UvA researchers show abrupt climatological effects of a solar minimum

7 May 2012

In a recent publication in 'Nature Geoscience', a team of researchers including Bas van Geel of the Institute for Biodiversity and Ecosystem Dynamics (IBED) show new evidence pointing to an important role of the variable Sun in regulating climate change.

A change in climate (cooler, wetter) around 850 B.C. is one of the phenomena that UvA biologist Bas van Geel has been studying in the last 40 years. Studies of fossil pollen, and remains of mosses and other plants allowed to ‘reconstruct’ the effects of climate. However, the primary focus of the research was to determine the cause of the climate change in question. In previous publications, Van Geel and co-workers identified an abrupt reduction in solar activity around 850 B.C. as the climate forcing factor.

An international team of scientists, including Van Geel, took further steps towards investigating solar forcing of climate change. In a recent article in the journal Nature Geoscience they published the results of a study of a sediment core from a crater lake in Germany. The sediment layers at the bottom of the lake were deposited in annual layers, i.e. each year is recognisable as a separate layer. The effects of the climate change around 850 B.C. were studied in detail. The abrupt climate shift coincided with a sudden increase in thickness of the annual sediment layers. The sudden reduction in solar activity must have led to a change in wind patterns. The resulting increase in wind subsequently caused mixing of the lake water in such a way that nutrients for diatoms were available for longer time periods than before. This led to larger amounts of diatom remnants in the sediment and thus thicker annual layers. A climate model that was used to model the collected data showed a clear relationship between reduced solar activity and the change in climate around 850 B.C.

The new data are of great relevance for the ongoing discussion about the current and future role of the sun in driving climate change. So far the opinion of the Intergovernmental Panel on Climate Change (IPCC) is that changes in solar activity are of minor importance compared to the effects of the increase in CO2 emission as a result of human activity. However, this opinion is based on research where only direct changes in solar radiation were considered as the influence of the Sun. Such changes in radiation are indeed small, but the results of the current study unmistakably point towards atmospheric mechanisms that amplify the effects of relatively small changes in solar activity. Present day observations show a decline in solar activity over the last few years that will presumably have an effect on the present day climate and the climate of the near future. The current study would suggest that in the next decades we may experience ‘global cooling’ rather than ‘global warming’.

Published by  IBED