Asami, R., Yamada, T., Iryu, Y., Quinn, T.M., Meyer, C.P. and Paulay, G. 2005. Interannual and decadal variability of the western Pacific sea surface condition for the years 1787-2000: Reconstruction based on stable isotope record from a Guam coral. Journal of Geophysical Research 110: 10.1029/2004JC002555.
What was done
The authors derived a monthly-resolved 213-year (1787-2000) time series of carbon and oxygen isotope variations that they measured in a 273-cm-long coral core retrieved from a Porites labata colony located on the northwestern coast of Guam (13°35'N, 144°50'E), where it had been exposed to open sea surface conditions over the entire period of its development.
What was learned
"On the basis of the Guam δ18Ocoral record," according to Asami et al., "the early 19th century (1801-1820) was the coolest in the past 210 years, which is consistent with SST reconstructions from a δ18Ocoral record from New Caledonia (Crowley et al., 1997)." This period, in their words, "was characterized by a decrease in solar irradiance (Lean et al., 1995; Crowley and Kim, 1996) and by a series of large volcanic eruptions in 1808-1809 and 1818-1822 (Crowley et al., 1997)." From that point on, they report that "the long-term δ18Ocoral trend is characterized by its overall depletion throughout the period," which is indicative of a gradual warming of approximately 0.75°C.
What it means
This temperature history, from a tropical region of the globe, is essentially identical to that of the extratropical Northern Hemispheric temperature record of Esper et al. (2002), which stands in stark contrast to the IPCC-endorsed record of Mann et al. (1998, 1999) - which is used to imply an unprecedented CO2-induced warming over the latter part of the 20th century - in that the Mann et al. record does not depict the existence of the Little Ice Age, which is clearly manifest at the beginning of the Guam coral record and at about the same time in the New Caledonia coral record. It also differs from the "hockeystick" record of Mann et al. in that it depicts essentially continuous warming from about 1815, just as the Esper et al. record does, whereas the Mann et al. record does not depict warming until after 1910, or about a century later. In addition, we note that a 0.75°C rise in temperature from the start of the warming until the end of the 20th century is not at all unusual, since it begins at one of the coldest points of the coldest multi-century period (the Little Ice Age) of the entire Holocene or current interglacial.
Why are these facts important? They are important because they indicate that the extreme cold of the early 1800s and the warming that followed it were not caused by changes in the air's CO2 concentration. Rather it was likely a reversal of the forces that produced the cold in the first place (Asami et al. mention a decrease in solar irradiance and large volcanic eruptions in this regard) that led to the subsequent warming, which represents nothing more nor less that the natural recovery of the earth from the global chill of one of the coldest portions of the coldest multi-century period of the past 10,000 years.
Crowley, T.J. and Kim, K.-Y. 1996. Comparison of proxy records of climate change and solar forcing. Geophysical Research Letters 23: 359-362.
Crowley, T.J., Quinn, T.M. and Taylor, F.W. 1997. Evidence for a volcanic cooling signal in a 335 year coral record from New Caledonia. Paleoceanography 12: 633-639.
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.
Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction of solar irradiance since 1610: Implications for climate change. Geophysical Research Letters 22: 3195-3198.
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Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures duing the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.Reviewed 12 October 2005