Present climate and biological change - Climate Change: Biological and Human Aspects - page 205

layers in cores of 13 lakes in north-eastern USA has determined how the degree of
storminess has varied over the Holocene. The results show a characteristic period of
about 3000 years and that this pattern appears consistent with long-term changes in
the Arctic Oscillation (Noren et al., 2002). Such oscillations are but one grouping of
many climatic intricacies that shape the complex detail of climate change.
Notwithstanding this complexity, or the natural vagaries and climate-related oscil-
lations, the Holocene's most recent hundred years (the 20th century) have largely
been a period of warming. Be it based on palaeoclimatic proxies (such as the high-
latitude dendrochronological records in Figure 2.1), or instrumental (satellite- or
ground-based), or a combination of both (Figure 5.2), late-20th-century warming was
unprecedented in the time since instrumentation began. Further, using palaeoproxies,
the late 20th century saw the greatest warming for 1000 years. Indeed multi-proxy
reconstructions of monthly seasonal surface temperatures for Europe back to 1500
show that the late 20th and early 21st centuries were warmer than any other time dur-
ing the past 500 years. This agrees with findings over the entire northern hemisphere.
European winters were 0.5 ◦ C warmer in the 20th century compared to the previous
four centuries. Conversely, summers have not been that much warmer, although indi-
vidual ones have had warmer spells. Finally, in the half millennium up to 2000 the
coldest European winter was that of 1708-9, which was not in contemporary times
but in the Little Ice Age (Luterbacher et al., 2004).
The 20th century's warming trend is almost certainly going to continue, which is of
tremendous concern. The Earth may be entering a climatic mode that is unprecedented
in human history. Indeed, it is likely that in the 21st century the Earth will achieve
temperatures not seen since the height of the last interglacial (140 000 years ago) and
just possibly temperatures not seen during the Quaternary (the past 2 million years).
Beyond this, further warming in the next century may result in temperatures not seen
since before the Quaternary, more than 2 million years ago. If so - and it is a real
possibility - it will transform our planet in ways that many have not yet contemplated.
The shift to this warmer world will not necessarily be smooth: it could involve 'critical
transitions', of which a profound change to the Broecker thermohaline circulation is
but one. It is because we are biological creatures that are involved in, and dependent
on, many biological systems, which in turn are affected by climate, that we need to
take note. Such climate change will certainly affect virtually everyone's lives, both
individually and on a societal level. To recognise the serious nature of this possibility
one needs only see how the more modest climatic changes have affected human
activities from the medieval climatic anomaly (MCA) to the present.
5.2 HumanchangearisingfromtheHoloceneclimate
5.2.1 Climaticimpactsonearlyhumancivilisations
Human civilisation (settlements and cities) arose early during the Holocene inter-
glacial. These settlements were small; minute by modern standards. Importantly,
sea-level rise following the last glacial continued for a few thousand years after the