What was done
A 3,000-year climatic history was derived for a high-altitude region on Mount Kenya in East Africa.  The proxy temperature record - from 1200-4200 years before present - was obtained via oxygen isotope analysis of biogenic opal extracted from a 180-cm sediment core retrieved from a shallow lake.

What was learned
The authors noted numerous small fluctuations in isotopic oxygen over the period of record.  Most notable, however, was a significant warming that occurred between 2,300 and 2,000 years ago, when temperatures rose about 4°C in just three centuries.  This finding is consistent with other proxy temperature records of the Mount Kenya region and its surroundings.

What it means
The authors' work demonstrates that sedimentary records of the oxygen isotope characteristics of biogenic opal can be used to reconstruct regional climate histories.  It also shows that large fluctuations in climate can occur over relatively short periods of time.  The importance of this latter point is especially significant in light of the current debate over carbon dioxide and global change, where measured temperatures over the last 100 years have indicated a global warming of about 0.5°C.  Many people cite this warming as a validation of general circulation model predictions of a CO2-induced greenhouse effect.  However, as shown in this article, over a period of time when carbon dioxide concentrations were relatively constant (2300-2000 years ago), temperatures warmed considerably more than they have over the past century.  Consequently, just because air temperatures and atmospheric CO2 have both increased over the last hundred years, it does not mean that the change in one must be causing the change in the other, as each has changed independently of the other in the past, as this one example demonstrates.