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 These include historical documents, together with natural archives of climate-sensitive phenomena, such as the growth or retreat of glaciers, tree rings, corals, sediments and ice cores.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.

By contrast, global temperatures over the last 10 000 years or so seem to have been much less variable, fluctuating by little more than one or two degrees. In short, the interglacial period in which we live, known as the Holocene, appears (on available evidence) to have provided the longest period of relatively stable global climate for at least 400 000 years. It is almost certainly no coincidence that this is also when many human societies developed agriculture and when the beginnings of modern civilisations occurred. We now shift the focus to the more recent past – the period during which human population growth and the coming of the industrial age began to make their mark on the composition of the atmosphere.

Viewed against the deeper-time data, the evidence for global warming is compelling. Figure b shows how modern instrumental records (shown in red) map onto the proxy records (blue). The grey region is the 95% confidence range in the annual data. Note that the uncertainty is much greater than for the period covered by the instrumental record, and increases further back in time.

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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