Could they be right? Certainly, there is a sense in which the Earth exists in the extended atmosphere of the Sun, and there are three means by which the Sun can, and does, influence the climate of the Earth:
- Solar radiation
- The solar wind
- The solar magnetic field
On some estimates, solar irradiance of the Earth has increased by 0.4% over the past 200-300 years, causing a temperature increase of about 0.4 degrees C (Lomborg, The Skeptical Environmentalist, p276). Significant, then, but not enough to entirely explain global warming.
The solar wind is the flow of charged particles, mainly protons and electrons, from the corona of the Sun. The Earth's magnetic field protects us from most of these particles, but disturbances in the solar wind can cause disturbances in the Earth's magnetic field, and this can, amongst other things, accelerate charged particles down the magnetic field lines at the magnetic poles of the Earth, causing the aurorae.
However, the Earth's atmosphere is bombarded by particles much more energetic than solar wind particles, called cosmic rays, or, to distinguish them from solar wind particles, galactic cosmic rays (GCRs). These are high kinetic-energy protons, electrons, positrons, and the nuclei of helium or even heavier elements. Such particles are probably produced by the supernovae explosions of high-mass stars. When cosmic ray particles hit the upper atmosphere of the Earth, they produce secondary particles, which can reach the lower atmosphere or ground level. When the Sun's magnetic field is weaker, a greater number of GCRs impinge upon the atmosphere of the Earth.
Now, Henrik Svensmark claims that secondary cosmic ray particles such as muons can liberate electrons from the atoms in the lower atmosphere, and these electrons help to make the condensation nuclei on which water droplets form. Hence, argues Svensmark, the greater the flux of cosmic rays, the greater the amount of low-altitude cloud cover over the Earth. Low-altitude clouds have a net cooling effect upon global temperatures, so if there were less cosmic rays, there would be less low-altitude cloud, and global temperatures would rise. The solar magnetic field ultimately determines the flux of cosmic rays to which the Earth is subjected, hence variations in the solar magnetic field can cause variations in the global temperature of the Earth.
Svensmark argues that a shorter sunspot cycle corresponds to more intense solar activity, less cosmic rays, less low-altitude cloud, and rising temperatures. And, to some extent, the evidence supports this claim: the sunspot cycle shortened in the first half of the twentieth century, in synchrony with rising global temperatures. The increase in greenhouse gas emissions in the first half of the twentieth century was negligible, so this rise in global temperature can only be explained by solar activity. However, in the second half of the twentieth century, global temperatures rose again by a comparable degree, but the length of the sunspot cycle remained largely constant. Given the rise in greenhouse gas emissions during the second half of the twentieth century, this suggests that global warming is the combined effect of solar activity and greenhouse gases. Svensmark and Calder may therefore be overplaying their case, but I await their book with interest.