An introduction to climate change - Climate Change: Biological and Human Aspects - page 27

clearance of tropical rain forests also had yet to begin. During this time atmospheric
carbon dioxide only changed a little (see Figure 1.5) and these small changes (if
not anthropogenic) may therefore have been due to global ecosystem responses to
temperature. They estimated that the sensitivity of the global carbon cycle was around
7.7 ppm CO 2 per degree Celsius (ppm CO 2 ◦ C − 1 ) warming with a likely range of 1.7-
21.4 ppm CO 2 ◦ C − 1 warming. This is a less than previous estimates of around 40 ppm
CO 2 ◦ C − 1 warming. Although we cannot discount the earlier estimates completely
until this more recent work has been verified in other ways, it does suggest that
amplification of other warming mechanisms is important (possibly ones such as
those, among others, given in Figure 1.8).
So much for the broad picture of interacting feedback cycles and flips between
semi-stable states. Not surprisingly, climatologists continually monitor research into
these positive- and negative-feedback systems. Indeed, climate-modelling research
has received huge investments in recent years, especially since the mid-1980s, when
computer technology became sufficiently sophisticated to accommodate models of
adequate complexity to provide tolerably useful (or at least interesting) output. The
models are good in that they do broadly reflect the global climate, but they (currently)
lack detail, both spatially and thermally, and there have been problems with some
outputs that simply do not tie up with what we know (see Chapter 5). For example, in
the 1990s global models were not particularly good at portraying climates at high lat-
itudes (which are warming far faster than models predict), whereas the models of the
early 21st century include more developed biological components but still have a long
way to go. Another more recent example is that ocean dynamics surprisingly do not
seem necessary to model El Nino event timing, although they do seem relevant for El
Ni no strength (Clement et al., 2011). Furthermore, most climate models erroneously
predict the existence of an Intertropical Convergence Zone (ITCZ) - a band near the
equator where the prevailing winds from the two hemispheres converge - in the South
Pacific, in addition to the real one observed north of the Equator: a problem known as
double ITCZ bias. However, computer models are continually getting better as they
are built with greater resolution (both vertically and horizontally) and incorporate
more of the features and processes operating on Earth. Indeed, as previously men-
tioned, since the late 1990s programmers have included biological processes in their
models, so continuing the trend of being able to increasingly match model outputs
with expectations based on reality. Biologists and geologists have much to contribute,
for what has taken place climatically is frequently recorded biologically and preserved
geologically. Not only do different species live under different climatic regimens but
species are affected by climate and these influences can be laid down in ways that are
long-lasting (for example, tree rings, to which we will return in Chapter 2).
Prior to the 1980s we had such a poor understanding of the way that the global
climate operates that there was great uncertainty as to whether the Earth was warming
or cooling. Indeed, when the first ice cores were analysed in the 1970s it was realised
that the last glacial period had lasted for roughly 100 000 years, whereas the previous
interglacial was just 10 000 years long. Furthermore, the change between the two was
sudden and not gradual. Consequently, in the 1970s, given that our current interglacial
(the Holocene; the time since the end of the last glacial to the present) has already
lasted about 11 700 years, there was for a while genuine concern that a new glacial was