Temperature response in the Altai lags solar forcing

An ice core drilled at the Belukha glacier in the Siberian Altai by a Swiss-Russian research team under the leadership of the Paul Scherrer Institute (PSI) in 2001 has now provided new findings in climate research. Oxygen isotopes in the ice were used to reconstruct the temperatures in the Altai over the past 750 years. The scientists discovered a strong link between regional temperatures and the solar activity in the period 1250–1850, concluding that the sun was an important driver of preindustrial temperature changes in the Altai. The observation that the reconstructed temperatures followed the solar forcing with a delay of 10 to 30 years is particularly interesting. The strong rise in temperature in the Altai between 1850 and 2000 can not be explained by solar activity changes, but rather by the increased concentration of the greenhouse gas CO₂ in the atmosphere. The researchers report on these findings in the online edition of the scientific journal Geophysical Research Letters.

The Altai mountains lie on the border between Russia, Kazakhstan, Mongolia, and China, in a region with a particularly pronounced continental climate. In 2001, an international research team under the leadership of Margit Schwikowski (Paul Scherrer Institute) drilled a 139 m-long ice core at the Belukha glacier, near the highest mountain of the Altai. Following extensive work in the laboratory, this core has now revealed its secrets.

Ice core acting as a thermometer

The ice core was cut into 3600 samples at –20°C in the PSI's cold room, and the ¹⁶O and ¹⁸O oxygen isotope content determined with an isotope mass spectrometer. It was demonstrated that the behaviour of the stable oxygen isotope ratio has closely followed the record of the temperature measured at a nearby weather station over the past 130 years. This parameter can therefore be used as a measure for temperature in the past. The deepest sample was dated to the year 1250, which means that the ice core contains climate information covering the past 750 years.

Solar activity influences temperature

The total solar irradiance is not a constant factor. It fluctuates periodically around a value of 1365 watts per square metre. The best-known cycle has an average duration of 11 years. It has only been possible to measure solar activity directly since 1978, but the number of sun spots – a measure of solar activity – has been observed through telescopes from as far back as the year 1610. Information about the solar activity before that time can be provided by other indirect methods: analysis of the cosmogenic radio-nuclides ¹⁰Be from polar ice cores, and ¹⁴C from tree rings, which are also dependent on solar activity. In the period between 1250 and 1850, the regional temperatures in the Altai showed a high correlation with the reconstructed solar activity. This indicates that the changes in solar activity during this time were a main driver of temperature changes.

The temperature follows the sun

Interestingly, the regional temperatures followed the solar forcing with a time lag of 10 to 30 years. The PSI researchers' study is the first in which such a delay has been observed over a period of more than 500 years. Since the influence of solar activity on climate has not yet been fully resolved, such observations provide an important contribution to its understanding. One possible mechanism discussed by various authors, which might explain this average lag of 20 years, is the indirect effect of the sun on temperature changes involving ocean-induced changes in atmospheric circulation. Ocean water warms up to a higher level in places where the solar radiation is most powerful, i.e. in the sub-tropics and the tropics. The heat energy is carried from the lower to the higher latitudes by the ocean, then released back into the atmosphere. Because of the high thermal capacity of the oceans and the variable velocities of their currents, these processes are subject to considerable delay. Changes in the North Atlantic atmospheric circulation system, which is responsible for temperature changes in the Altai, may be initiated 20 years earlier by changes of solar radiation in the tropical oceans.

Strong temperature increase in the 20th century can not be explained by the sun

Our study distinguishes between the pre-industrial era (1250–1850) and the period covered by the past 150 years, emphasises Anja Eichler, scientist at the Paul Scherrer Institute. While changes in the solar activity were a main driver of temperature variations in the pre-industrial period, the temperatures in the Altai have shown a much higher rate of increase than that of solar activity during the past 150 years. The strong increase in the industrial period, however, correlates with the increase in the concentration of the greenhouse gas CO₂ over this time. The results of our regional study indicate that changes in solar activity explain less than half of the increase in temperature in the Altai since 1850. This agrees with global studies, based on reconstructed northern hemispheric temperatures, says the researcher.

This work was undertaken in a collaborative project between the Paul Scherrer Institute and the Eawag – Swiss Federal Institute of Aquatic Science and Technology, the Oeschger Centre for Climate Change Research, and the Department of Chemistry and Biochemistry at the University of Bern, together with the Institute for Water and Environmental Problems at Barnaul (Russia).

About the Paul Scherrer Institute

The Paul Scherrer Institute develops, builds and operates large and complex research facilities, and makes them available to the national and international scientific community. Its own work concentrates on solid-state research and material sciences, elementary particle physics, biology and medicine, energy research and environmental research. With a staff of 1300 and an annual budget of approximately CHF 260 million, this is Switzerland's largest research institution.