NCPA - National Center for Policy Analysis

Global Temperatures, Solar Activity And Cosmic Radiation

February 8, 2001

Greenhouse gases are responsible for less than half the rise in global temperatures over the past century, a scientist with the European Space Agency says. Physicist Paal Brekke says natural processes involving changes in the Sun could have at least as powerful an effect on global temperature as increased emissions of carbon dioxide (CO2).

Climate scientists have already looked at changes related to sunspot activity -- the 11-year cycle of flares on the Sun's surface -- and long-term changes in the Sun's brightness, which has a cycle that lasts for centuries. They have discounted the effect of both on the temperature increase over the last century because those cycles either happen over too short a timescale, or have too weak an effect.

But so far they have omitted to take two other factors into account:

  • Changes in the amount of ultraviolet radiation from the Sun do indeed affect the ozone layer.
  • The Sun's magnetic field and solar wind -- streams of electrons and protons from the Sun -- protects the entire Solar System by partially shielding it from cosmic rays (very energetic particles and radiation from outer space).

This shield does not stop all the cosmic rays, and its effectiveness varies with long-term changes in the Sun's activity, which can rise and fall on a timescale of centuries.

Satellite data show the amount of low clouds over the Earth closely follows the amount of cosmic rays reaching the Earth. The cloud cover, in turn, affects the amount of sunlight reaching the Earth's surface. Global warming due to this effect over the last century could be comparable to the amount of warming due to the greenhouse effect.

Source: Paal Brekke, "The Sun and Climate Change," BBC News Online: In Depth: Climate Change, November 16, 2000.


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