Oppo et al. (1998) studied sediments from Ocean Drilling Project site 980 on the Feni Drift (55.5°N, 14.7°W, 2179 m below sea level).  Working with a core covering the period from 500,000 to 340,000 years ago, they analyzed ð¹⁸O and ¹³C data derived from benthic foraminifera and ð¹⁸O data obtained from planktonic foraminifera to develop histories of deep water circulation and sea surface temperature.  In doing so, they discovered a number of persistent climatic oscillations with periods of 6000, 2600, 1800 and 1400 years that traversed the entire length of their sediment core record, extending through glacial and interglacial periods alike.  They also cited evidence from several other studies that have revealed similar climatic oscillations throughout the last glaciation and deglaciation, the Holocene, and even the early Pleistocene.  These sea surface temperature variations, which were found to be in phase with deep ocean circulation changes, were greatest during periods of ice sheet growth and disintegration (4 to 4.5°C), intermediate during glacial maxima (3°C), and least during warm interglacial periods (0.5 to 1°C).

Raymo et al. (1998) studied various physical and chemical characteristics of an ocean sediment core obtained from a water depth of nearly 2,000 meters at a site south of Iceland.  They found that millennial-scale oscillations of climate were occurring well over one million years ago in a region of the North Atlantic that has been shown to strongly influence circum-Atlantic, and possibly global, climate.  These oscillations appeared to be similar in character and timing to the Dansgaard-Oeschger cycles of the most recent glacial epoch.  What is more, because the climate of the early Pleistocene was too warm to support the growth and development of large ice sheets characteristic of the late Pleistocene, and because similar millennial-scale climate oscillations are evident in both time periods, Raymo et al. suggest 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."

McManus et al. (1999) examined a half-million-year-old deep-sea sediment core in the eastern North Atlantic in a project designed to infer changes in climate over the last five glacial-interglacial cycles.  Significant temperature oscillations on a millennial time scale were noted throughout the record, but they were of much greater amplitude during glacial episodes.  Sea-surface temperature, for example, oscillated with an amplitude of 1 to 2°C during warm interglacials, but varied by as much as 4 to 6°C during colder glacial times.  The scientists concluded that climatic "variability on millennial time scales has thus been the rule, rather than the exception."

Bianchi and McCave (1999) analyzed grain-sizes of deep-sea sediment cores extracted from the northeast Atlantic Ocean that covered the last 11,000 years.  They demonstrated that the cores' sediment grain-size was related to the flow rate of the thermohaline circulation in this region, which in turn was shown to be related to well-known climatic events in Europe.  Of particular note was a climatic cycle running throughout the record with a periodicity of about 1,500 years.  Its millennial-scale oscillations, according to the two scientists, were comparable to the Little Ice Age and Medieval War Period, and were, in their opinion, a "recurrent feature of earlier parts of Holocene climatic history."

At the turn of the millennium, Keigwin and Boyle (2000) briefly reviewed what was known about the millennial-scale oscillation of earth's climate that is evident in proxy climate data pertaining to the last deglaciation and which has continued with reduced amplitude through the Holocene.  They also described its association with concomitant changes (demonstrable during the last deglaciation but tenuous during the Holocene) in the thermohaline circulation of the North Atlantic Ocean, noting that the Little Ice Age (LIA) was the most recent cold phase of this persistent climatic phenomenon that may have been induced by variations in the production rate of North Atlantic Deep Water.  In addition, they report that "mounting evidence indicates that the LIA was a global event, and that its onset was synchronous within a few years in both Greenland and Antarctica."  In the Northern Hemishpere, this cold run of weather was expressed as a 1°C cooling between approximately 1500 and 1900 AD, with a cooling of about 1.7°C in Greenland.

What is ultimately responsible for this approximate 1500-year cycle of global climate change that has been intensely studied in the region of the North Atlantic Ocean and demonstrated to prevail throughout glacial and interglacial periods alike?  This is the question Bond et al. (2001) set out to answer in a study of ice-rafted debris found in three North Atlantic deep-sea sediment cores and cosmogenic nuclides sequestered in the Greenland ice cap (¹⁰Be) and Northern Hemispheric tree rings (¹⁴C).

Based on arduous analyses of deep-sea sediment cores that yielded the variable-with-depth amounts of three proven proxies for the prior presence of overlying drift-ice, the scientists were able to discern and, with the help of an accelerator mass spectrometer, date a number of recurring alternate periods of relative cold and warmth that wended their way throughout the entire 12,000-year expanse of the Holocene.  The mean duration of the several complete climatic cycles thus delineated was 1340 years, the cold and warm nodes of the latter of which oscillations, in the words of Bond et al., were "broadly correlative with the so called 'Little Ice Age' and 'Medieval Warm Period'."

The signal accomplishment of the scientists' study was the linking of these millennial-scale climate oscillations - and their imbedded centennial-scale oscillations - with similar-scale oscillations in cosmogenic nuclide production, which are known to be driven by contemporaneous oscillations in the energy output of the sun.  In fact, Bond et al. were able to report that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum."  In light of this observation they concluded that "a solar influence on climate of the magnitude and consistency implied by our evidence could not have been confined to the North Atlantic," suggesting that the cyclical climatic effects of the variable solar inferno are likely experienced throughout the world.

At this point of their paper, the international team of scientists had pretty much verified a number of things we had regularly reported on our website over the preceding years, i.e., that in spite of the contrary claims of a host of climate alarmists, the Little Ice Age and Medieval Warm Period were (1) real, (2) global, (3) solar-induced, and (4) but the latest examples of uninterrupted alternating intervals of relative cold and warmth that stretch back in time through glacial and interglacial periods alike.

Because these several subjects are of such great significance, particularly to the global warming debate that currently rages over the climate model-predicted consequences of anthropogenic CO2 emissions, Bond and his band of researchers went on to cite additional evidence in support of the implications of their work.  With respect to the global extent of the climatic impact of the solar radiation variations they detected (topics 2 and 3 above, with 1 implied), they made explicit reference to confirmatory studies conducted in Scandinavia, Greenland, the Netherlands, the Faroe Islands, Oman, the Sargasso Sea, coastal West Africa, the Cariaco Basin, equatorial East Africa, and the Yucatan Peninsula, demonstrating thereby that "the footprint of the solar impact on climate we have documented extend[s] from polar to tropical latitudes."  Also in support of topic 3, they noted that "the solar-climate links implied by our record are so dominant over the last 12,000 years … it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the LIA could not have been a coincidence," further noting that their findings support previous suggestions that both the Little Ice Age and Medieval Warm Period "may have been partly or entirely linked to changes in solar irradiance."

Another point reiterated by Bond et al. is that the oscillations in drift-ice they observed "persist across the glacial termination and well into the last glaciation, suggesting that the cycle is a pervasive feature of the climate system."  At two of their coring sites, in fact, they identified a series of such cyclical variations that extended throughout all of the previous interglacial and were "strikingly similar to those of the Holocene."

So how do small changes in solar radiation inferred from cosmogenic nuclide variations bring about such significant and pervasive shifts in earth's global climate?  In answer to this question, which has long plagued proponents of a solar-climate link, Bond et al. describe a scenario whereby solar-induced changes high in the stratosphere are propagated downward through the atmosphere to the earth's surface, where they may provoke changes in North Atlantic Deep Water formation that alter the global thermohaline circulation.  In light of the plausibility of this scenario, they suggest that "the solar signals thus may have been transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint."

Concluding their remarkable paper, Bond et al. say the results of their study "demonstrate that the earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output," noting that their work thus "supports the presumption that solar variability will continue to influence climate in the future."  Hence, their study provides ample ammunition for defending the premise that the global warming of the past century or so was likely nothing more than the solar-mediated recovery of the earth from the chilly conditions of the Little Ice Age, and that any further warming of the planet that might occur would likely be nothing more than a continuation of the same solar-driven cycle that has successfully ushered the globe into the Modern Warm Period.

Just a year after the appearance of Bond et al.'s paper, the first formal workshop of the Ice Sheet-Ocean Interaction working group, which was set up within the International Marine Past Global Changes Study (IMAGES) program, was held in December of 2002 "to answer questions," in the words of Dokken et al. (2003), "concerning Heinrich events through an active interchange of ideas and information among the fields of glaciology, glacial geology, sendimentology, geochemistry, and paleoceanography."  As a prologue to their report of the more speculative ideas discussed at the meeting, Dokken et al. list a number of well established facts that harmonize with the findings of many of the papers discussed above.  First, "large temperature variations on land, in the air, and at the ocean surface, and highly variable flux of ice-rafted debris (IRD) delivered to the North Atlantic Ocean show that rapid climate fluctuations took place during the last glacial period."  Second, "these quasi-periodic, high-amplitude climate variations followed a sequence of events recognized as a rapid warming, followed by a phase of gradual cooling, and terminating with more rapid cooling and increased flux of IRD to the North Atlantic Ocean."  Third, each of these climatic oscillations, dubbed Dansgaard/Oechger (D/O) cycles "lasted 1500 years, and was followed by an almost identical sequence [where] approximately every fourth cycle culminated in a more pronounced cooling with a massive discharge of IRD into the North Atlantic Ocean [a Heinrich event that produces an ocean-bottom Heinrich layer of IRD] over an interval of 500 years."  Fourth, Heinrich events "seem to be part of an almost regular system; this suggests some external climate forcing."

The role of the North Atlantic branch of the global thermohaline circulation in this climate cycle was discussed contemporaneously by Ganopolski and Rahmstorf (2001, 2002) and Alley and Rahmstorf (2002), who suggested that it possesses two potential modes of operation during glacial times: a cold stable mode and a warm marginally unstable mode, the latter of which typically lasts for but a few hundred years.  The cold stable mode is characterized by deep-water formation south of Iceland; while the warm unstable mode is characterized by deep-water formation in the Nordic Seas and shares many characteristics with the circulatory mode of the current interglacial, although it is not quite as strong.

All else being equal, the cold stable mode of the ocean's thermohaline circulation would be expected to persist throughout an entire glacial period in their view.  However, as the three scientists (GRA) note, a weak real-world forcing with a periodicity on the order of 1500 years produces small cyclical variations in freshwater input to high northern latitudes at approximately the same periodicity; and these perturbations, when in the declining phase, often, but not always, initiate a transition to the warm unstable mode of thermohaline circulation, which includes a shift in the location of deep-water formation from south of Iceland to the Nordic Seas.  This new mode of circulation (warm unstable, which is accompanied by rapidly warming air temperatures) then persists for a few hundred years before reverting back (because of its inherent instability) to the cold stable mode of circulation (and its accompanying colder air temperatures).

An interesting aspect of this scenario is that the cyclical perturbation that leads to the change in the ocean's mode of thermohaline circulation is directly responsible for only a small fraction of the change in deep-water formation that is required to trigger the rapid warming events.  By applying the concept of stochastic resonance to the problem, however, Ganopolski and Rahmstorf (2002) have demonstrated the possibility that background noise in the system "triggers the events and thus amplifies the weak cycle into major climatic shifts with global reverberations."  Alternatively, Hunt and Malin (1998) have suggested that ice-load-induced earthquakes may have produced the six Heinrich events known to have occurred over the past 75,000 years; but this hypothesis does not explain their high degree of evenly-spaced repeatability, nor does it explain the more frequent occurrence of DO events or their Holocene analogues.

These several observations, some empirical and some theoretical, suggest a number of important things.  First, very weak forcing factors may well have the potential to produce large changes in earth's climate under certain circumstances; and one such forcing factor that presents itself to our minds within this context is solar variability, particularly as a consequence of the work of Bond et al.  This possibility has also presented itself to GRA.  Ganopolski and Rahmstorf (2001), for example, state that the low-amplitude cycle in freshwater forcing responsible for the large-amplitude cyclical changes in glacial climate could be "ultimately due to solar variability," while Alley and Rahmstorf (2002) say that "a possible cause could be a weak periodic variation in the output of the sun."

An interesting thing about the Holocene, however, is the fact that Ganopolski and Rahmstorf (2002) report that - in their model, at least - its climate "is not susceptible to regime switches by stochastic resonance with plausible parameter choices and even unrealistically large noise amplitudes, and neither is it in conceptual models."  Also, as they correctly report - and of even more significance, since the observation is based on real-world data - "there is no evidence for regime switches during the Holocene."

This, thus, is the other important lesson to be learned from these several studies: Holocene climate - both in theory and point of fact - is not susceptible to catastrophic changes.  Indeed, the Holocene is only known to have experienced much more modest climatic oscillations of the Medieval Warm Period-to-Little Ice Age-to-Modern Warm Period type, which are serious enough when in the cooling mode, but actually welcome when in the warming mode.

A final question worth considering within this context, especially in light of the specter of global warming-induced extinctions raised by Thompson et al. (2004), which is discussed in our editorial of 14 Jan 2004, is what happens to oceanic ecosystems during the climatic transitions that bring about significant millennial-scale climate regime shifts.  Do any sea-dwelling species go extinct?

Cannariato et al. (1999) investigated this question by studying the character, magnitude and speed of biotic responses of benthic foraminifera to millennial-scale climate oscillations.  In their analysis of an ocean sediment core retrieved from the Northeast Pacific, they detected a number of rapid climatic switches throughout the course of its 60,000-year record, representing periods of "extreme environmental variability," as they describe them; but they report that no extinctions were observed and that the benthic ecosystems "appear to be both resilient and robust in response to rapid and often extreme environmental conditions."  In fact, they note that major faunal turnovers often occurred within decades throughout the record "without extinction or speciation."

In conclusion, we are about as convinced as we can be that modest millennial-scale climatic oscillations are an unavoidable fact of life on earth, and that climate-alarmist predictions of catastrophic CO2-induced global warming are totally out of sync with reality.  Also, there is no question in our minds but that the historical increase in global temperature over the past two centuries, as described by Esper et al. (2002), is solar-induced and represents a return to climatic conditions akin to those of the Medieval Warm Period.  We welcome this modest climatic transition; and we welcome the contemporaneous increase in atmospheric CO2 concentration, which poses no threat of additional warming or loss of plant and animal species, but holds out the promise of significantly enhanced biological productivity.

References
Alley, R.B.S. and Rahmstorf, S.  2002.  Stochastic resonance in glacial climate.  EOS, Transactions, American Geophysical Union 83: 129, 135.

Bianchi, G.G. and McCave, I.N.  1999.  Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland.  Nature 397: 515-517.

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.

Cannariato, K.G., Kennett, J.P. and Behl, R.J.  1999.  Biotic response to late Quaternary rapid climate switches in Santa Barbara Basin: Ecological and evolutionary implications.  Geology 27: 63-66.

Dokken, T., Andrews, J., Hemming, S., Stokes, C. and Jansen, E.  2003.  Researchers discuss abrupt climate change: Ice sheets and oceans in action.  EOS, Transactions, American Geophysical Union 84: 189, 193.

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.

Ganopolski A. and Rahmstorf, S.  2001.  Rapid changes of glacial climate simulated in a coupled climate model.  Nature 409: 153-158.

Ganopolski, A. and Rahmstorf, S.  2002.  Abrupt glacial climate changes due to stochastic resonance.  Physical Review Letters 88: 038501.

Hunt, A.G. and Malin, P.E.  1998.  Possible Triggering of Heinrich Events by Ice-Load-Induced Earthquakes.  Nature 393: 155-158.

Keigwin, L.D. and Boyle, E.A.  2000.  Detecting Holocene changes in thermohaline circulation.  Proceedings of the National Academy of Sciences USA 97: 1343-1346.

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.

Oppo, D.W., McManus, J.F. and Cullen, J.L.  1998.  Abrupt climate events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments.  Science 279: 1335-1338.

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.

Thomas, C.D., Cameron, A., Green, R.E., Bakkenes, M., Beaumont, L.J., Collingham, Y.C., Erasmus, B.F.N., Ferreira de Siqueira, M., Grainger, A., Hannah, L., Hughes, L., Huntley, B., van Jaarsveld, A.S., Midgley, G.F., Miles, L., Ortega-Huerta, M.A., Peterson A.T., Phillips, O.L. and Williams, S.E.  2004.  Extinction risk from climate change.  Nature 427: 145-148.