How changes in Earth's Magnetic Field could effect climate

Joan Feynman (yes, the sister of Richard) and Alexander Ruzmaikin   (both of NASA JPL) in a paper published long ago (2000 - before the CO2 mania hit full stride), wrote one of the most important papers regarding climate.

Abstract.
 High energy cosmic rays   may influence the formation of clouds, and thus can have an impact on weather and climate. Cosmic rays in the solar wind are incident on the magnetosphere boundary and are then transmitted through the magnetosphere and atmosphere to reach the upper troposphere.

 The flux to the troposphere will depend both on the intensity and spectrum of the cosmic rays at the outer boundary of the magnetosphere (magnetopause) and on the configuration of the magnetosphere through which they propagate. Both the incident flux and the magnetospheric transmission have changed systematically during this century due to systematic changes in the solar wind. We show that, early in the century the region of the troposphere open to cosmic ray precipitation was usually confined to a relatively small high-latitude region. As the century progressed there was a systematic increase in the size of this region by over 7". We suggest that these changes contributed to climate change during the last 100 years.

Thus changes in the magnetic field lead to changes in the size and position of the oval where cosmic rays may reach the troposphere and effect earth's climate.

When a cosmic ray particle enters  the magnetosphere its trajectory is highly influenced by the Earth’s magnetic field and the configuration of the magnetosphere. The main field of the Earth changes relatively slowly with time, but geomagnetic activity, which reflects the configuration of the magnetosphere, changes on time scales of minutes to days.

The transmission of a high energy charged proton in the Earth’s field depends on the energy of the particle and the direction from which it comes. A useful concept is the cosmic ray cutoff, i.e. the lowest latitude that a proton of a given energy will penetrate if it is vertically incident on the Earth’s field. Three regions of transmission can be distinguished.

There is a low latitude region where the field has a dipole configuration which is insensitive to solar wind conditions. The trajectory of a particle incident on this region can be calculated in a straightforward manner but only protons of very high energy will be able to penetrate to the troposphere. Since the flux of such particles is very small, effectively few cosmic rays reach the troposphere in this region.

There is also a high latitude region (the polar cap) in which the Earth’s field lines are essentially open to the solar wind and particles of all rigidities can find their way to the atmosphere.

Between these two regions is a third region in which the transmission is dependent on the rigidity of the particle and the direction of incidence at the magnetopause. The paths of these particles are very complex. The auroral oval marks the transition between the open polar cap and the closed dipolar inner magnetosphere. Changes in the position of the auroral oval will be reflected in changes in the positions at which galactic cosmic rays impinge on the troposphere. Such changes will influence cloud formation and through that mechanism, may be expected to change the weather and climate.

Have we had such a change in the position of the Earth's field lines? Perhaps a change in the position of the Magnetic North Pole?

Yes, we have. The largest change in the position of the North Pole ever recorded. That happens to co-incide almost exactly with changes in earth temperature. The theory seems to merit much further consideration.

Have there been changes in cloud cover (that cosmic rays are thought to seed)?

Yes, we have. Note that the red line is earth temperature (Hadcrut 3), and that it's scale is inverted. More clouds = cooler. Less clouds = warmer.

The paper is here.