January 8, 2010

Is Earth’s Temperature Controlled by the Sun?

Filed under: Climate Forcings, Solar

Nicola Scafetta is an atmospheric scientist with Duke University’s highly regarded Department of Physics, and in a recent article in the Journal of Atmospheric and Solar-Terrestrial Physics, he provides an excellent introduction for us stating “Estimating the solar contribution to global mean air surface temperature change is fundamental for evaluating the anthropogenic contribution to climate change. This is regarded as one of the most important issues of our time. While some theoretical climate model studies indicate that the solar variability has little effect on climate (these studies estimate that less than 10% of the global warming observed since 1900 is due to the sun), several empirical studies suggest that large climatic variations are well synchronized with solar variations and, therefore, climate is quite sensitive to solar changes.”

No doubt about it – a considerable debate rages in the climate community about the role of the sun in controlling the temperature of the earth. There are many leading climate scientists who believe that the sun’s impact is negligible while others believe the sun’s control on earth’s temperature is substantial. Both groups publish regularly, and there is no end of empirical and theoretical evidence to support both camps. If you think the debate is over in the world of climate change, look at the literature on solar control of climate, and you will immediately conclude the debate is as lively as ever.

Scafetta set out to introduce “an empirical bi-scale climate model characterized by both fast and slow characteristic time responses to solar forcing” to the issue. The research plan looked rather straightforward, but one enormous complication is described in the article. Scafetta notes “Determining how solar activity has changed on decadal and secular scales is necessary to estimate the solar contribution to climate change. Unfortunately, how solar activity has changed in time is not known with certainty.” That’s right, even if we restrict the time of analysis to 1978 to present, the trends and variations in total solar irradiance (TSI) are amazingly uncertain … go figure.

Two major camps have evolved in this regard with one group concluding that the TSI has not changed much from 1980 to 2000 while another group concludes that TSI has increased over that same period. To no surprise to us at World Climate Report, Scafetta notes that models adopted by the IPCC in their 2007 assessment “assumed that TSI did not change significantly since 1950 and that, consequently, the sun could not be responsible for the significant warming observed since 1975.”

After conducting his analyses with the two popular TSI time series, Scafetta concludes “When taken into account the entire range of possible TSI satellite composite since 1980, the solar contribution to climate change ranges from a slight cooling to a significant warming, which can be as large as 65% of the total observed global warming.” Several other interesting quotes come from the piece including “The IPCC (2007) claim that the solar contribution to climate change since 1950 is negligible may be based on wrong solar data in addition to the fact that the EBMs and GCMs there used are missing or poorly modeling several climate mechanisms that would significantly amplify the solar effect on climate.” If you are wondering, EBMs and GCMs are energy balance models and general circulation models used widely by the IPCC. He also boldly states “the hockey stick temperature graph is unlikely because the Medieval Warm Period and the Little Ice Age, which occurred during a maximum and a minimum of solar activity, respectively, are supported by numerous historical facts and data from several regions of the Earth.”

Our second article appeared recently in Geophysical Research Letters written by three scientists with the National Space Institute with the Technical University of Denmark. Svensmark et al. have been interested in solar influences on climate for many years, and to be sure, the lead author (Henrik) has been associated with the global warming “skeptics” for well over a decade. He continues to be one of the most prolific scientists in the greenhouse debate, and the latest article will not sit well with the deniers who cannot accept that there may be a strong solar control of the Earth’s climate.

Svensmark et al begin their piece stating “Explosive events on the sun provide natural experiments for testing hypotheses about solar influences on the Earth. A conspicuous effect is the sudden reduction, over hours to days, in the influx of galactic cosmic rays (GCRs), first noticed by Scott E. Forbush in 1937. Such Forbush decreases (FDs) are now understood to be the result of magnetic plasma clouds from solar coronal mass ejections that pass near the Earth and provide a temporary shield against GCRs. Whether or not any consequences of these events are perceptible in the weather has been a subject of debate for 50 years”. Do you notice how many scientists insist there is an ongoing debate on subjects involving climate change?

The research team examined 26 Forbush decreases from 1987 to 2007, and they examined a variety of data bases to search for a climate response on the Earth. They clearly state in their abstract that “We find that low clouds contain less liquid water following Forbush decreases, and for the most influential events the liquid water in the oceanic atmosphere can diminish by as much as 7%. Cloud water content as gauged by the Special Sensor Microwave/Imager (SSM/I) reaches a minimum ~7 days after the Forbush minimum in cosmic rays, and so does the fraction of low clouds seen by the Moderate Resolution Imaging Spectroradiometer (MODIS) and in the International Satellite Cloud Climate Project (ISCCP). Parallel observations by the aerosol robotic network AERONET reveal falls in the relative abundance of fine aerosol particles which, in normal circumstances, could have evolved into cloud condensation nuclei.” They conclude “Our results show global-scale evidence of conspicuous influences of solar variability on cloudiness and aerosols.”

These results show us that the story behind the solar control of climate is a complicated one with many unknowns. Furthermore, simply linking the total output of the sun to the temperature of the earth may fail to account for the interactions between cosmic rays and cloudiness and aerosols. As noted by Scafetta, there are those who continue to deny the role the sun plays in explaining temperature variations at a global scale – but clearly, there is still a lot more to be learned.


Scafetta, N. 2009. Empirical analysis of the solar contribution to global mean air surface temperature change. Journal of Atmospheric and Solar-Terrestrial Physics, 71, 1916–1923.

Svensmark, H., T. Bondo, and J. Svensmark. 2009. Cosmic ray decreases affect atmospheric aerosols and clouds. Geophysical Research Letters, 36, L15101, doi:10.1029/2009GL038429.

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