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Ocean Temperatures (The Past Few Centuries) -- Summary
In order to understand the present -- and potentially predict the future -- it is helpful to have a correct understanding of the past; and nowhere is this more important than in the ongoing debate over the impact of anthropogenic CO2 emissions on global climate. In this summary, therefore, we briefly review what has been learned about this subject based on proxy sea surface temperature data pertaining to the past few centuries.

Asami et al. (2005) developed a 213-year (1787-2000) monthly-resolved time series of carbon and oxygen isotope data that they obtained from a coral core retrieved from a Porites labata colony located on the northwestern coast of Guam, where the colony had been exposed to open sea surface conditions over the entire period of its development. On the basis of this record, they determined that "the early 19th century (1801-1820) was the coolest in the past 210 years, which is consistent with sea surface temperature [SST] reconstructions derived from a δ18O coral 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)." But from that point on, they report that "the long-term δ18O coral trend is characterized by its overall depletion throughout the period," which is indicative of a gradual warming of approximately 0.75C.

This temperature history, from a tropical region of the globe, is essentially identical to the extratropical Northern Hemispheric temperature record of Esper et al. (2002), which stands in stark contrast to the IPCC-endorsed temperature reconstruction of Mann et al. (1998, 1999), which does not depict the existence of the Little Ice Age. However, this multi-century cool period is clearly manifest at the beginning of the Guam record, and it is evident at about the same time in the New Caledonia record. In addition, the Guam SST history also differs from the Mann et al. temperature history in that it depicts close to continuous warming from about 1815, just as the Esper et al. record does, whereas the Mann et al. record does not depict any warming until after 1910, or about a century later. What is more, we note that the 0.75C rise in temperature from the start of the warming until the end of the 20th century, although substantial in the Guam record, is not at all unusual, since it begins at one of the coldest points of the coldest multi-century period of the entire Holocene or current interglacial.

In another study from a tropical region, Winter et al. (2000) determined SSTs for the periods 1700-1705, 1780-1785 and 1810-1815 from a study of oxygen isotope data obtained from coral skeletons of Montastrea faveolata located on the southwestern shore of Puerto Rico, while similar isotope data that were obtained for the period 1983-1989 and contemporary SSTs that were directly measured at the University of Puerto Rico's marine station at La Parguera were used to calibrate the temperature reconstruction technique and provide a current baseline against which to compare the researchers' Little Ice Age results. This work revealed that the SSTs they derived were significantly cooler during the three Little Ice Age periods than they were at the time of their study. In fact, they say their results indicate that "the Caribbean experienced cooling during the Little Ice Age with temperature estimated to be at least [our italics] 2-3 cooler than found during the present decade."

These results augmented the accumulating evidence that the Little Ice Age was indeed real and widespread, in contradiction of the climate-alarmist attempt to rewrite climate history and relegate this significant cold period to something so small and localized that it did not even appear in the Mann et al. temperature reconstruction of the past millennium. The data presented by Winter et al., on the other hand, as well as the data contained in several papers they cited, led them to conclude that "the Little Ice Age may have been more global in extent than previously expected." In addition, it may have been much colder than previously believed; for they added that the cooling suggested by their data "represents about half of the sea surface temperature cooling recorded in Barbados corals during the Last Glacial Maximum." And such cooling -- equivalent to descending half the way into a full-fledged ice age -- is something that cannot readily be swept under the rug and remain unnoticed for long.

Dima et al. (2005) introduced their study of ocean temperatures by noting that previous investigations of a Rarotonga coral-based SST reconstruction from the Cook Islands in the South Pacific Ocean focused on documenting and interpreting decadal and interdecadal variability without separating distinct modes of variability within this frequency band (Linsley et al. 2000, 2004; Evans et al. 2001). Hence, they reanalyzed the original coral record using Singular Spectrum Analysis in an effort to determine the dominant periods of multi-decadal variability in the series over the period 1727-1996. The results of their analysis revealed two dominant multi-decadal cycles, with periods of about 25 and 80 years. These modes of variability were determined to be similar to multi-decadal modes found in the global SST field of Kaplan et al. (1998) for the period 1856-1996. The ~25-year cycle was found to be associated with the well-known Pacific Decadal Oscillation, whereas the ~80-year cycle was determined to be "almost identical" to a pattern of solar forcing found by Lohmann et al. (2004), which, according to Dima et al., "points to a possible solar origin" of this mode of SST variability. Consequently, the results of their study provide an intriguing glimpse into the cyclical world of oceanic climatic change, demonstrating the existence of two strong multi-decadal modes of SST variability that are clearly natural in origin. As a result, one must be extremely cautious in interpreting the significance of SST trends over the past several decades. Before they can be attributed to anthropogenic activities, they must have all known modes of natural variability removed from them.

Looking back on the study of Linsley et al. (2000), which was referenced by Dima et al., we find they retrieved a core of continuous coral from a massive colony of Porites lutea on the southwest side of Rarotonga in the Cook Islands, within which they measured Sr/Ca ratios on 1-mm sections spanning the entire core (representing 271 years of growth), as well as δ18O values at the same resolution from 1726 to 1770 and from 1950 to 1997, the latter of which intervals was used for calibration purposes and utilized Integrated Global Ocean Service System Products SST data. This analysis revealed the existence of a quarter-century period centered on about the year 1745 when SSTs in the vicinity of Rarotonga were at least 1.5C warmer than they are today.

The results of this study raise serious questions about the validity of the temperature history of Mann et al., which is being used as a lever to nudge the nations of the earth into accepting a planet-wide scheme for reducing anthropogenic CO2 emissions. This latter history is derived from but a few records, which by virtue of their paucity cannot be construed to represent the globe as a whole; and the study of Linsley et al. demonstrates that superb data from one previously-unsampled part of the planet tell a very different story from the Mann et al. hockeystick graph. It would seem only prudent, therefore, to delay judgment on this important matter until more long-term coral-derived temperature data are acquired. In the words of Cane and Evans (2000), who provided an interesting perspective on the Linsely et al. paper, "fewer than 100 comparable records should be enough to fill in the big picture" with respect to modes of decadal SST variability.

Working in another part of the tropical South Pacific Ocean a couple of years later -- in an attempt to obtain one of those much needed records -- Hendy et al. (2002) reconstructed an even longer 420-year SST history based on Sr/Ca measurements of several coral cores taken from massive Porites colonies in the central portion of Australia's Great Barrier Reef. The earliest portion of this region's reconstructed temperature history, from 1565 to about 1700, corresponds to the coldest period of the Little Ice Age as recorded in the Northern Hemisphere. Five-year blocks of mean Great Barrier Reef SSTs during this cold period were sometimes 0.5 to 1.0C or more below the region's long-term mean. Over the following century, however, South Pacific SSTs were much warmer, as were temperatures in the Northern Hemisphere, at least according to the original IPCC temperature history, which we believe was considerably more realistic than the reconstruction of Mann et al. In the South Pacific, in fact, SSTs during this period were consistently as warm as -- and many times even warmer than -- those of the early 1980s, where the coral record ended. Then, during the late 1800s, the South Pacific once again experienced colder conditions that coincided with the "last gasp" of the Little Ice Age in the Northern Hemisphere, after which the Current Warm Period made its presence felt in both regions.

These observations do two important things. First, the largely synchronous temperature trends of the South Pacific Ocean and Northern Hemisphere lend credence to our belief that the Little Ice Age was a truly global phenomenon and not -- as climate alarmists are prone to proclaim -- a minor regional anomaly of lands bordering on the North Atlantic Ocean. Second, the fact that the data of Hendy et al., as well as the data of Linsley et al., portray mid-18th century South Pacific SSTs as being equally as warm as -- or even warmer than -- the latter part of the 20th century lends credence to our belief that the climate of the modern world is in no way unusual, unnatural or unprecedented.

Enlarging on this thought, we note that the mid-18th century warmth of the tropical and subtropical South Pacific Ocean occurred at essentially the same time as the significant peak in Northern Hemispheric temperature that is strikingly evident in the data of Jones et al. (1998), which are reproduced in the paper of Hendy et al. This observation suggests to us that if the data of Hendy et al. and Linsley et al. are representative of the great expanse of Southern Hemispheric ocean, the mid-18th century mean temperature of the entire globe may well have been about the same as it is now.

If true -- and there is nothing standing in the way of marine scientists from proving or disproving this hypothesis than the conducting of more coral-based paleoclimate studies throughout the Southern Hemisphere -- this observation would likely spell the end of the CO2-induced global warming hysteria, which is so ardently propagated by climate alarmists. The atmosphere's CO2 concentration during this period of potentially equal-to-present warmth was only about 280 ppm, i.e., the baseline value that had prevailed for several centuries prior to the Industrial Revolution. And if the planet was as warm in the mid-18th century as it is now, especially with there having been so much less CO2 in the air during that earlier time period, there is no compelling reason to believe that today's warmth is due to the subsequent increase in the atmosphere's CO2 concentration.

In light of these observations, we urge all who have the capacity to encourage and/or conduct such crucial research to do so as swiftly as possible. As Cane and Evans (2000) noted with respect to the study of Linsley et al., "their temperature record is persuasive," and "more records of this length and quality would go a long way toward clarifying our picture of the modes of decadal variability." And, we would add, they would go a long way toward helping to settle the greatest environmental question ever to plague mankind.

Providing an even longer and different proxy temperature record from the Northern Hemisphere, Jacoby et al. (2004) extracted cores from century-old oak trees growing within one km of the Pacific Ocean on Kunashir Island (the southernmost large island in the Kurile Island chain belonging to Russia, which is located between the Sea of Okhotsk and the northwest Pacific Ocean) and developed them into a four-century tree-ring width index series that was correlated strongly with island summer air temperature. As the scientists who did the work describe it, "the recorded temperature data and the tree-ring data show similar correlation patterns with sea-surface temperatures of the North Pacific." More specifically, they found that "the tree-ring series explains more than 33% of the variance of the July-September Pacific Decadal Oscillation and has similar spectral properties, further supporting the concept of multidecadal variation or shifts in North Pacific climate, for four centuries."

Of even more interest to us, however, was the fact that the Kunashir June-September mean maximum temperature reconstruction exhibited no long-term trend (neither cooling nor warming) over the entire period from 1600 to 2000, nor did it show any net temperature change over the 20th century. Also of note, the peak warmth of the last hundred years occured right at the mid-century mark, after which temperatures decreased considerably to end up right about where they started the century. Consequently, the reconstructed long-term temperature history of Kunashir Island bears absolutely no resemblance to the IPCC-endorsed temperature record of Mann et al., which boasts a temperature increase from 1910 to the end of the record that its creators describe as being unprecedented over the entire past millennium. And with Kunashir temperatures being "strongly influenced by the surrounding sea-surface temperatures," as Jacoby et al. determined, we cannot help but wonder just how much more of the Pacific Ocean fails to follow the politically-correct temperature trend, while marching unfalteringly to the rhythmic beat of the Pacific Decadal Oscillation. Being faithful to the latter drummer, is it not strange that Kunashir temperatures are unfaithful to the former? Perhaps not, seeing the former is unfaithful to reality.

In conclusion, multi-century proxy SST data that have been obtained at various places throughout the world over the past couple of decades provide absolutely no evidence that can be construed to support the theory of CO2-induced global warming as framed by the world's climate alarmists.

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.

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.

Dima, M., Felis, T., Lohmann, G. and Rimbu, N. 2005. Distinct modes of bidecadal and multidecadal variability in a climate reconstruction of the last centuries from a South Pacific coral. Climate Dynamics 25: 329-336.

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.

Evans, M.N., Cane, M.A., Schrag, D.P., Kaplan, A., Linsley, B.K., Villalba, R. and Wellington, G.M. 2001. Support for tropically-driven Pacific decadal variability based on paleoproxy evidence. Geophysical Research Letters 28: 3689-3692.

Hendy, E.J., Gagan, M.K., Alibert, C.A., McCulloch, M.T., Lough, J.M. and Isdale, P.J. 2002. Abrupt decrease in tropical Pacific sea surface salinity at end of Little Ice Age. Science 295: 1511-1514.

Jacoby, G., Solomina, O., Frank, D., Eremenko, N. and D'Arrigo, R. 2004. Kunashir (Kuriles) oak 400-year reconstruction of temperature and relation to the Pacific Decadal Oscillation. Palaeogeography, Palaeoclimatology, Palaeoecology 209: 303-311.

Jones, P.D., Briffa, K.R., Barnett, T.P. and Tett, S.F.B. 1998. High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with general circulation model control-run temperatures. The Holocene 8: 455-471.

Kaplan, A., Cane, M.A., Kushnir, Y., Clement, A.C., Blumenthal, M.B. and Rajagopalan, B. 1998. Analyses of global sea surface temperature 1856-1991. Journal of Geophysical Research 103: 18,567-18,589.

Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction of solar irradiance since 1610: Implications for climate change. Geophysical Research Letters 22: 3195-3198.

Linsley, B.K., Wellington, G.M. and Schrag, D.P. 2000. Decadal sea surface temperature variability in the subtropical South Pacific from 1726 to 1997 A.D. Science 290: 1145-1148.

Linsley, B.K., Wellington, G.M., Schrag, D.P., Ren, L., Salinger, M.J. and Tudhope, A.W. 2004. Geochemical evidence from corals for changes in the amplitude and spatial pattern of South Pacific interdecadal climate variability over the last 300 years. Climate Dynamics 22: 1-11.

Lohmann, G., Rimbu, N. and Dima, M. 2004. Climate signature of solar irradiance variations: analysis of long-term instrumental, historical, and proxy data. International Journal of Climatology 24: 1045-1056.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Winter, A., Ishioroshi, H., Watanabe, T., Oba, T. and Christy, J. 2000. Caribbean sea surface temperatures: Two-to-three degrees cooler than present during the Little Ice Age. Geophysical Research Letters 27: 3365-3368.

Last updated 17 June 2009