Select Committee on Science and Technology Minutes of Evidence

Memorandum submitted by Professor David Quentin Bowen, Professor of Quaternary Geology, Department of Earth Sciences, Cardiff University


  It has long been thought by some that variability in the radiative output of the Sun may be the main driver of climate change. But because the observational record of sunspot activity only goes back a few centuries it was not possible to validate this. Recently, however, new discoveries and new analytical tools have been able to show that variability in the radiative output of the Sun coincided with climate change over the last 40,000 years—including the last century and last millennium.


  1.  The past radiative activity of the Sun is indicated by records of variability in the production rates of two cosmogenic isotopes Radiocarbon (14C) and Beryllium 10 (10Be).

  2.1  Cosmogenic isotopes are produced by cosmic rays in the atmosphere. When the Sun's radiative output is high it creates a strong solar magnetic field. This effectively shields the Earth from cosmic rays to such an extent that the production rate of cosmogenic isotopes in the Earth's atmosphere is reduced. Conversely, when the Sun's radiative output is lower, production rates of 14C and 10Be are higher, because the solar magnetic field is weaker and cosmic ray activity in the atmosphere is increased.

  2.2  Past production rates of 14C (which is mixed in the atmosphere and taken up in photosynthesis) is measured from annual tree rings. Past production rates of 10Be (which is rained on to ice sheet surfaces at polar latitudes where there is little defence by the Earth's magnetic field) is measured from annual layers in ice cores.

  3.  Variability in production rates for 14C and 10Be coincide with changes in climate. The 14C record covers the past 8,000 years and that for 10Be covers the period between 3,000 and 40,000 years ago. When production rates were low (indicating stronger radiative forcing from the Sun) the climate was warmer. When production rates were high (indicating weaker solar radiative forcing) the climate was colder and wetter. Three diagrams (overleaf) show these relationships for different periods.

  4.  During the twentieth century the continuously rising record of atmospheric CO2 concentrations does not parallel the temperature record. Between about 1940 and 1970, when CO2 was rising, global temperatures were declining. Yet during this period solar activity was also declining. The rise in temperature after 1970 coincides with stronger solar radiative forcing.

Figure 1

Figure 2

  Greenland Ice Sheet Project 2 10Be record correlated with the tree ring records of 14C, between 8,000 and 5,000 years ago (from: Stuiver & Reimer 1993).

Figure 3

  10Be record from the Antarctic ice sheet correlated with North American and European tree ring records of 14C during the last millennium (from: Bard et al. 1997).

Figure 4

  5.1  Cycles of climate change in the Greenland Ice Sheet Project 2 ice core (GISP2) occur at 6,100, 1,450 and 2,200 years. The 1,450-year cycle underpins every major climatic change for the last 110,000 years. These cycles run through the last ice age as well as the present interglacial, but were amplified during the ice age because of the Earth's unstable geographical configuration. The current interglacial, however, has been anything but stable. It could be argued that until the mechanism and origin of these cycles is understood it may not be possible to predict the future. Two main mechanisms have been proposed.

  5.2  First, an ocean oscillator mechanism, popularly known as the "ocean conveyor". Bond et al (1999) believe this oscillator is responsible for the 1,450-year cycle which currently coincides with ongoing warming and recovery from the Little Ice Age. Second, transmission of climate changes that originate in low latitudes through the atmosphere: for example, changes in sea surface temperatures in the Tropical Pacific Ocean send global signals of climate change through the atmosphere as El iño episodes make clear (Cane & Clement 1999). Both of these, however, may be complementary and it is plausible that both are forced by solar variability.


  6.  It is not unreasonable to propose that the correlation of 14C and 10Be production rates with climate change points to variability in solar radiative forcing as the prime candidate for causing past and ongoing change.

  7.  Attempts to predict the future that do not take into account the facts of climate evolution on different time scales, especially the last 40,000 years, for which new evidence is available, cannot be said to address the current issue of climate change in a comprehensive way. They are, for example, not incorporated into current computer models. The relatively new discoveries outlined here may prove to be insurmountable hurdles in the quest to show a discernible human influence on climate variability.

29 January 2000


  Bard E, Raisbeck HM, Yiou F & Jouzel J (1997) Solar modulation of cosmogenic isotope production over the last millennium: comparison between 14C and 10Be records. Earth and Planetary Science Letters. 150, 453-462.

  Bond GC, Showers W, Elliot M, Evans M, Lotti R, Hajdas I, Bonani G and Johnson S (1999) The North Atlantic;s 1-2 kyr Climate Rhythm: Relation to Heinrich Events, Dansgaard/Oeschger Cycles and the Little Ice Age. In: Clark PU, Webb RS, Keigwin LD (eds). Mechanisms of Global Climate Change at Millennial Time Scales. American Geophysical Union, Washington DC, 35-58.

  Cane M & Clement AC (1999) A role for the tropical Pacific coupled ocean-atmosphere system on Milankovitch and millennial timescales: global impacts. In: Clark PU et al (op cit). 373-383.

  Finkel RC & Nishiizumi K (1997) Beryllium 10 concentrations in the Greenland Ice Sheet Project 2 ice core from 3 to 40 ka. Journal of Geophysical Research. 102 (C12), 26, 699-26, 706.

  Friis-Christensen E & Lassen K (1991) Length of the Solar Cycle: an indication of solar activity closely associated with climate. Science. 254, 698-700.

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