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Medieval Warm Period (Solar Influence - Temperature) -- Summary
The degree to which the sun has influenced earth's climate over the 20th century is a topic of heated discussion in the area of global climate change. The debate derives from the fact that although numerous studies have demonstrated significant correlations between certain measures of solar activity and various climatic phenomena (Reid, 1991, 1997, 1999, 2000, for example), the magnitude of the solar radiative forcing reported in these studies is generally so small that it is difficult to see how it could possibly have produced climatic effects of the magnitude observed. Supporters of solar effects theories counter this concern by proposing that various positive feedback mechanisms likely amplify the solar perturbations 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 solar-climate linkages and that supports the emerging belief that small changes in solar activity did indeed produce the Medieval Warm Period, which was probably the warmest interval of the past millennium.

Parker (1999) starts us off by reporting that during times of much reduced solar activity (such as the Maunder Minimum of the Little Ice Age) and much increased activity (such as the 12th century Solar Maximum of the Medieval Warm Period), solar brightness variations on the order of ΔB/B = 0.005 typically occur. Then, adding that the mean temperature (T) of the northern portion of the earth varied by 1 to 2C between these two periods (so that ~1.5K/~300K = 0.005), he states that "we cannot help noting that ΔT/T = ΔB/B."

Hong et al. (2000) developed a 6000-year high-resolution δ18O record from plant cellulose deposited in a peat bog in the Jilin Province of China from which they inferred the corresponding temperature history of that location, after which they compared this record with a previously-derived δ14C tree-ring record representative of the intensity of solar activity over this period. Of particular note was their finding of "an obvious warm period represented by the high δ18O from around AD 1100 to 1200 which may correspond to the Medieval Warm Epoch of Europe." They also report that "at that time, the northern boundary of the cultivation of citrus tree (Citrus reticulata Blanco) and Boehmeria nivea (a perennial herb), both subtropical and thermophilous plants, moved gradually into the northern part of China, and it has been estimated that the annual mean temperature was 0.9-1.0C higher than at present." Last of all, they report "there is a remarkable, nearly one to one, correspondence between the changes of atmospheric δ14C and the variation in δ18O of the peat cellulose," which led them to conclude that at the site of their study, the temperature history of the past 6000 years (including the Medieval Warm Period) was "forced mainly by solar variability."

About the same time, Perry and Hsu (2000) developed a simple solar-luminosity model by summing the amplitude of solar radiation variance for fundamental harmonics of the eleven-year sunspot cycle and used it to estimate total solar-output variations over the past 40,000 years, after which the results of this exercise were compared with geophysical, archaeological and historical evidence of climate variation throughout the Holocene. They learned from this exercise that the model output was well correlated with the amount of 14C found in well-dated tree rings gong back to the time of the Medieval Warm Period (about AD 1100), which finding, in their words, "supports the hypothesis that the sun is varying its energy production in a manner that is consistent with the superposition of harmonic cycles of solar activity." In addition, present in both of the records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1,300 years.

Also the same year, Bard et al. (2000) created a 1200-year history of cosmonuclide production in earth's atmosphere from 10Be measurements of South Pole ice (Raisbeck et al., 1990) and the atmospheric 14C record as measured in tree rings (Bard et al., 1997), after which they converted the history to Total Solar Irradiance (TSI) values by "applying a linear scaling using the TSI values published previously for the Maunder Minimum," when cosmonuclide production was 30-50% above the modern value. This work resulted in an extended TSI record that suggests that "solar output was significantly reduced between AD 1450 and 1850, but slightly higher or similar to the present value during a period centered around AD 1200." As a result, they say "it could thus be argued that irradiance variations may have contributed to the so-called "little ice age" and "medieval warm period" and that they "would tend to force global effects."

One of the most compelling and comprehensive studies of the subject ever to be carried out was conducted by Bond et al. (2001), who focused their attention on ice-rafted debris found in three North Atlantic deep-sea sediment cores as well as cosmogenic nuclides sequestered in the Greenland ice cap (10Be) and Northern Hemispheric tree rings (14C). 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 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; and the cold and warm nodes of the last full cycle of this oscillation, 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 researchers' 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. 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.

At this point of their paper, the international team of scientists had pretty much verified a number of things we have regularly reported on our website over the past several years, i.e., that in spite of the contrary claims of a host of climate alarmists, the Little Ice Age and Medieval Warm Period were (1) real, (2) global, (3) solar-induced, and (4) but the latest examples of uninterrupted alternating intervals of relative cold and warmth that stretch back in time through glacial and interglacial periods alike.

Because these several subjects 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 (topics 2 and 3 above, with 1 implied), 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." Also in support of topic 3, they noted 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 Little Ice Age 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."

Shindell et al. (2001) used a version of the Goddard Institute for Space Studies GCM to estimate climatic differences between the period of the Maunder Minimum in solar irradiance (mid-1600s to early 1700s) and a century later, when solar output was relatively high for several decades. For the globe as a whole, they reported a mean annual near-surface air temperature difference on the order of 0.3 to 0.4C between the model-simulated climates of the two periods, which they say is about the magnitude of change suggested by historical and proxy climate data. Much larger temperature differences between the two periods (on the order of 1 to 2C) were observed in model reconstructions for Northern Hemispheric continents in winter; and similar differences were also observed in the historical and proxy climate records of those regions.

Although the GCM employed in their study was by no means perfect (none are) and omitted several phenomena believed to be of importance to correctly simulating earth's climate, such as changes in ocean circulation, Shindell et al. say their results "provide evidence that relatively small solar forcing may play a significant role in century-scale Northern Hemisphere winter climate change," specifically stating that "colder winter temperatures over the Northern Hemispheric continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) and warmer temperatures during the 12th through 14th centuries (the putative Medieval Warm Period) may have been influenced by long-term solar variations."

Rigozo et al. (2001) reconstructed sunspot numbers for the last 1000 years using a sum of sine waves derived from spectral analysis of the time series of sunspot number RZ for the period 1700-1999; and from this record they derived the strengths of a number of parameters related to several aspects of solar variability over the past millennium. This work, in their words, revealed that "the 1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclides variation records, and, specifically, the epochs of the Oort, Wolf, Sporer, Maunder, and Dalton Minimums, as well [as] the Medieval and Modern Maximums," the latter of which they describe as "starting near 1900."

The mean sunspot number for the Wolf, Sporer and Maunder Minimums was 1.36. For the Oort and Dalton Minimums it was 25.05; while for the Medieval Maximum it was 53.00, and for the Modern Maximum it was 57.54. Compared to the average of the Wolf, Sporer and Maunder Minimums, therefore, the mean sunspot number of the Oort and Dalton Minimums was 18.42 times greater; while that of the Medieval Maximum was 38.97 times greater, and that of the Modern Maximum was 42.31 times greater. Similar strength ratios for the solar radio flux were 1.41, 1.89 and 1.97, respectively, while for the solar wind velocity the corresponding ratios were 1.05, 1.10 and 1.11, and for the southward component of the interplanetary magnetic field they were 1.70, 2.54 and 2.67.

Interestingly, both the Medieval and Modern Maximums in sunspot number and solar variability parameters stand out head and shoulders above all other periods of the past thousand years, with the Modern Maximum slightly besting the Medieval Maximum. Due to the many empirical evidences for climate modulation by solar variability, therefore, it is only to be expected, on this basis, that current temperatures might be higher than at any other time during the past millennium. Since other factors also come into play, however, and since the Medieval and Modern Maximums were not all that different, this conclusion may not be precisely correct. In any event, the observations of this study suggest no need whatsoever for invoking variations in the air's CO2 content as a cause of temperature variations during any period of the past thousand years.

Viau et al. (2002) analyzed a set of 3,076 14C dates from the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America. The results of their statistical analyses indicated there were nine millennial-scale oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1650 years were recorded in the pollen records. The most recent of the vegetation transitions was centered at approximately 600 years BP. This event, in their words, "culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago." Prior to that event, a major transition that began approximately 1600 years BP represents the climatic amelioration that "culminat[ed] in the maximum warming of the Medieval Warm Period 1000 years ago." And so it goes, on back through the Holocene and into the preceding late glacial period, with the times of all major pollen transitions being "consistent with ice and marine records."

According to Viau et al., "the large-scale nature of these transitions and the fact that they are found in different proxies confirms the hypothesis that Holocene and late glacial climate variations of millennial-scale were abrupt transitions between climatic regimes as the atmosphere-ocean system reorganized in response to some forcing." In addition, they say that "although several mechanisms for such natural forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents," and they add that "these transitions are identifiable across North America and presumably the world."

Xu et al. (2002) studied plant cellulose 18O variations in cores retrieved from peat deposits west of Hongyuan County at the northeastern edge of the Qinghai-Tibetan Plateau in China. Following the demise of the Roman Warm Period, their data revealed three consistently cold events that were centered at approximately AD 500, 700 and 900, during the Dark Ages Cold Period. Then, from 1100-1300, they report that "the δ18O of Hongyuan peat cellulose increased, consistent with that of Jinchuan peat cellulose and corresponding to the 'Medieval Warm Period'," after which they observed the three especially cold parts of the 'Little Ice Age,' i.e., 1370-1400, 1550-1610 and 1780-1880. Last of all, power spectrum analyses of their data revealed periodicities of 79, 88 and 123-127 years, which led the researchers to conclude "that the main driving force of Hongyuan climate change is from solar activities."

In a study of a precisely dated δ18O record with better than decadal resolution that they derived from a stalagmite recovered from Spannagel Cave in the Central Alps of Austria, Mangini et al. (2005) developed a highly-resolved record of temperature at approximately 2500 meters above sea level over the past 2000 years, based on a transfer function they derived from a comparison of their δ18O data with the reconstructed temperature history of post-1500 Europe that was developed by Luterbacher et al. (2004).

The lowest temperatures of the past two millennia, according to the new record, occurred during the Little Ice Age (AD 1400-1850), while the highest temperatures were found in the Medieval Warm Period (MWP: AD 800-1300). Furthermore, Mangini et al. say that the highest temperatures of the MWP were "slightly higher than those of the top section of the stalagmite (AD 1950) and higher than the present-day temperature." In fact, at three different points during the MWP, their data indicate temperature spikes in excess of 1C above present (1995-1998) temperatures.

Mangini et al. additionally report that their temperature reconstruction compares well with reconstructions developed from Greenland ice cores (Muller and Gordon, 2000), Bermuda Rise ocean-bottom sediments (Keigwin, 1996), and glacier tongue advances and retreats in the Alps (Holzhauser, 1997; Wanner et al., 2000), as well as with the Northern Hemispheric temperature reconstruction of Moberg et al. (2005). Considered together, they say these several data sets "indicate that the MWP was a climatically distinct period in the Northern Hemisphere," emphasizing that "this conclusion is in strong contradiction to the temperature reconstruction by the IPCC, which only sees the last 100 years as a period of increased temperature during the last 2000 years."

In a second severe blow to IPCC dogma, Mangini et al. found "a high correlation between δ18O and δ14C, that reflects the amount of radiocarbon in the upper atmosphere," and they note that this correlation "suggests that solar variability was a major driver of climate in Central Europe during the past 2 millennia." In this regard, they report that "the maxima of δ18O coincide with solar minima (Dalton, Maunder, Sporer, Wolf, as well as with minima at around AD 700, 500 and 300)," and that "the coldest period between 1688 and 1698 coincided with the Maunder Minimum." Also, in a linear-model analysis of the percent of variance of their full temperature reconstruction that is individually explained by solar and CO2 forcing, they found that the impact of the sun was fully 279 times greater than that of the air's CO2 concentration, noting that "the flat evolution of CO2 during the first 19 centuries yields almost vanishing correlation coefficients with the temperature reconstructions."

In light of these several real-world observations, it would seem almost impossible to deny that there was indeed a Medieval Warm Period of vast geographical extent, that it was at least as warm as the Current Warm Period (and probably even warmer), and that it was caused by some aspect of solar activity. Hence, there is absolutely no need to invoke the historical increase in the air's CO2 content as a cause of the world's current warmth; the sun suffices nicely in this regard.

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Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus 52B: 985-992.

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.

Holzhauser, H. 1997. Fluctuations of the Grosser Aletsch Glacier and the Gorner Glacier during the last 3200 years: new results. In: Frenzel, B. (Ed.) Glacier Fluctuations During the Holocene. Fischer, Stuttgart, Germany, pp. 35-58.

Hong, Y.T., Jiang, H.B., Liu, T.S., Zhou, L.P., Beer, J., Li, H.D., Leng, X.T., Hong, B. and Qin, X.G. 2000. Response of climate to solar forcing recorded in a 6000-year δ18O time-series of Chinese peat cellulose. The Holocene 10: 1-7.

Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1503-1508.

Luterbacher, J., Dietrich, D., Xoplaki, E., Grosjean, M. and Wanner, H. 2004. European seasonal and annual temperature variability trends, and extremes since 1500. Science 303: 1499-1503.

Mangini, A., Spotl, C. and Verdes, P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a 18O stalagmite record. Earth and Planetary Science Letters 235: 741-751.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

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Last updated 31 May 2006