Rapid climate changes in the late Pleistocene

The following consist of two parts. The first is an appreciation by E. Linacre of a recently published book Climate Process and Change by E. Bryant (1), and the second part is his rejoinder. National Public Radio (USA) also covered the topic, on 30 October 1997. Listen here.

Important conclusions on climates within the past 200,000 years can be reached from analysis of ice cores from Greenland and Antarctica (1). For instance, annual layering of the ice indicates age, in the same way that tree rings do (Section 15.3). The amount of chloride within the layers indicates the temperature, since the salt comes from sea spray blown by the stronger winds of colder periods. Comparable inferences may be made from the variations with depth of calcium, sulphates and nitrogen oxides, and also from the oxygen-isotope ratio (Note 15.L).

The evidence shows that (warm) interglacial periods arise within a few hundred years, but a glacial period takes much longer to form. The Last Interglacial involved three warm periods between 133 kBP and 70 kBP, separated by two abrupt interruptions of coldness, each lasting two to six thousand years (2-6 ka).

In the most recent Ice Age (sometimes called the Wurm glacial) there were 22 warm events, called ‘interstadials’, at the beginning and end of which there could be regional-mean temperature changes of up to 7 K within a decade. Such changes appear especially in the ice-core record of the last part of the Wurm glacial, from 34 - 17 kBP, when there were six major ‘Dansgaard-Oeschger oscillations’, each lasting about 1,000 years. At each flip from one regime to another or flop back again, temperatures in Greenland would alter by 7 K, sometimes within a year, the snowfall varied by a factor of two and atmospheric dustiness a hundred-fold, showing huge changes of wind strength. Supporting evidence for these flip-flops between alternative patterns of global circulation comes from cores of ocean sediment, which indicate parallel fluctuations of sea-surface temperature. The Wurm ended within about 3 decades, at around 11.5 kBP, when temperatures rose suddenly by 7 K. Since then, the climate of the Holocene (Interglacial) has been relatively stable.

One concludes, provisionally, that large changes of climate can occur naturally, indicating a surprisingly tender, temporary stability in either glacial or interglacial conditions. Rapid swings between two (or more?) tenuous states of equilibrium during the Pleistocene are well documented. The implication is that fairly modest changes induced by human society may be enough to trigger another swing.

PART 2, by Associate Professor Edward Bryant, University of Wollongong, New South Wales, Australia

I believe that …

1. Holocene climate compared to a Glacial one is more stable and less variable.
2. Small changes that have occurred during the Holocene have occurred rapidly and have had an impact on society. For example the descent into the Little Ice Age and Medieval Climatic deterioration of the 13th century were swift.
3. There are sensitive regions where climate can change quickly and where changes have a regional if not more widespread effect. One of these regions is the North Atlantic Ocean, in particular the area near southern Greenland.
4. Human effects are unlikely to perturb Holocene climate as dramatically as natural effects.
5. Human effects on climate include urban heat islands, sulphate aerosol pollution, dust, fossil fuel and biomass burning.
6. Human effects on climate will be greatest in certain regions, especially urban areas (which invariably will experience a warming trend), and will result in very site-specific changes.
7. These localised changes will be greater and have a more dramatic effect on society than global changes.
8. Cooling climate impacts on society more than does warming.
9. The globalised industrial revolution that our society has lived through during the past two centuries has never been road-tested for a cooling climate.

(1) Bryant, E. 1997. Climate Process and Change (Cambridge Univ. Press), 802pp.