We begin with the study of Wiles et al. (2004), who derived a composite Glacier Expansion Index (GEI) for Alaska based on "dendrochronologically-derived calendar dates from forests overrun by advancing ice and age estimates of moraines using tree-rings and lichens," after which they compared this history of glacial activity with "the ¹⁴C record preserved in tree rings corrected for marine and terrestrial reservoir effects as a proxy for solar variability" and with the history of the Pacific Decadal Oscillation (PDO) derived by Cook (2002).

Results of the study showed Alaska ice expansions "approximately every 200 years, compatible with a solar mode of variability," specifically, the de Vries 208-year solar cycle; and by merging this cycle with the cyclical behavior of the PDO, Wiles et al. obtained a dual-parameter forcing function that was even better correlated with the Alaskan composite GEI, with major glacial advances clearly associated with the Sporer, Maunder and Dalton solar minima.

In introducing the rational for their study, Wiles et al. say that "increased understanding of solar variability and its climatic impacts is critical for separating anthropogenic from natural forcing and for predicting anticipated temperature change for future centuries." In this regard, it is most interesting that they make no mention of possible CO2-induced global warming in discussing their results, presumably because there is no need to do so. Alaskan glacial activity, which, in their words, "has been shown to be primarily a record of summer temperature change (Barclay et al., 1999)," appears to be sufficiently well described within the context of solar and PDO variability alone. And four years later, Wiles et al. (2008) reconfirmed this Alaska solar-climate link in a separate study.

Nearby in the Columbia Icefield area of the Canadian Rockies, Luckman and Wilson (2005) used new tree-ring data to present a significant update to a millennial temperature reconstruction published for this region in 1997. The new update employed different standardization techniques, such as the regional curve standardization method, in an effort to capture a greater degree of low frequency variability (centennial to millennial scale) than reported in the initial study. In addition, the new data set added over one hundred years to the chronology (AD 950-1994).

The updated proxy indicator of temperature showed considerable decadal- and centennial-scale variability, where generally warmer conditions prevailed during the 11th and 12th centuries, between about AD 1350-1450 and from about 1875 through the end of the record, while persistent cold conditions prevailed between 1200-1350, 1450-1550 and 1650-1850, with the 1690s being exceptionally cold (more than 0.4°C colder than the other intervals).

The revised Columbia Icefield temperature reconstruction provides further evidence for natural climate fluctuations on centennial-to-millennial timescales and demonstrates, once again, that temperatures during the Current Warm Period are no different from those observed during the Medieval Warm Period (11-12th centuries) or the Little Medieval Warm Period (1350-1450). And since we know that atmospheric CO2 concentrations had nothing to do with the warm temperatures of those earlier periods, we cannot rule out the possibility that they also have nothing to do with the warm temperatures of the modern era.

But if not CO2, then what? According to Luckman and Wilson, the Columbia Icefield reconstruction "appears to indicate a reasonable response of local trees to large-scale forcing of climates, with reconstructed cool conditions comparing well with periods of known low solar activity," which is a nice way of suggesting that the sun is the main driver of these low frequency temperature trends.

Heading south to the warmer regions of North America, Barron and Bukry (2007) extracted sediment cores from three sites on the eastern slope of the Gulf of California. By examining these high-resolution records of diatoms and silicoflagellate assemblages, they were able to reconstruct sea surface temperatures there over the past 2000 years. So what did they find?

In all three of the sediment cores, the relative abundance of Azpeitia nodulifera (a tropical diatom whose presence suggests the occurrence of higher sea surface temperatures), was found to be far greater during the Medieval Warm Period than at any other time over the 2000-year period studied, while during the Current Warm Period its relative abundance was actually lower than the 2000-year mean, also in all three of the sediment cores. In addition, the first of the cores exhibited elevated A. nodulifera abundances from the start of the record to about AD 350, during the latter part of the Roman Warm Period, as well as between AD 1520 and 1560, during what we have denominated the Little Medieval Warm Period. What is more, by analyzing radiocarbon production data, Barron and Bukry determined that "intervals of increased radiocarbon production (sunspot minima) correlate with intervals of enhanced biosilica productivity," leading the two researchers to conclude that "solar forcing played a major role in determining surface water conditions in the Gulf of California during the past 2000 yr." As for how this was accomplished, Barron and Burky say that "reduced solar irradiance (sunspot minima) causes cooling of winter atmospheric temperatures above the southwest US," and that "this strengthens the atmospheric low and leads to intensification of northwest winds blowing down the Gulf, resulting in increased overturn of surface waters, increased productivity, and cooler SST."

Staying warm, but moving across the continent, Richey et al. (2007) constructed "a continuous decadal-scale resolution record of climate variability over the past 1400 years in the northern Gulf of Mexico" from a box core recovered in the Pigmy Basin, northern Gulf of Mexico [27°11.61'N, 91°24.54'W]," based on "paired analyses of Mg/Ca and δ¹⁸O in the white variety of the planktic foraminifer Globigerinoides ruber and relative abundance variations of G. sacculifer in the foraminifer assemblages."

Results revealed that "two multi-decadal intervals of sustained high Mg/Ca indicate that Gulf of Mexico sea surface temperatures (SSTs) were as warm or warmer than [our italics] near-modern conditions between 1000 and 1400 yr B.P.," while "foraminiferal Mg/Ca during the coolest interval of the Little Ice Age (ca. 250 yr B.P.) indicate that SST was 2-2.5°C below modern SST." In addition, they found that "four minima in the Mg/Ca record between 900 and 250 yr. B.P. correspond with the Maunder, Sporer, Wolf, and Oort sunspot minima," providing additional evidence that the historic warmth of Earth's past was likely solar-induced, which suggests that the Current Warm Period may well be deriving its warmth from the same source.

Also in the Gulf of Mexico, Poore et al. (2003) developed a 14,000-year record of Holocene climate based primarily on the relative abundance of the planktic foraminifer Globigerinoides sacculifer found in two sediment cores. In reference to North Atlantic millennial-scale cool events 1-7 -- which were identified by Bond et al. (2001) as belonging to a pervasive climatic oscillation with a period of approximately 1500 years -- Poore et al. say of their own study that distinct excursions to lower abundances of G. sacculifer "match within 200 years the ages of Bond events 1-6," noting that "major cooling events detected in the subpolar North Atlantic can be recognized in the GOM record." They additionally note that "the GOM record includes more cycles than can be explained by a quasiperiodic 1500-year cycle," but that such centennial-scale cycles with periods ranging from 200 to 500 years are also observed in the study of Bond et al., noting further that their results "are in agreement with a number of studies indicating the presence of substantial century-scale variability in Holocene climate records from different areas," specifically citing the reports of Campbell et al. (1998), Peterson et al. (1991) and Hodell et al. (2001). Last of all, they discuss evidence that leads them to conclude that "some of the high-frequency variation (century scale) in G. sacculifer abundance in our GOM records is forced by solar variability."

In still another example of a solar-temperature connection, Lund and Curry (2004) analyzed a planktonic foraminiferal δ¹⁸O time series obtained from three well-dated sediment cores retrieved from the seabed near the Florida Keys (24.4°N, 83.3°W) that covered the past 5200 years. As they describe it, isotopic data from the three cores "indicate the surface Florida Current was denser (colder, saltier or both) during the Little Ice Age than either the Medieval Warm Period or today," and that "when considered with other published results (Keigwin, 1996; deMenocal et al., 2000), it is possible that the entire subtropical gyre of the North Atlantic cooled during the Little Ice Age ... perhaps consistent with the simulated effects of reduced solar irradiance (Rind and Overpeck, 1993; Shindell et al., 2001)." In addition, they report that "the coherence and phasing of atmospheric ¹⁴C production and Florida Current δ¹⁸O during the Late Holocene implies that solar variability may influence Florida Current surface density at frequencies between 1/300 and 1/100 years," demonstrating once again a situation where both centennial- and millennial-scale climatic variability is explained by similar-scale variability in solar activity, which suggests there is no need to invoke the historical increase in the air's CO2 content to explain the increase in temperature that represents earth's transition from the Little Ice Age to the Current Warm Period.

Finally, we conclude with the study of Li et al. (2006), who "recovered a 14,000-year mineral-magnetic record from White Lake (~41°N, 75°W), a hardwater lake containing organic-rich sediments in northwestern New Jersey, USA." According to these researchers, a comparison of the White Lake data with climate records from the North Atlantic sediments "shows that low lake levels at ~1.3, 3.0, 4.4, and 6.1 ka [1000 years before present] in White Lake occurred almost concurrently with the cold events at ~1.5, 3.0, 4.5, and 6.0 ka in the North Atlantic Ocean (Bond et al., 2001)," and that "these cold events are associated with the 1500-year warm/cold cycles in the North Atlantic during the Holocene" that have "been interpreted to result from solar forcing (Bond et al., 2001)." Consequently, it is clear that broad-scale periods of warmth have occurred over and over again throughout the Holocene -- and beyond (Oppo et al., 1998; Raymo et al., 1998) -- forced solely by variable solar activity, which suggests that the Current Warm Period was also instigated by this same recurring phenomenon and not the CO2 output of the Industrial Revolution.

References

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