Last week I referred to a recent article in the Earth science trade rag Eos concerned with natural climate variability, and how one distinguishes this from anthropogenic effects. In the 6 September 2011 issue of the same journal, Nathan Schwadron and Harlan Spence look at the interplanetary space environment, and ask to what degree this affects the terrestrial ecosphere and climate.
Experts on space weather and plasma physics, Schwadron and Spence are particularly interested in the flux of ultra-high-energy charged particles known as galactic cosmic rays (GCRs) incident on Earth. Charged particles are affected by magnetic fields, so the GCR flux is heavily influenced by the level of solar activity, which gives rise to and determines the behaviour of the interplanetary magnetic field, and its interaction with Earth’s internally-generated magnetic field. Solar activity varies on different timescales, including the 11-year Schwabe cycle, the Gleissberg cycle of 70–100 years (an amplitude modulation of the Schwabe cycle), and also much longer cyclical variations and secular trends.
Of particular interest to climate scientists are anomalies such as the Maunder Minimum in solar activity of 1645–1715, which coincided with unusually cold winters in Europe and North America. Measurements of an isotope of Beryllium found in Antarctic ice cores shows a correlation between the weak Sun of the Maunder Minimum, and a reduction in solar magnetic field strength and rise in GCR activity much larger than we are currently experiencing.
Peaks in the Beryllium record have also been detected 35,000 and 60,000 years ago, but the cause of these changes remains unknown. It has been suggested that the maxima are related to geomagnetic field disruptions and reversals, but this doesn’t explain the regional rather than global nature of the ostensibly related climate change.
What is clear is that many of the long-term variations in cosmic ray flux may be due to changes in the interplanetary environment and beyond. This may add a further complication in understanding the Earth system, but Schwadron and Spence argue that it is essential in a detailed study of an interlinked planetary ecosystem that supports and brings about life.
The writers conclude by asking whether we are heading towards a new Maunder-like minimum in solar activity. All we can say with confidence is that, while such large changes may not occur over the next decade, history suggests that the interplanetary environment over the coming century will be quite different from that of the past 50 years.