By Andy May and Javier
The evidence for a persistent irregular climate cycle with a period of 2400 ±200 years is strong. There is compelling evidence of a solar cycle of about the same length and phase; suggesting that the solar cycle might be causing the climate cycle. We will present a summary of the evidence, beginning with the original paleontological evidence, followed by the cosmogenic radionuclide (10Be or Beryllium-10 and 14C or Carbon-14) evidence. For more information, a bibliography of many papers discussing topics relevant to the Bray (Hallstatt) cycle can be found here. Only a small portion of the relevant papers are mentioned in this summary post.
In the November 16, 1968 issue of Nature, James R. Bray first proposed the idea of a 2600-year solar-driven climate cycle based primarily upon evidence of Holocene global glacier advances and retreats. We prefer to call this period the Bray Cycle after him, but the same cycle is often called the Hallstatt Cycle. In this post, we will use both names interchangeably to refer both to the climate cycle and the solar cycle. Bray only considered the maximum advance of a glacier field or a major re-advance that reached the near vicinity of the maximum. He used glacier fields in North America, Greenland, Eurasia, New Zealand and South America in the study. The glacial advances were dated using tree rings, lichenometry and radiocarbon dating. Glacial events for the last 13,700 years suggested an optimum interval of 2600 years. He used a “solar index,” based upon sunspots, sunspot cycle length and auroral records that covered the period from 700BC to the present day to show the cause might be a solar cycle. Over this period, the chi-square statistic tied the glacial events to solar activity with a score of 28.6 (P<0.001).
One inescapable conclusion, from the evidence presented, is that solar variability is an important cause of climate change in the centennial to millennial time frame. Therefore, it must have contributed more to recent warming since the last Bray low ended at the end of the Little Ice Age than the IPCC suggests.
This post is in response to Willis Eschenbach’s posts entitled “Sharpening a Cyclical Shovel” and “The Cosmic Problem with Rays.” His posts were in response to our previous posts on natural climate cycles: Impact of the ~ 2400 yr solar cycle on climate and human societies, Periodicities in solar variability and climate change: A simple model, and Solar variability and the Earth’s climate.