The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 165

Greenland, as well as a pocket in Russia north of the Caspian Sea, the climate was as
much as 3 ◦ C warmer than at the end of the 20th century. At lower latitudes this warm
period manifested itself not so much with a warmer climate but in other ways as more
energy entered parts of the climatic system, such as the monsoon's summer circula-
tion. So, there was more precipitation in many parts of the subtropics. Consequently,
the tropics during the maximum were warmer and wetter than today. However, due
to current global warming, from the various IPCC scenarios (see section 5.3.1), it is
likely that we will reach temperatures equivalent to the Holocene climatic maximum
well before the mid-21st century and possibly as soon the end of its first quarter.
It is worth noting that in the past the Holocene climatic maximum (9000-5000
years ago) has been described as the Holocene climatic optimum. Many palaeocli-
matologists have ceased to use this term as the use of the word optimum does not
mean that the climate was in any sense climatologically or biologically optimal, but
that the time represented a thermal maximum for the pre-21st-century Holocene.
Having said this, we are still in the Holocene, and present-day global warming is
taking us beyond what was the Holocene climatic maximum, and so it may be that
terminology will again change in a decade or two.
In addition to a warmer climate, and resulting sea-level rise, the Holocene saw
humans contribute their own ecological impact. The degree these various factors of
climate, sea-level rise and humans together impacted on species varied with location
and, of course, the species in question. The resulting picture was complex. Yet,
just one of these factors was unique to the end of the last glacial and the current
interglacial and that was the spread of modern humans ( H. sapiens ). There is no single
mechanism responsible for the extinctions of every species in every location in the
last glacial and the current interglacial. As noted earlier in section 4.3, some Pliocene
large animals (megafauna) survived to quite recently in geological terms. Mammoths
( Mammuthus spp.) survived up to as recently as 4000 years ago due to isolation on
Wrangel Island. Another species, the Irish elk ( Megaloceros giganteus ), survived
to about 7000 years ago. Before 20 000 years ago the Irish elk was widespread
from Ireland to Russia (west-east) and Scandinavia to the Mediterranean (north-
south) but at the time of the LGM (18 000-22 000 years ago) they appear to have
become more restricted, probably to refugia in the shrub steppes of central Asia.
From there the Irish elk recolonised much of their lost ground as the Alpine and
Scandinavian ice sheets retreated. It is possible that the productive shrub and open
grass community necessary for the Irish elk persisted in the eastern Urals (Stuart
et al., 2004). The picture is further complicated in that some large herbivore species
(possibly including the Irish elk) not only responded to changes in their food-plant
communities but themselves affected those plant communities and plant productivity,
through nutrient redistribution and grazing. Further, by trampling tundra mosses and
stimulating grasses through grazing the woolly mammoth might have helped maintain
and even expand its own habitat (Pastor and Moen, 2004). Just as a prolonged period
of climate change (albeit vulcanism-induced) put pressure on the dinosaurs (the
extinction of which was then sealed with an asteroid impact), so climate change
appears to have put pressure on Pliocene megafauna which were finally made extinct
by human hunting; after all, the megafauna had survived a number of previous
glacial-interglacial climatic shifts.