The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 157

since 1850 are estimated as being due to changes in the Sun's energy output, although
there is considerable uncertainty associated with this (IPCC, 2001).
If our Holocene interglacial is, in Milankovitch terms, similar to the one 420 000
years ago following Termination V, then the Holocene could last 25 000 years. This
would mean that, some 10 000 years in, we are not quite half way through our present
interglacial. This contrasts with palaeoclimatologists' views back in 1972, when
comparisons were made with the Eemian and they thought that all interglacials were
as short, suggesting that the next glacial was imminent (Kukla et al., 1972). However,
more recent analyses (Berger and Loutre, 2002) of future Milankovitch curves and
climate models suggest that we are indeed at least only half way through our current
interglacial, just as the latest ice-core evidence suggests. Furthermore, continuing
current carbon dioxide emissions throughout this century could conceivably mean
extending our current interglacial, possibly as much as another 50 000 years, but
almost certainly another 30 000 years, assuming depletion over the coming centuries
of all economically recoverable fossil fuels without any carbon abatement. Indeed, if
we trigger something analogous to the Initial Eocene Thermal Maximum/Palaeocene-
Eocene Thermal Maximum (IETM/PETM) then the warming might extend 100 000
years or more (see section 3.3.9). Either way, the next glacial is not now thought to
be imminent.
Why the shapes of the temperature curves of the Milankovitch-similar Holocene
and Termination V interglacials are different is a good question. The aforementioned
reasons are all likely in that the cause lies in the other variables in the system. But
there are other possibilities too. For example, the Earth's plate tectonic arrangement,
while only slightly different in the last interglacial, was more different still four
glacial-interglacial cycles ago. For instance, the Himalayas were lower 420 000 years
ago and this would have affected albedo and atmospheric circulation. Even so, these
differences over hundreds of thousands of years are paltry compared to the difference
in plate tectonics over tens of millions of years mentioned in Chapter 3. Another
difference between the Termination V interglacial and ours is that our interglacial is
four more along the series of Quaternary interglacials that have seen a broad trend
of deepening intervening glacials. So, the Termination V interglacial may not be
analogous in all ways to the Holocene, but both Milankovitch theory and ice-core
evidence suggest that their durations may be similar.
4.6.2 TheAllerød,BøllingandYoungerDryas(14600-11600yearsago)
The end of the last glacial in the northern hemisphere was marked by a series of major
events of sufficient strength to be reflected in the southern hemisphere, albeit more
weakly. This is reflected in Figure 4.12, which portrays ice-core records of the past
15 000 years. Figure 4.12a shows detail of the deuterium record from Vostok, Antarc-
tica, which reflects regional temperature change. Figure 4.12b shows detail of the
northern hemisphere's Greenland ice core showing its electrical conductivity meas-
urements (ECMs). Here conductivity of ice is improved with acid ions and decreases
when these acids are neutralised by dust. Ammonia - such as from nearby biomass
burning (water-soluble ammonia is easily absorbed by water vapour and then washed
out of the air) - will also neutralise acid, but the ammonia record in Greenland's