Present climate and biological change - Climate Change: Biological and Human Aspects - page 202

the increase in global evapotranspiration seemed to cease until 2008. This change was
driven primarily by moisture limitation in the southern hemisphere, particularly in
Africa and Australia. In these regions, microwave satellite observations indicate that
soil moisture decreased from 1998 to 2008. So, increasing soil-moisture limitations on
evapotranspiration largely explain the recent decline of the global land evapotranspir-
ation trend. Whether the changing behaviour of evapotranspiration is representative
of natural climate variability or reflects a more permanent re-organisation of the land
water cycle is a key question for Earth system science.
Another question regarding the longer-term prospects for the global climate is
that of where we are in the natural glacial-interglacial cycle. Has the anthropogenic
release of greenhouse gas affected this cycle? By the 1990s two things had become
clear. First, in the northern hemisphere at least, there was a marginal cooling trend of
0.2 ◦ C between 1900. This cooling trend

was both slight and only just above the background noise of climatic variability.
Nonetheless, it might suggest that, as hinted above, in the absence of 20th-century
global warming, the beginning of the end of the current Holocene interglacial would
be upon us soon. (The fear in the 1970s had been that there would be global cooling
into a glacial, and not global warming.) However, as noted above, this long-term
cooling trend ended with the 20th-century warming. The second thing that became
clear in the 1990s was that both glacials and interglacials are complex, with vagaries
of their own (such as Bond cycles and associated Heinrich events within a glacial).
Furthermore, as noted in section 4.6.1, glacials and interglacials have their own
individual characteristics. Just because the Eemian interglacial (which in some texts
is referred to as the Ipswichian or Sangamon) 130 000 years ago was roughly similar
in length to its predecessor interglacial 220 000 years ago, this does not necessarily
mean that our current Holocene interglacial will be a similar length.
As noted in Chapter 1, the pacemaker of glacials and interglacials is the Mil-
ankovitch solar radiation curve. Again (see Chapter 1) this curve is made up of three
cycles of varying lengths and so it is likely that, unless the Milankovitch circum-
stances are similar, each glacial and interglacial will have its own characteristics.
Indeed, as discussed in the last chapter, the closest of the interglacials similar to our
own in a Milankovitch sense was the one following Termination V (five glacials ago),
called the Hoxnian interglacial, which began 425 000 years ago (see Figure 4.11). It
is this interglacial, and not the Eemian, that should be used as an analogue for our
current Holocene interglacial (Augustin et al., 2004).
Projections into the future using Milankovitch curves (Berger and Loutre, 2002)
suggest that, unlike the Eemian, the Holocene interglacial will be exceptionally long.
Looking back at the Waalian interglacial (sometimes called the Pastonian) following
Termination V, we can also see from the Antarctic ice-core record that that interglacial
lasted some 30 000 years, from 425 000 to 395 000 years ago (Augustin et al., 2004).
In other words, today, some 10 000 years into our current Holocene interglacial,
without global warming it is unlikely that we would soon see a return to full glacial
conditions. Without global warming it is thought that we might have an overall cooling
over the next 25 000 years, only after which would there then follow a sharper return
to glacial conditions. However, assuming that we will continue to add substantial
amounts of carbon dioxide to the atmosphere over the next couple of centuries, we

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