Memorandum submitted by Professor David
Quentin Bowen, Professor of Quaternary Geology, Department of
Earth Sciences, Cardiff University
INTRODUCTION
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.
EVIDENCE
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.
SUMMARY
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
REFERENCES
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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