Linsley et al. (2000) produced a proxy SST record for the period 1726 to 1997, which they obtained from a 3.5-meter coral core retrieved from the southwest side of Rarotonga, located at 21.5°S and 159.5°W in the Cook Islands.  Their analysis revealed that SSTs in the vicinity of Rarotonga were at least 1.5°C warmer than they are today during a quarter-century period centered approximately on the year 1745.  Such natural warming, were it to occur over a similar time period today, would likely be deemed proof of
CO2-induced global warming by climate alarmists, when, of course, it obviously would not be.

Winter et al. (2000) likewise examined proxy SSTs from north of the equator, which they derived from oxygen isotope data obtained from coral skeletons of Montastrea faveolata located on the southwestern shore of Puerto Rico.  When compared to current temperatures, SSTs for the periods 1700-1705, 1780-1785 and 1810-1814 were found to be significantly cooler.

Moving back further in time, other studies continue to document the natural variability of oceanic temperatures when conditions were both colder and warmer than at present.  Gagan et al. (1998), for example, reported that the temperature of the tropical ocean at the Great Barrier Reef about 5350 years ago was 1.2°C warmer than the mean that prevailed throughout the early 1990s.  Barber et al. (1999) found temperature drops of 1.5 to 3°C at marine and terrestrial sites around the northeastern North Atlantic Ocean approximately 8200 years ago; and Ruhlemann et al. (1999) report a significant warming in the western tropical Atlantic during two dramatic cooling events: Heinrich event H1 (16,900 to 15,400 years ago) and the younger Dryas event (12,900 to 11,600 years ago).

Perhaps the greatest demonstrations of natural oceanic temperature variability, however, come from the studies of McManus et al. (1999), Herbert et al. (2001) and Raymo et al. (1998).  In the study of McManus et al. (1999), the authors examined a half-million-year-old deep-sea sediment core in the eastern North Atlantic to infer changes in climate over the last five glacial-interglacial cycles.  They found a number of significant temperature oscillations throughout the record, but they were of much greater amplitude during glacial episodes than during interglacials, varying between 4 to 6°C during colder glacial times, and between 1 to 2°C during warmer interglacials.

Herbert et al. (2001) analyzed proxy sea surface temperatures over the past 550,000 years from several marine sediment cores obtained along the western coast of North America from the southern tip of the Baja Peninsula all the way up to Oregon.  Analysis of the SST data revealed that "the previous interglacial (isotope stage 5e) produced surface waters several degrees warmer than today," such that "waters as warm as those now at Santa Barbara occurred along the Oregon margin."  Furthermore, from the SST histories presented in their paper, it can be seen that SSTs for this region during the current interglacial have not reached the warm peaks witnessed in all four of the preceding interglacial periods, falling short by a margin of 1 to 4°C.

Lastly, Raymo et al. (1998) examined the characteristics of an ocean sediment core obtained from a water depth of nearly 2,000 meters at a site south of Iceland spanning a time interval of one million years.  They found that millennial-scale climate oscillations similar in character and timing to the Dansgaard-Oeschger cycles of the most recent glacial epoch were occurring in this region well over one million years ago, leading them to conclude that millennial-scale climate oscillations "may be a pervasive and long-term characteristic of Earth’s climate, rather than just a feature of the strong glacial-interglacial cycles of the past 800,000 years."

In contemplating these findings, it is clear that neither the glacial nor the independent millennial-scale climate oscillations of the past million-plus years were driven by variations in atmospheric CO2 concentration.  Hence, there would appear to be little reason to attribute the observed warming of the past century or so to the concurrent increase in the air’s CO2 content or to expect that any further rise in the air's CO2 content would trigger significant warming in the future.  Nevertheless, many people do just that.

In a detailed analysis of a vast array of oceanic temperatures spanning the globe and reaching from the surface down to a depth of 3000 meters, Levitus et al. (2000) found a 0.06°C temperature increase between the mid-1950s and mid-1990s.  Because of the fact that the oceans of the globe have a combined mass some 2500 times greater than that of the atmosphere, this number, as small as it looks, is truly significant.  But is it correct?

Although their data extended back in time several years beyond the point at which they specified the warming to begin, Levitus et al. computed the linear trend in temperature between the lowest valley of their oscillating time series and its highest peak, ensuring that they would obtain the largest warming possible.  In addition, the strong oscillatory behavior of the oceanic temperature trend they uncovered all but insures that the next decade will be one of oceanic cooling.  Hence, over a moderately longer time period, stretching into both the past and future, global ocean warming would be computed to be much less than what Levitus et al. reported; and the extended length of record would make the rate of warming smaller still.  Yet in spite of these readily evident facts, NASA's James Hansen is quoted by Kerr (2000) as saying that the new ocean-warming data "imply that climate sensitivity is not at the low end of the spectrum" that has typically been considered plausible.

But the warped hype does not end with the magnitude of the warming; it continues with its cause.  Climate modeler Jerry Mahlman, for example, states – according to Kerr – that the study of Levitus et al. "adds credibility to the belief that most of the warming in the 20th century is anthropogenic."  Yet Levitus et al. clearly state that "we cannot partition the observed warming to an anthropogenic component or a component associated with natural variability."

Which brings one back to the subject of climate sensitivity.  To calculate such a parameter one must have values for both a climate forcing and a climate response.  And if one can't even identify the source of the forcing, much less its magnitude, it is clearly impossible to calculate a sensitivity.

So maybe the discovery of Levitus and colleagues wasn't the Holy Grail of current climatology after all; but it was a piece of the puzzle, and a good one at that.  Nevertheless, there are many additional pieces yet to be discovered; and even when they are all in hand, we will still have to fit them together.  Even so, some exuberance is in order; but we would do well to be more temperate in our evaluation of each new scientific finding related to global climate change.  Knowledge must precede wisdom; and we're still just scratching the surface of the prerequisite for what we really need.

References
Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D. and Gagnon, J.-M.  1999.  Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes.  Nature 400: 344-348.

Gagan, M.K., Ayliffe, L.K., Hopley, D., Cali, J.A., Mortimer, G.E., Chappell, J., McCulloch, M.T. and Head, M.J.  1998.  Temperature and surface-ocean water balance of the mid-Holocene tropical western Pacific.  Science 279: 1014-1017.

Herbert, T.D., Schuffert, J.D., Andreasen, D., Heusser, L., Lyle, M., Mix, A., Ravelo, A.C., Stott, L.D. and Herguera, J.C.  2001.  Collapse of the California Current during glacial maxima linked to climate change on land.  Science 293: 71-76.

Kerr, R.A.  2000.  Globe's "missing warming" found in the ocean.  Science 287: 2126-2127.

Levitus, S., Antonov, J.I., Boyer, T.P. and Stephens, C.  2000.  Warming of the world ocean.  Science 287: 2225-2229.

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.

McManus, J.F., Oppo, D.W. and Cullen, J.L.  1999.  A 0.5-million-year record of millennial-scale climate variability in the North Atlantic.  Science 283: 971-974.

Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J.  1998.  Millennial-scale climate instability during the early Pleistocene epoch.  Nature 392: 699-702.

Ruhlemann, C., Mulitza, S., Muller, P.J., Wefer, G. and Zahn, R.  1999.  Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation.  Nature 402: 511-514.

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.