Millennial-scale solar variation may be affecting climate

One of the lessons that climatologists have learned in the past few decades is that the earth’s climate varies on many different time scales.  We are starting to understand the origins of some of these cycles, but the patterns are complex and it is no easy task.  There is a long way to go before we will have a complete understanding of all of the factors that drive the earth's climate.

It has been shown with some clarity that the relatively long-term cycles – in the order of tens of thousands to hundreds of thousands of years – are related to the earth’s orbital and rotational variations.  These are the Milankovitch cycles; namely the angle of tilt of the rotational axis, the direction of that tilt relative to the orbit around the sun, and the eccentricity of the earth’s orbit around the sun.  These cycles played a major role in the many advances and retreats of the Pleistocene ice sheets.

The sun itself is subject to fluctuations in output – on several different time scales – but until recently there has been no clear link between these variations and the earth’s climate.  Most of us are familiar with the 11 and 22 year sun-spot cycles, however it is evident that the output of the sun fluctuates on several other time scales as well, one of which has a period of close to 1500 years.

Oceanographers from the Lamont Doherty Earth Observatory in New York have recently presented data which link the 1500-year solar cycles to climate fluctuation over the 12,000 years (Bond et al., 2001, see also Kerr, 2001). Of course we don't have direct data on either of these phenomena for that period of time, and for each we must rely on proxy data - changes in other variables which respond to the sun's output and the climate.

The proxy data used for climate in the study of Bond et al. is the composition of North Atlantic sea-floor sediments.  During cold periods glaciers in Canada and Greenland advance significantly, producing large numbers of icebergs that float across the northern Atlantic shedding rocky debris onto the sea floor as they melt.  A 1500-year cycle is evident from this data set.

The proxy data for solar irradiance come from the ¹⁴C contents of tree wood.  Solar radiance is correlated with solar wind output, and changes in the solar wind affect the production of ¹⁴C.  When the sun and its solar wind are relatively weak, the solar wind is less effective at deflecting the cosmic radiation that leads to the production of ¹⁴C from ¹⁴N in the upper atmosphere, so ¹⁴C levels increase.

A high ¹⁴C content in tree wood is an indication of a weak sun, and Bond et al. have shown a strong correlation between the ¹⁴C data and the sea-floor sediment data.

The 1500 year solar variation can also be tied to the Little Ice Age, a period of relatively cold temperatures, extending from approximately 1400 to nearly 1900.  The model therefore predicts continued warming for the next two hundred years, and a return to a cold period around the year 3100.

References

G. Bond, S. Hoffmann, R. Lotti-Bond, J. Beer, R. Muscheler, M. Evans, B. Kromer, W. Showers, I. Hajdes, G. Bonani, Persistent Solar Influence on North Atlantic Climate During the Holocene, Science (Nov, 2001)

R. Kerr, A variable sun paces millennial climate, Science, Vol. 294, p. 1442-1443, (Nov, 2001)

Steven Earle, 2001. Return to Earth Science News