Lessons from the deep-past
Whilst marine biologists
are coming to recognise the value of historical and archaeological data “the
contribution of archaeology to the present debate over the future of the planet
is just not discussed in most publications”. (Wickham-Jones 2010, 3). This is an oversight, as ancient data offer a
way to mitigate against the Shifting Baseline Syndrome of modern evidence and
therefore help to build a stronger case.
For instance, the Copenhagen Diagnosis (please read this) which examines the evidence
for global temperature rise, the melting of land and sea ice (glaciers,
ice-caps, ice sheets) and sea level rise is based almost exclusively on recent
data, obtained since 1980. This may seem like a long time-frame but climate is
not static and change is to be expected over the short-term.
Admittedly there is a
section in the volume entitled ‘lessons from the past’ but even here the
time-frame is limited to the last two thousand years. Given that the world is
currently estimated to be 4.54 billion years (± 0.05), there is considerable
scope to examine climatic data that would provide a secure baseline to make substantiated
claims about the aberrant nature of present day climate.
This is where evidence
from archaeology and paleoenvironmental reconstruction are particularly
important as they provide empirical data about long-term changes in global
climate and temperature.
Past conditions are
reconstructed from a variety of proxy data. These include historical
documents, together with natural archives of climate-sensitive phenomena, such
as the growth or retreat of glaciers, tree rings, corals, sediments and ice
cores. In general, the proxy data record becomes more sparse and more imprecise
the further back in time we go. Nevertheless, it has proved possible to produce
a reasonably reliable reconstruction of how global temperature has varied
throughout most of the Earth's history
For example, dendroclimatology
depends on the fact that trees in many parts of the world experience an annual
growth cycle. Each year's growth (the thickness and/or density of a ring)
depends on the local temperature and moisture conditions, creating a unique
record that can then be matched with overlapping records from other trees to
produce longer time series. Annual records typically go back 500 to 700 years.
In a few cases, the preservation of fossil trees has allowed continuous records
from 11 000 years ago to the present to be constructed.
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In a similar way, cyclical
responses lead to annual banding in corals, which can provide information about
sea-surface temperatures, sea level and other ocean conditions – typically back
to some 400 years ago.
Layered sediments on lake
and ocean floors are another rich source. The types of pollen trapped in lake
sediments reveal shifting patterns of vegetation, and thus indirect information
about temperature and moisture conditions. Records can go back some 100 000
years. In marine sediments, analysis of microfossils can provide data on
seawater temperature and salinity (salt content), atmospheric CO2
and ocean circulation.
Less common deposits of
coarse debris can point to the break up of ice sheets and the release of
detritus from melting icebergs. Marine sediments provide information from time
periods ranging from 20 000 years to 180 million years ago.
Finally, long ice cores
drilled out of the Greenland and Antarctic ice sheets yield a wealth of
information. For example, past temperatures can be determined by oxygen
isotope analysis . 99% of the oxygen on Earth is the isotope 16O;
most of the rest is 18O. Because water molecules containing the
different isotopes (i.e. H2 16O and H2 18O)
have slightly different physical properties, it turns out that the 18O/16O
ratio in ice locked up on land is affected by the ambient temperature at the
time when the ice formed. Thus, fluctuations in the oxygen isotope ratio in an
ice core provide a proxy for temperature changes back through time (see below)
Temperature changes over
the past 400 000 years reconstructed from the Vostok ice core, the longest
continuous ice-core record to date.
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This record tells us that
the Earth entered into the most recent comparatively cold period of its history
(known as the Pleistocene Ice Age) around 2.6 million years ago. Drilled in
Antarctica, the Vostok ice core provides a temperature record that goes back
several hundreds of thousands of years. Beyond about 10 000 years ago, it tells
a story of an unstable climate oscillating between short warm interglacial
periods and longer cold glacial periods about every 100 000 years –
with global temperatures varying by as much as 5 to 8 °C – interspersed by many
more short-term fluctuations.
However, the “long-term
hemispheric trend is best described as a modest and irregular cooling from AD
1000 to around 1850–1900, followed by an abrupt 20th century warming” (IPCC,
2001a).
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