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Global Climate and the Solar Cycle

EGU Journal Highlights - Atmospheric Chemistry and Physics - December 2006

How much of global climate change is caused by fluctuations of the Sun's radiation? A reliable assessment of human-induced global warming requires an answer to this question. Now it appears that the influence of the solar cycle on Earth's climate is much less than most atmospheric scientists assumed so far.

This is the argument of Jan Kazil, Edward R. Lovejoy, Mary C. Barth and Keran O'Brien from NOAA, NCAR, the University of Colorado and Northern Arizona University. They published their findings in the on-line journal Atmospheric Chemistry and Physics of the European Geosciences Union.

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During the minimum phase of the Sun's decadal activity cycle, the solar magnetic field weakens and allows more galactic cosmic rays to reach Earth's atmosphere. It appears that global cloud cover and reflectivity could be enhanced at solar minimum compared to solar maximum, and more sunlight reflected back to space.

This is caused by a complex chain of events whereby more ions lead to an increased aerosol production in the atmosphere. In this paper Kazil and colleagues show that this effect accounts for a variation in warming of the Earth by the Sun of no more than 0.22 W/m2 in the course of a solar cycle. A second finding supports the theory that aerosol particles observed near the surface of tropical oceans may have their origin at higher altitudes, where they form from convectively lifted near-surface air.

Clouds are brighter than the Earth's surface and reflect a considerable amount of the incoming solar radiation back to space. Hence, they strongly influence the planet's temperature and climate. Aerosols, which are small particles in the air, are essential for the formation of cloud droplets, and changes in aerosol concentrations and properties affect the reflectivity of clouds. Aerosols are either directly emitted into the atmosphere or form from gas phase constituents such as sulphuric acid.

Atmospheric ions are likely to act as agents for the formation of liquid aerosols via the formation of "seed particles" (called condensation nuclei) because they greatly stabilize small clusters of molecules with respect to evaporation.

The main source of ions in the atmosphere are galactic cosmic rays, whose intensity is modulated by the decadal solar activity cycle. It has therefore been suggested that the change in ion production resulting from the modulation of galactic cosmic rays by the solar cycle influence atmospheric aerosol concentrations. This, in turn, leads to a variation in cloud cover, and consequently the amount of sunlight reflected back to space.

Kazil et al. used a computer model which describes the formation of sulphuric acid / water aerosol particles from ions on global scales. Their model simulations show that at solar minimum, when the ion production is at its maximum, warming of the Earth by sunlight is reduced by at most 0.22 W/m2. This is due to the enhanced cloud reflection, relative to solar maximum when ion production is at its minimum. This upper limit to the effect is less than the concurrent reduction by 0.24 W/m2 in warming of the Earth by sunlight due to the decrease of solar brightness from solar maximum to minimum.

This finding indicates only a weak effect of galactic cosmic rays on clouds due to aerosol formation from ions, and hence on the Earth's climate. These results, however, do not preclude the possibility that other mechanisms connect solar variability and climate.

A second finding is that over tropical oceans, aerosol generation from the gas phase near the surface is inefficient compared with that at higher altitudes. Here, aerosol particles form readily because of the combination of low temperatures and frequent injection of near-surface air by convection. This supports the theory that aerosol particles observed near the surface of tropical oceans may have their origin at higher altitudes, where they form due to convective lifting of near-surface air.

Aerosol nucleation over oceans and the role of galactic cosmic rays
J. Kazil, E. R. Lovejoy, M. C. Barth, and K. O'Brien
Atmospheric Chemistry and Physics, Volume 6, Number 12, pp. 4905-4924.
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Author's address:

Dr Jan Kazil
Cooperative Institute for Research in Environmental Sciences
University of Colorado
Boulder, CO, USA
+1 303 497-7994

Dr. Frederik M. van der Wateren | idw
Further information:

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