Monday, February 14, 2011

New Paper: Solar irradiance at Earth surface varies up to 24 times more than expected

A new peer-reviewed paper published in the journal Atmospheric Chemistry and Physics finds that measurements of solar irradiance at ground level at the South Pole show variations of up to 24 times more than would be expected over the course of a solar cycle. While satellite measurements find that total solar irradiance only varies 0.1% from a solar minimum to solar maximum, the ground-level measurements analyzed by the authors show a change of 1.8 ± 1.0% in the UV-A (320–400 nm) spectrum and 2.4 ± 1.9% in the visible (400–600 nm) spectrum over the course of a solar cycle.
Regressions based on all 17 solstice periods indicate approximate 1.8% and 2.4% decreases in ground-level irradiance for the wavelength regions 320–400 nm and 400–600 nm, respectively, from solar maximum to solar minimum. The associated uncertainty ranges are approximately 0.8–2.7% for the UV-A and 0.5%–4.3% for the visible.
Changes in extraterrestrial irradiance over the solar cycle surely contribute a portion of the variability deduced at the polar surface for the 320–400 nm region, although the magnitude of this contribution is uncertain. However, the inferred solar cycle dependence in the 400–600 nm visible band is too large to be of extraterrestrial origin unless one adopts values at the lowest end of the error range.
The UV-A and visible portions are the most energetic and significant portions of the solar spectrum heating the Earth. While the authors are uncertain of the origin of this variability at the surface, they note that it is "too large to be of extraterrestrial origin." Climate models assume that the solar irradiance reaching the Earth's surface only varies 0.1% over solar cycles in accordance with satellite measurements, but as shown by this paper, that may be an incorrect assumption. Another recent study has shown that solar UV activity has increased almost 50% over the past 400 years. The antiquated assumption in climate science that the effect of the Sun upon the Earth's climate is a constant (they even call it "the solar constant") is in dire need of reassessment. The IPCC, however, is only mandated to assess anthropogenic climate change and only pays lip service to the role of the Sun.

Full paper
Atmos. Chem. Phys., 11, 1177-1189, 2011

Solar irradiance at the earth's surface: long-term behavior observed at the South Pole

J. E. Frederick and A. L. Hodge
Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA

Abstract. This research examines a 17-year database of UV-A (320–400 nm) and visible (400–600 nm) solar irradiance obtained by a scanning spectroradiometer located at the South Pole. The goal is to define the variability in solar irradiance reaching the polar surface, with emphasis on the influence of cloudiness and on identifying systematic trends and possible links to the solar cycle. To eliminate changes associated with the varying solar elevation, the analysis focuses on data averaged over 30–35 day periods centered on each year's austral summer solstice. The long-term average effect of South Polar clouds is a small attenuation, with the mean measured irradiances being about 5–6% less than the clear-sky values, although at any specific time clouds may reduce or enhance the signal that reaches the sensor. The instantaneous fractional attenuation or enhancement is wavelength dependent, where the percent deviation from the clear-sky irradiance at 400–600 nm is typically 2.5 times that at 320–340 nm. When averaged over the period near each year's summer solstice, significant correlations appear between irradiances at all wavelengths and the solar cycle as measured by the 10.7 cm solar radio flux. An approximate 1.8 ± 1.0% decrease in ground-level irradiance occurs from solar maximum to solar minimum for the wavelength band 320–400 nm. The corresponding decrease for 400–600 nm is 2.4 ± 1.9%. The best-estimate declines appear too large to originate in the sun. If the correlations have a geophysical origin, they suggest a small variation in atmospheric attenuation with the solar cycle over the period of observation, with the greatest attenuation occurring at solar minimum.

Related: A very simple climate model incorporating only "sunspot integral" and ocean oscillations has a correlation coefficient of .96 to global temperature, as compared to a correlation coefficient of .44 for CO2:

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