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Solar Influence on Temperature (Northern Hemisphere) -- Summary
Climate alarmists frequently claim that earth's climate is becoming more variable and extreme as a result of CO2 induced global warming. With respect to temperature, we have shown elsewhere on our website that its modern frequency and severity fall well within the range of natural variability (see Natural Variability of Climate and Temperature Trends in our Subject Index). In the present review, we examine the issue of attribution, specifically investigating the natural role or influence of the sun on Northern Hemisphere temperatures.

We begin our discussion with the study of Bond et al. (2001), who examined ice-rafted debris found in three North Atlantic deep-sea sediment cores and cosmogenic nuclides (10Be and 14C) sequestered in the Greenland ice cap (10Be) and Northern Hemispheric tree rings (14C) in an effort to learn what is responsible for the approximate 1500-year cycle of global climate change that has been intensely studied in the region of the North Atlantic Ocean and demonstrated to prevail throughout glacial and interglacial periods alike.

Based on arduous analyses of the deep-sea sediment cores that yielded the variable-with-depth amounts of three proven proxies for the prior presence of overlying drift-ice, the scientists were able to discern and, with the help of an accelerator mass spectrometer, date a number of recurring alternate periods of relative cold and warmth that wended their way through the entire 12,000-year expanse of the Holocene. The mean duration of the several complete climatic cycles thus delineated was 1340 years, the cold and warm nodes of the latter of which oscillations, in the words of Bond et al., were "broadly correlative with the so called 'Little Ice Age' and 'Medieval Warm Period'."

The signal accomplishment of the scientists' study was the linking of these millennial-scale climate oscillations - and their imbedded centennial-scale oscillations - with similar-scale oscillations in cosmogenic nuclide production, which are known to be driven by contemporaneous oscillations in the energy output of the sun. In fact, Bond et al. were able to report that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum." In light of this observation they concluded that "a solar influence on climate of the magnitude and consistency implied by our evidence could not have been confined to the North Atlantic," suggesting that the cyclical climatic effects of the variable solar inferno are experienced throughout the world.

Because these findings are of such great significance, particularly to the global warming debate that currently rages over the climate model-predicted consequences of anthropogenic CO2 emissions, Bond and his band of researchers went on to cite additional evidence in support of the implications of their work. With respect to the global extent of the climatic impact of the solar radiation variations they detected, they made explicit reference to confirmatory studies conducted in Scandinavia, Greenland, the Netherlands, the Faroe Islands, Oman, the Sargasso Sea, coastal West Africa, the Cariaco Basin, equatorial East Africa, and the Yucatan Peninsula, demonstrating thereby that "the footprint of the solar impact on climate we have documented extend[s] from polar to tropical latitudes." They also note that "the solar-climate links implied by our record are so dominant over the last 12,000 years ... it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the LIA could not have been a coincidence," further noting that their findings support previous suggestions that both the Little Ice Age and Medieval Warm Period "may have been partly or entirely linked to changes in solar irradiance."

Another point reiterated by Bond et al. is that the oscillations in drift-ice they studied "persist across the glacial termination and well into the last glaciation, suggesting that the cycle is a pervasive feature of the climate system." At two of their coring sites, in fact, they identified a series of such cyclical variations that extended throughout all of the previous interglacial and were "strikingly similar to those of the Holocene." Here they could also have cited the work of Oppo et al. (1998), who observed similar climatic oscillations in a sediment core that covered the span of time from 340,000 to 500,000 years before present, and that of Raymo et al. (1998), who pushed back the time of the cycles' earliest known occurrence to well over one million years ago.

So how do the small changes in solar radiation inferred from the cosmogenic nuclide variations bring about such significant and pervasive shifts in Earth's global climate? In answer to this question, which has long plagued proponents of a solar-climate link, Bond et al. describe a scenario whereby solar-induced changes high in the stratosphere are propagated downward through the atmosphere to the Earth's surface, where they likely provoke changes in North Atlantic Deep Water formation that alter the global Thermohaline Circulation. In light of the plausibility of this scenario, they suggest that "the solar signals thus may have been transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint."

Concluding their landmark paper, the researchers say the results of their study "demonstrate that the Earth's climate system is highly sensitive to extremely weak perturbations in the Sun's energy output," noting that their work "supports the presumption that solar variability will continue to influence climate in the future." It is readily evident, therefore, that the study of Bond et al. provides ample ammunition for defending the premise that the global warming of the past century or so may well have been nothing more than the solar-mediated recovery of the earth from the chilly conditions of the most recent Little Ice Age, and that any further warming of the planet that might occur would likely be nothing more than a continuation of the same solar-mediated cycle that is destined to usher the globe into the next Medieval-like or Modern Warm Period. Nevertheless, we continue by examining other studies that demonstrate the power of the Sun in moderating temperatures of the Northern Hemisphere.

Assembling a wide range of lacustrine, tree-ring, ice-core and marine records that reveal a Northern Hemispheric -- and possibly global -- cooling event of less than 200 years duration with a 50-year cooling-peak centered at approximately 10,300 years BP, Bjorck et al. (2001) searched for signs of various forcing factors that might have been the cause of this dramatic climatic excursion. According to the authors, the onset of the cooling event broadly coincided with rising 10Be fluxes, which are indicative of either decreased solar or geomagnetic forcing; and since they note that "no large magnetic field variation that could have caused this event has been found," they postulate that "the 10Be maximum was caused by distinctly reduced solar forcing." They also note that the onset of the Younger Dryas is coeval with a rise in 10Be flux, as is the Preboreal climatic oscillation.

Pang and Yau (2002) assembled and analyzed a vast amount of data pertaining to phenomena that have been reliably linked to variations in solar activity, including frequencies of sunspot and aurora sightings, the abundance of carbon-14 in the rings of long-lived trees, and the amount of beryllium-10 in the annual ice layers of polar ice cores. In the case of sunspot sightings, the authors used a catalogue of 235 Chinese, Korean and Japanese records compiled by Yau (1988), a catalogue of 270 Chinese records compiled by Zhuang and Wang (1988), and a time chart of 139 records developed by Clark and Stephenson (1979), as well as a number of later catalogues that made the overall record more complete.

Over the past 1800 years, the authors identified "some nine cycles of solar brightness change," which include the well-known Oort, Wolf, Sporer, Maunder and Dalton Minima. With respect to the Maunder Minimum -- which occurred between 1645 and 1715 and is widely acknowledged to have been responsible for some of the coldest weather of the Little Ice Age -- they report that the temperatures of that period "were about one-half of a degree Celsius lower than the mean for the 1970s, consistent with the decrease in the decadal average solar irradiance." Then, from 1795 to 1825, came the Dalton Minimum, along with another dip in Northern Hemispheric temperatures. Since that time, however, the authors say "the sun has gradually brightened" and "we are now in the Modern Maximum," which is likely responsible for the warmth of the Current Warm Period.

The authors say that although the long-term variations in solar brightness they identified "account for less than 1% of the total irradiance, there is clear evidence that they affect the Earth's climate." And so they do. Pang and Yau's dual plot of total solar irradiance and Northern Hemispheric temperature from 1620 to the present (their Fig. 1c), indicates that the former parameter (when appropriately scaled, but without reference to any specific climate-change mechanism) can account for essentially all of the net change experienced by the latter parameter up to about 1980. After that time, however, the IPCC surface air temperature record rises dramatically, although radiosonde and satellite temperature histories largely match what would be predicted from the solar irradiance record. Could these facts thus be interpreted as evidence for the corruptness of the post-1980 global or Northern Hemispheric surface air temperature record? With many other reasons to doubt the IPCC temperature history, they well could be.

In a separate study, Rohling et al. (2003) "narrow down" temporal constraints on the millennial-scale variability of climate evident in ice-core δ18O records by "determining statistically significant anomalies in the major ion series of the GISP2 ice core," after which they conduct "a process-oriented synthesis of proxy records from the Northern Hemisphere."

With respect to the temporal relationships among various millennial-scale oscillations in Northern Hemispheric proxy climate records, the authors conclude that a "compelling case" can be made for their being virtually in-phase, based on (1) "the high degree of similarity in event sequences and structures over a very wide spatial domain," and (2) the fact that their process-oriented synthesis "highlights a consistent common theme of relative dominance shifts between winter-type and summer-type conditions, ranging all the way across the Northern Hemisphere from polar into monsoonal latitudes." These findings, they additionally note, "corroborate the in-phase relationship between climate variabilities in the high northern latitudes and the tropics suggested in Blunier et al. (1998) and Brook et al. (1999)."

On another note, the authors report that although individual cycles of the persistent climatic oscillation "appear to have different intensities and durations, a mean periodicity appears around ~1500 years (Mayewske et al., 1997; Van Kreveld et al., 2000; Alley et al., 2001)." They further report that "this cycle seems independent from the global glaciation state (Mayewski et al., 1997; Bond et al., 1999)," and that "10Be and delta 14C records may imply a link with solar variability (Mayewski et al., 1997; Bond et al., 2001)."

Lastly, we come to the study of Usoskin et al. (2003), who note that "sunspots lie at the heart of solar active regions and trace the emergence of large-scale magnetic flux, which is responsible for the various phenomena of solar activity" that may influence earth's climate. They also indicate that "the sunspot number (SN) series represents the longest running direct record of solar activity, with reliable observations starting in 1610, soon after the invention of the telescope." Hence, by comparing long-term SN and temperature data, it may be possible to determine if there is a correlation between the two parameters that might indicate a dependence of earth's temperature upon variations in solar activity.

In order to compare SN data with the millennial-scale temperature reconstruction of Mann et al. (1999), which is the climate history upon which climate alarmists base their claims of impending CO2-induced climatic catastrophe, the directly-measured SN record must be extended back in time at least another 600 years, which is precisely what Usoskin et al. did, using records of 10Be cosmonuclide concentration derived from polar ice cores to reconstruct average SN from AD 850 to the present. In accomplishing this task, they employed detailed physical models that they say were "developed for each individual link in the chain connecting the SN with the cosmogenic isotopes," and they combined these models in such a way that "the output of one model [became] the input for the next step."

Most interestingly, the reconstructed SN history of the past millennium looks very much like the infamous "hockeystick" temperature history of Mann et al. (1999). It slowly declines over the entire time period -- with numerous modest oscillations associated with well-known solar maxima and minima -- until the end of the Little Ice Age, whereupon it rises dramatically. Usoskin et al. report, for example, that "while the average value of the reconstructed SN between 850 and 1900 is about 30, it reaches values of 60 since 1900 and 76 since 1944." In addition, they report that "the largest 100-year average of the reconstructed SN prior to 1900 is 44, which occurs in 1140-1240, i.e., during the medieval maximum," but they note that "even this is significantly less than the level reached in the last century." Hence, they readily and correctly conclude, on the basis of their work, that "the high level of solar activity since the 1940s is unique since the year 850."

Usoskin et al.'s results also raise some interesting questions; for if their SN reconstruction is correct, it could logically be claimed that the hockeystick temperature history of Mann et al. (1999) is also largely correct. Is this the good news for which climate alarmists have been desperately seeking? Probably not; for such reasoning implies that the hockeystick nature of the SN trend is responsible for the hockeystick nature of the temperature trend. And this conclusion would leave precious little room for attributing the modern rise in anthropogenic CO2 emissions as responsible for the modern rise in global temperature.

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Blunier, T., Chapellaz, J., Schwander, J., Dallenbach, A., Stauffer, B., Stocker, T.F., Raynaud, D., Jouzel, J., Clausen, H.B., Hammer, C.U. and Johnsen, S.J. 1998. Asynchrony of Antarctic and Greenland climate change during the last glacial period. Nature 394: 739-743.

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Last updated 6 May 2009