioned period known as
the Holocene climatic maximum (see also Figure 5.3). In Britain trees grew on land
180-300 m higher than the present-day tree line, and species such as lime ( Tilia spp.)
and elm ( Ulmus spp.) were more common than today. Trees also grew in the Orkney
Islands, Faroe Islands and Iceland. Winters then were probably far milder than today
(Lamb, 1965).
However, there is some ambiguity in the climate record as some parts of the Earth
appear to have cooled for part of the climate maximum period, especially in high
latitudes (Antarctica, Greenland and eastern Canada). One suggestion is that this
high-latitude cooling decreased precipitation and reduced terrestrial biomass. The
carbon from this biomass then entered the atmosphere and indeed a small increase
of 10 ppm in atmospheric carbon dioxide is detected between 7000 and 5000 years
ago. (However, this is small by late-20th-century increases of more than 60 ppm over
the century.) The Holocene climatic maximum increase in greenhouse forcing may
in part account for the climate maximum and this period of time may represent a
redistribution of carbon between various pools (Steig, 1999). Such problems are the
stuff of some current research.
Palaeoclimatic records for the last 4500 years generally indicate that temperatures
were lower than the Holocene climatic maximum. There was a general cooling of
1-2 ◦ C, known as the Iron Age neoglaciation, which took place between 4500 and
2500 years ago.
There was then a return to warmer conditions, but not nearly as warm as the
Holocene climatic maximum, around the beginnings of the Roman Empire. Changes
of just a degree, or at most around two, mark the nature of much of the climatic changes
seen within the Holocene interglacial. (Remember that regional warming and cooling
can be greater than the global average change. Furthermore, a comparatively small
global change in temperature might become manifest in greater or less seasonality.)
A return to a cooler climate took place 1500-1000 years ago ( 500-1000), which

coincided with the historical period in Europe known as the Dark Ages. This was
followed by the medieval climatic anomaly (MCA) or medieval warm period (MWP;
broadly around

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900-1300). The MCA is also sometimes called the medieval
climatic optimum but human ecologists and environmental scientists are wary of this
term as it implies that this time was somehow optimal. During the MCA European
temperatures reached some of the warmest levels for the last 4000 years, since the
Holocene climatic maximum. Although not as warm as the Holocene maximum was
for Europe, the MCA saw Europe roughly a degree warmer than in the last quarter
of the 20th century. Estimates for continental Europe's climate are mainly 1.0-1.4 ◦ C,
and for Greenland 2-4 ◦ C, warmer during the MCA compared to the end of the 20th
century.

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