Temperature in North America
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
We begin our review of the influence of the sun on North American temperatures 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. 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 dataset added more than one hundred years to the chronology and now covers the period AD 950-1994.
The updated proxy indicator of temperature showed considerable decadal- and centennial-scale variability, where generally warmer conditions prevailed during the eleventh and twelfth 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 (eleventh—twelfth 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 2,000 years. 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 greater during the Medieval Warm Period than at any other time over the 2,000-year period studied, while during the Current Warm Period its relative abundance was actually lower than the 2,000-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. 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 authors 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 Bukry 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.”
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 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.
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 identified by Bond et al. (2001) as belonging to a pervasive climatic oscillation with a period of approximately 1,500 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, 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 5,200 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.
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).”
It is clear that broad-scale periods of warmth in North America have occurred over and over again throughout the Holocene—and beyond (Oppo et al., 1998; Raymo et al., 1998)—forced by variable solar activity. This suggests that the Current Warm Period was also instigated by this recurring phenomenon, not the CO2 output of the Industrial Revolution.
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Barron, J.A. and Bukry, D. 2007. Solar forcing of Gulf of California climate during the past 2000 yr suggested by diatoms and silicoflagellates. Marine Micropaleontology 62: 115-139.
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.
Campbell, I.D., Campbell, C., Apps, M.J., Rutter, N.W. and Bush, A.B.G. 1998. Late Holocene ca.1500 yr climatic periodicities and their implications. Geology 26: 471-473.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Cook, E.R. 2002. Reconstructions of Pacific decadal variability from long tree-ring records. EOS: Transactions, American Geophysical Union 83: S133.
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Wiles, G.C., Barclay, D.J., Calkin, P.E. and Lowell, T.V. 2008. Century to millennial-scale temperature variations for the last two thousand years indicated from glacial geologic records of Southern Alaska. Global and Planetary Change 60: 115-125.
Wiles, G.C., D’Arrigo, R.D., Villalba, R., Calkin, P.E. and Barclay, D.J. 2004. Century-scale solar variability and Alaskan temperature change over the past millennium. Geophysical Research Letters 31: 10.1029/2004GL020050.