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Solar Activity, Cosmic Rays, Low Clouds and Climate Change
Reference
Usoskin, I.G., Marsh, N., Kovaltsov, G.A., Mursula, K. and Gladysheva, O.G. 2004. Latitudinal dependence of low cloud amount on cosmic ray induced ionization. Geophysical Research Letters 31: 10.1029/2004GL019507.

Background
When solar activity is high, the magnetic field that is carried by the solar wind intensifies, providing more shielding of the earth from low-energy galactic cosmic rays. This effect may lead to a decrease in ion production in the lower atmosphere, possibly resulting in the creation of fewer cloud condensation nuclei and less low-level cloud cover; and this phenomenon, in turn, may allow more solar radiation to impinge upon the earth and drive global warming.

What was done
In an important test of a critical portion of this hypothesis, the authors compared the spatial distributions of low cloud amount (LCA) and cosmic ray-induced ionization (CRII) over the globe for the period 1984-2000. In doing so, they used observed LCA data obtained from the ISCCP-D2 database limited to IR radiances, while they employed CRII values calculated by Usoskin et al. (2004a) at 3 km altitude, which corresponds roughly to the limiting altitude below which low clouds form.

What was learned
Usoskin et al. determined that "the LCA time series can be decomposed into a long-term slow trend and inter-annual variations, the latter depicting a clear 11-year cycle in phase with CRII." In addition, they say there is "a one-to-one relation between the relative variations of LCA and CRII over the latitude range 20-55S and 10-70N," and that "the amplitude of relative variations in LCA was found to increase polewards, in accordance with the amplitude of CRII variations."

What it means
The findings of the five-member team of Finnish, Danish and Russian scientists provide substantial evidence for a solar-cosmic ray linkage (the 11-year cycle of CRII) and a cosmic ray-cloud linkage (the in-phase cycles of CRII and CLA), making the full solar activity/cosmic ray/low cloud/climate change hypothesis appear to be rather robust.

Reference
Usoskin, I.G., Gladysheva, O.G. and Kovaltsov, G.A. 2004a. Cosmic ray-induced ionization in the atmosphere: spatial and temporal changes. Journal of Atmospheric and Solar-Terrestrial Physics 66: 1791-1796.

Reviewed 15 March 2006