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Dickensian Winters in New Zealand

Recent determinations of past surface temperatures for the last few centuries in New Zealand have been made using a relatively new technique, the borehole temperature method.

From about 1780 to 1880, the climate in the northern hemisphere was particularly cold -- winters in Britain at this time are sometimes referred to as the "Dickensian winters", as they were described by Charles Dickens in his novels. During winters at this time, the River Thames sometimes froze over and people were able to skate on it. In Holland, Dutch artists also captured these winters on canvas. Average temperatures were about 0.8^oC colder than now, as shown on the graph at right. %IMGR|1079|A=%The information used to produce this plot comes from temperature measurements as well as from indirect scientific methods and written and oral sources.

Until now, in New Zealand the only evidence for Dickensian winters has been from glacier studies which indicate that a significant cold period ended in the late 1800s. Meteorological records in New Zealand do not cover the period, going back only to 1850. They show that warming has occurred over the last century and a half -- an increase in temperatures of about 0.8 ^oC since 1850. However, recent New Zealand results from the borehole temperature method, which is used to obtain past surface temperatures, show that the Dickensian winters did indeed occur in New Zealand.

The borehole temperature method is based on the characteristics of thermal diffusion in a solid, such as the earth, in which heat takes a finite time to diffuse within the solid from one region to another region. For example, ground surface temperature fluctuations propagate downwards into the earth as a temperature wave. Because the process is slow, the near-surface earth contains a history of the most recent surface temperatures; deeper underground the earth contains the history of surface temperatures from further back in time.

It is thus possible to deduce the record of climatic changes for the past three or four centuries from the analysis of temperatures measured within vertical boreholes. This method has been developed and applied extensively in the northern hemisphere, where the results show Dickensian winters very clearly, and has recently been applied in New Zealand.

The results for New Zealand show that Dickensian winters, a broad, distinct minimum in temperatures about two centuries ago, have occurred at four sites in lowland and coastal environments, two in the South Island and two in the North Island. These sites have a considerable geographic spread.

The amount of warming over the last century can also be determined from the measurements. This averages 0.9^oC for the two South Island sites, both of which are located in natural vegetation, and about 3^oC at the two North Island sites, both of which have been deforested this century. The amount of warming at the two South Island sites is attributed to warming of the New Zealand climate, and agrees with NZ meteorological records. However the North Island sites show an additional ground temperature rise due to the effects of deforestation.

Peter Whitehead works for GNS