Morellon, M., Valero-Garces, B., Gonzalez-Samperiz, P., Vegas-Vilarrubia, T., Rubio, E., Rieradevall, M., Delgado-Huertas, A., Mata, P., Romero, O., Engstrom, D.R., Lopez-Vicente, M., Navas, A. and Soto, J. 2011. Climate changes and human activities recorded in the sediments of Lake Estanya (NE Spain) during the Medieval Warm Period and Little Ice Age. Journal of Paleolimnology 46: 423-452.
The authors write that "in the context of present-day global warming, there is increased interest in documenting climate variability during the last millennium," since "it is crucial to reconstruct pre-industrial conditions to discriminate anthropogenic components (i.e., greenhouse gases, land-use changes) from natural forcings (i.e., solar variability, volcanic emissions)."
What was done
Morellon et al. conducted a multi-proxy study of several short sediment cores they recovered from Lake Estanya (42°02'N, 0°32'E) in the Pre-Pyrenean Ranges of northeast Spain, which "provides a detailed record of the complex environmental, hydrological and anthropogenic interactions occurring in the area since medieval times." More specifically, they say that "the integration of sedimentary facies, elemental and isotopic geochemistry, and biological proxies (diatoms, chironomids and pollen), together with a robust chronological control, provided by AMS radiocarbon dating and 210Pb and 137Cs radiometric techniques, enabled precise reconstruction of the main phases of environmental change, associated with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the industrial era."
What was learned
The thirteen researchers identified the MWP as occurring in their record from AD 1150 to 1300, noting that their pollen data reflect "warmer and drier conditions," in harmony with the higher temperatures of the Iberian Peninsula over the same time period that have been documented by Martinez-Cortizas et al. (1999), the higher temperatures of the Western Mediterranean region found by Taricco et al. (2008), and the global reconstructions of Crowley and Lowery (2000) and Osborn and Briffa (2006), which "clearly document warmer conditions from the twelfth to fourteenth centuries," which warmth, in the words of Morellon et al. is "likely related to increased solar irradiance (Bard et al., 2000), persistent La Niņa-like tropical Pacific conditions, a warm phase of the Atlantic Multidecadal Oscillation, and a more frequent positive phase of the North Atlantic Oscillation (Seager et al., 2007)."
Following hard on the heels of the MWP, Morellon et al. note the occurrence of the LIA, which they recognize as occurring from AD 1300 to 1850. And here they report that, on the Iberian Peninsula, "lower temperatures (Martinez-Cortizas et al., 1999) characterize this period," which "coincided with colder North Atlantic (Bond et al., 2001) and Mediterranean sea surface temperatures (Taricco et al., 2008) and a phase of mountain glacier advance (Wanner et al., 2008)." And following the LIA they identify the transition period of AD 1850-2004 that takes the region into the Current Warm Period.
In discussing all three of these distinctive periods, they say that "a comparison of the main hydrological transitions during the last 800 years in Lake Estanya and solar irradiance (Bard et al., 2000) reveals that lower lake levels dominated during periods of enhanced solar activity (MWP and post-1850 AD) and higher lake levels during periods of diminished solar activity (LIA)." And within the LIA, they note that periods of higher lake levels or evidence of increased water balance occurred during the solar minima of Wolf (AD 1282-1342), Sporer (AD 1460-1550), Maunder (AD 1645-1715) and Dalton (AD 1790-1830).
What it means
In light of these several observations it would appear that the multi-centennial climate oscillation uncovered by Morellon et al. has been driven by a similar oscillation in solar activity, as well as by multi-decadal solar activity fluctuations superimposed upon that longer-period oscillation. And these relationships suggest that there is no compelling need to attribute 20th-century global warming to the concomitant increase in the air's CO2 content. Natural variability appears quite capable of explaining it all.
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.
Crowley, T.J. and Lowery, T.S. 2000. How warm was the Medieval Warm Period? Ambio 29: 51-54.
Martinez-Cortizas, A., Pontevedra-Pombal, X., Garcia-Rodeja, E., Novoa-Muņoz, J.C. and Shotyk, W. 1999. Mercury in a Spanish peat bog: Archive of climate change and atmospheric metal deposition. Science 284: 939-942.
Osborn, T.J. and Briffa, K.R. 2006. The spatial extent of 20th-century warmth in the context of the past 1200 years. Science 311: 841-844.
Seager, R., Graham, N., Herweijer, C., Gordon, A.L., Kushnir, Y. and Cook, E. 2007. Blueprints for medieval hydroclimate. Quaternary Science Reviews 26: 2322-2336.
Taricco, C., Ghil, M. and Vivaldo, G. 2008. Two millennia of climate variability in the Central Mediterranean. Climate of the Past Discussions 4: 1089-1113.
Wanner, H., Beer, J., Butikofer, J., Crowley, T.J., Cubasch, U., Fluckiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J.O., Kuttel, M., Muller, S.A., Prentice, I.C., Solomina, O., Stocker, T.F., Tarasov, P., Wagner, M. and Widmann, M. 2008. Mid- to late Holocene climate change: an overview. Quaternary Science Reviews 27: 1791-1828.Reviewed 8 February 2012