How much of an influence the sun exerts on earth’s climate has long been a topic of heated discussion in the area of global climate change. The primary reason for differing opinions on the subject derives from the facts that (1) numerous studies have demonstrated a correlation between various measures of solar activity and climatic phenomena, and (2) the amount of solar radiative forcing reported in these studies is generally found to be so small that it is difficult to see how it could possibly produce climatic effects of the magnitude observed. Supporters of solar effects theories thus generally contend that various feedback mechanisms may amplify the initial solar perturbation to the extent that significant changes in climate do indeed result. In this summary we highlight some of the recent scientific literature that demonstrates the viability of such solar-climate linkages.
Many solar-climate studies utilize tree-ring records of 14C as a measure of solar activity, because solar activity (including variations in the number of sunspots and the brightness of the sun) influences the production and amount of 14C, such that periods of higher solar activity yield a lower production and atmospheric burden of 14C
(Perry and Hsu, 2000). This being the case, it can be appreciated that as trees remove carbon from the atmosphere and sequester it in their tissues, they are recording a history of solar activity that could be influencing earth’s atmosphere-ocean system. Thus, the history of 14C contained in tree rings has been examined by a number of authors as a proxy indicator of solar activity and compared with various indices of climate.
A good example of this type of work is the study of Hong et al. (2000), who developed a 6000-year high-resolution delta18O record from plant cellulose deposited in a peat bog in the Jilin Province of China (42° 20' N, 126° 22' E) from which they inferred the temperature history of that location over the past six millennia. In comparing this record with changes in atmospheric 14C derived from tree rings, the authors found a "remarkable, nearly one to one, correspondence," which led them to conclude that the temperature history of this region over the past 6000 years was "forced mainly by solar variability." Similar conclusions have been drawn by Karlén (1998), who analyzed changes in the sizes of glaciers and the altitude of the alpine tree-limit, as well as variations in the width of tree rings in Scandinavia, over the last 10,000 years. When comparing these data with tree-ring 14C anomalies, they too observed strong correlations.
Other 14C studies reveal a solar influence in Oman and Mexico. Neff et al. (2001) investigated the relationship between a 14C tree-ring record and a delta18O proxy record of monsoon rainfall intensity as recorded in calcite delta18O data obtained from a stalagmite in northern Oman for the period 9,600-6,100 years ago, reporting an "extremely strong" relationship between the two data sets. Comparison of lake sediment core data taken from Mexico’s Yucatan Peninsula and a 14C tree-ring record covering the past 2600 years revealed similar periodicities, leading Hodell et al. (2001) to conclude that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing."
More examples of a solar forcing of drought come from Yu and Ito (1999), Dean and Schwalb (2000), Black et al. (1999) and Verschuren et al. (2000). Yu and Ito (1999) report that recurring intervals of drought in the Great Plains of North America occur with periodicities of 100, 130, 200 and 400 years and that they line up "in surprising detail" with several solar indices, leading them to seriously consider "solar variability as the major cause of century-scale drought frequency in the northern Great Plains." Dean and Schwalb (2000) report similar solar-related drought conditions with periodicities of 200 and 400 years for the Great Plains; and Black et al. (1999) report finding a solar related influence on climate variability in the North Atlantic, which, they contend, "may play a role in triggering changes in the frequency and persistence of drought over North America." In addition, Verschuren et al. (2000) report a solar-drought link for equatorial east Africa, noting that all three of the severest drought events of the past 700 years there were "broadly coeval with phases of high solar radiation, and the intervening periods of increased moisture were coeval with phases of low solar radiation."
Many other researchers have also commented on a solar-climate link.
Vaganov et al. (2000) reported finding a significant correlation between solar activity and temperature over the past 600 years in the Asian subarctic, while
Domak et al. (2001) make a case for similar solar influences over the past 13,000 years, based upon radiocarbon and spectral analyses of data from an ocean sediment core on the inner continental shelf of the western Antarctic Peninsula. In addition,
Rozelot (2001) examined variations in the sun’s radius and compared them to temperature records of the past four centuries, finding that "warm periods on Earth correlate well with smaller apparent diameter of the Sun and colder ones with a bigger Sun."
With respect to more recent solar and climatic history, Lockwood et al. (1999) determined that, contemporaneously with the warming of the earth, the sun’s total magnetic flux rose by a factor of 1.41 over the period 1964-1996 and by a factor of 2.3 since 1901. Commenting on this finding, Parker (1999) noted that the doubling of the magnetic field of the sun over the past century was accompanied by a doubling of the number of sunspots, and that one consequence of the latter phenomenon is a much more vigorous sun that is slightly brighter, which caused him to wonder "to what extent the solar brightening has contributed to the increase in atmospheric temperature and CO2." Likewise, Broecker (1999) wonders if cycles in the solar wind might not have been responsible for the warming of the 1980s and 90s.
How do small changes in solar activity, as discussed above, influence climate? Chambers et al. (1999), Van Geel et al. (1999), Tobias and Weiss (2000) and
Solanki et al. (2000) have each identified viable "multiplier effects" that can operate on solar rhythms in such a way that minor variations in solar activity can be reflected in more significant variations within the earth’s atmosphere. Principal among these phenomena is the effect of cosmic rays on cloud cover. Recently, for example,
Kniveton and Todd (2001) reported "evidence of a statistically strong relationship between cosmic ray flux, precipitation and precipitation efficiency over ocean surfaces at mid to high latitudes." Not surprisingly, however, a review of the models used by the Intergovernmental Panel on Climate Change to predict future greenhouse gas-induced global warming revealed such processes to be inadequately represented and even ignored (Chambers et al., 1999).
In view of these many observations, and with respect to the claims that have been raised about potential CO2-induced global warming in many governmental and political circles, we agree with Parker (1999) that "it is essential to check to what extent the facts support these conclusions [about CO2 and global warming] before embarking on drastic, perilous and perhaps misguided plans for global action."
References
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