Nutshell

The Svensmark hypothesis in a nutshell

Illustration from Svensmark, “The Adventurous Journey of Spaceship Earth” DTU Yearbook 2009

• Cosmic rays, high-energy particles raining down from exploded stars, knock electrons out of air molecules.
  
• The electrons help clusters of sulphuric acid and water molecules to form, which can grow into cloud condensation nuclei – seeds on which water droplets form to make clouds.
• Low clouds made with liquid water droplets cool the Earth’s surface.
• Variations in the Sun’s magnetic activity alter the influx of cosmic rays to the Earth.
• When the Sun is lazy, magnetically speaking, there are more cosmic rays and more low clouds, and the world is cooler.
• When the Sun is active fewer cosmic rays reach the Earth and, with fewer low clouds, the world warms up.
• The Sun became unusually active during the 20th Century and as a result “global warming” occurred.
• Recently (2006-2010) the Sun has been unusually lazy and “global warming” seems to have gone into reverse, as expected by the Svensmark hypothesis.
• Coolings and warmings of around 2 deg. C have occurred repeatedly over the past 10,000 years, as the Sun’s activity and the cosmic ray influx have varied.
• Over many millions of year, much larger variations of up to 10 deg. C occur as the Sun and Earth, travelling through the Galaxy, visit regions with more or fewer exploding stars.

For objections to the Svensmark hypothesis and answers to them, see Falsification tests

Nigel Calder