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Climate Oscillations (Millennial Variability: Hemispheric-to-Global-Scale) -- Summary
In addition to reports that deal with specific parts of the world, scientific publications periodically appear that address the millennial-scale variability of climate on a much wider hemispheric- or global-scale.  Hence, in this brief review we report on a few of the studies of this nature that have appeared in recent years.

Broecker et al. (1999) analyzed the results of a number of prior studies of different oceanic water-movement tracers, including heat content, salt content, radioactive carbon-14, chlorofluorocarbon-11, and the sum of phosphate plus oxygen, each of which factors tells us something about the origin of the water found at various places in the global ocean abyss.  They concluded that the planet's primary locations of oceanic deep water formation (the North Atlantic and Southern Oceans, believed by many to drive the global thermohaline circulation) have each been supplying about 15 sverdrups (15 x 106 m3/sec) of new deep water to this oceanic "conveyor belt" system for most of the past 800 years.  Over the last several decades, however, their analysis suggests that the contribution of the Southern Ocean has decreased to only about a third of its mean 800-year rate.

This conclusion, if correct, could have significant implications for earth's millennial-scale oscillation of climate.  Broecker et al. suggest, for example, that the Little Ice Age was a consequence of two related phenomena: more intense deep water formation in the Southern Ocean and reduced deep water formation in the North Atlantic.  If true, the slowdown in the rate of Southern Ocean deep water formation over the past several decades may well have been responsible for the concomitant warming of the Northern Hemisphere.

These ideas are incredibly important, for they suggest that the recent warming of the Northern Hemisphere may have been totally unrelated to the concurrent increase in the air's CO2 content; for the atmospheric burden of carbon dioxide varied but little over the entire climatic cycle that brought us out of the Dark Ages Cold Period and into the Medieval Warm Period, and out of the Medieval Warm Period and into the Little Ice Age, whereas there is evidence that the global thermohaline circulation may have been intimately involved with both of these temperature transitions.

In a closely related development, Broecker (2001) asks in the title of an enlightening Science paper: "Was the Medieval Warm Period Global?"  He answers in the affirmative, recounting the evidence for a series of climatic warmings, spaced at roughly 1500-year intervals, that were similar in magnitude and duration to the warming the earth has experienced since the end of the Little Ice Age; and he indicates that these warmings - all of which were obviously caused by something other than anthropogenic CO2 emissions - have occurred with great regularity throughout the Holocene, as well as throughout the preceding glacial period.

This evidence alone should be sufficient to demonstrate there is nothing unique or unusual about the modest temperature increase the earth has experienced since the end of the Little Ice Age.  Indeed, this run-of-the-mill warming, which began about 1860, is nothing more than a garden-variety climate change of the type that has occurred over and over again without any help from humanity.  Hence, it is truly disingenuous of climate alarmists to claim that this temperature increase bears the fingerprints of man.

Broecker next introduces us to the science of reconstructing surface air temperature histories from borehole temperature data, which technique reveals that the magnitude of the temperature drop over Greenland from the peak warmth of the Medieval Warm Period to the coldest part of the Little Ice Age was approximately 2C.  In addition, he notes that as many as six thousand borehole records from all continents of the world confirm that the earth was a significantly warmer place a thousand years ago than it is today.

In spite of this voluminous and widespread evidence for the reality and global extent of the millennial-scale climatic oscillation that produced all of the major multi-century climate anomalies of the past two millennia - the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and Modern Warm Period - there are still some scientists who refuse to recognize reality, opting instead to rewrite climatic history in an attempt to make us believe that these major global episodes of warmer and cooler temperature never existed (Mann et al., 1998, 1999; Mann and Jones, 2003).  However, a continuous stream of empirical studies testifies to the hollowness of their contention on a rather regular basis, one of the more influential of which is that of Esper et al. (2002).

So what did Esper et al. do?  In the simplest of terms, they employed an analysis technique that allows accurate long-term climatic trends to be derived from individual tree-ring series that are of much shorter duration than the potential climatic oscillation being studied; and they applied this technique to over 1200 tree-ring series derived from 14 different locations scattered over the extratropical region of the Northern Hemisphere.

Two separate chronologies were thus developed: one from trees that exhibited age trends that are weakly linear and one from trees with age trends that are more nonlinear.  The results, in their words, were "two nearly independent tree-ring chronologies covering the years 800-1990," which were "very similar over the past ~1200 years."  These tree-ring histories were then calibrated against Northern Hemispheric (0 to 90N) mean annual instrumental temperatures from the period 1856-1980 to make them compatible with the temperature reconstructions of Mann et al.

What do the results show?  The biggest difference between the Esper et al. and Mann et al. temperature histories is the degree to which the coolness of the Little Ice Age is expressed.  The Little Ice Age is much more evident in the record of Esper et al., and its significantly lower temperatures are what make the Medieval Warm Period stand out more dramatically in their temperature reconstruction.  Also, they note that "the warmest period covers the interval 950-1045, with the peak occurring around 990."  This finding, they say, "suggests that past comparisons of the Medieval Warm Period with the 20th-century warming back to the year 1000 have not included all of the Medieval Warm Period and, perhaps, not even its warmest interval."

In commenting on these findings, Briffa and Osborn (2002) make several interesting and important points.  First, they acknowledge that "the last millennium was much cooler than previously interpreted" and that "an early period of warmth in the late 10th and early 11th centuries is more pronounced than in previous large-scale reconstructions."  In fact, the Esper et al. record makes it abundantly clear that the peak warmth of the Medieval Warm Period was fully equivalent to the warmth of the present.

This fact reaffirms the point raised long ago by Idso (1988), i.e., that there is no need to invoke CO2-induced global warming as a cause of the planet's recovery from the global chill of the Little Ice Age.  "Since something other than atmospheric CO2 variability was ... clearly responsible for bringing the planet into the Little Ice Age," as he phrased it, "something other than atmospheric CO2 variability may just as well have brought the planet out of it."  And that something else, as suggested by Esper et al., is probably "the 1000- to 2000-year climate rhythm (1470 500 years) in the North Atlantic, which may be related to solar-forced changes in thermohaline circulation," as has recently been described in compelling detail by Bond et al. (2001).

Briffa and Osborn also note that Esper et al.'s record clearly shows that the warming of the 20th century was actually "a continuation of a trend that began at the start of the 19th century."  In addition, the Esper et al. record indicates that the Northern Hemisphere warmed in a consistent near-linear fashion over this entire 200-year period, contrary to the climate-alarmist claim of unprecedented warming over only the last century.  Hence, the new data do great damage to the claim that CO2-enhanced greenhouse warming is responsible for the temperature increase that brought us out of the Little Ice Age, since the increase in the atmosphere's CO2 concentration over this period was highly non-linear, rising by only 10 to 15 ppm over the 19th century, but by fully 70 to 75 ppm over the 20th century, with no analogous increase in the latter period's rate of warming.

Finally, Briffa and Osborn say that "we need to know why it was once so warm and then so cool, before we can say whether 21st-century warming is likely to be nearer to the top or the bottom of the latest IPCC [predicted temperature] range."  Actually, we probably already know the answer to this question: the extremes of warmth and coolness to which they refer were likely caused by "solar-forced changes in thermohaline circulation," as suggested by Esper et al. and described by Bond et al.  In any event, it is becoming ever more clear with each passing day that these significant climatic changes were not caused by changes in the air's CO2 content.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Irka Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

Briffa, K.R. and Osborn, T.J.  2002.  Blowing hot and cold.  Science 295: 2227-2228.

Broecker, W.S.  2001.  Was the Medieval Warm Period global?  Science 291: 1497-1499.

Broecker, W.S., Sutherland, S. and Peng, T.-H.  1999.  A possible 20th-century slowdown of Southern Ocean deep water formation.  Science 286: 1132-1135.

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.

Idso, S.B.  1988.  Greenhouse warming or Little Ice Age demise: A critical problem for climatology.  Theoretical and Applied Climatology 39: 54-56.

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.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.