Millennial-Scale Climate Variability: It's the Norm!

Harada et al. (2012a), in their preface to a special issue of Deep-Sea Research II, briefly highlight what the issue has to say about "the causal relationships connecting atmospheric circulation, the sea surface environment, the intermediate-deep ocean circulation and the carbon cycle," a good understanding of which, in their words, "is crucial to clarifying the interactions and feedback mechanisms between the global climate system and the physics and climatology of the western North Pacific and its adjacent seas." And in so doing, they touch on several contributions that clearly reveal the normality of the millennial-scale cycling of climate that has alternately brought the Earth into and out of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age and the Current Warm Period.

As one example, the four editors of the special issue point to records from the central Okhotsk Sea that reveal "increases and decreases in primary production under warm and cold climatic conditions respectively," where they say that marine productivity "responded to a millennial timescale climate change," noting that "on a millennial timescale, reductions of primary productivity during the past 77,000 years were concurrent with cooling events associated with abrupt North Atlantic Heinrich events," citing Gobarenko et al.( 2012).

A second example is what they call the "strong connection between episodes of enhanced or reduced terrestrial plant content and millennial timescale climate change [that] is also evident in the Okhotsk Sea sediment record during the last deglaciation," where "the down-core profile of molecular lipids derived from terrestrial plants is linked to fluctuations of river discharge related to climate change," citing Seki et al. (2012).

Last of all, Harada et al. (2012b) show that "statistical multivariate analysis of a new compilation of alkenone-derived sea surface temperatures from the western North Pacific and the Japan and Okhotsk Seas reveals a coherent mode of millennial-scale variability that is closely linked to deglacial changes of the Atlantic Meridional Overturning Circulation." And they even cite the results of a couple of climate model studies (Chikamoto et al., 2012; Menviel et al., 2012) that yield results that are harmonious with the many palaeodata-driven studies of the special issue.

Taken together, these new studies, plus a host of other studies we have reviewed on our website (see Millennial-Scale Oscillations of Temperature in our Topical Archive), clearly demonstrate that the warming experienced over the past century or so is in no way unusual, unnatural or unprecedented, nor that it need to have been driven by anthropogenic CO₂ emissions. Earth's climate, for a very long time, has simply been doin' what comes naturally ... and doin' it well!

Additional References
Chikamoto, M.O., Menviel, L., Abe-Ouchi, A., Ohgaito, R., Timmermann, A., Okazaki, Y., Harada, N., Oka, A. and Mouchet, A. 2012. Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models. Deep-Sea Research II 61-64: 114-126.

Gorbarenko, A.S., Harada, N., Malakhov, I.M., Velivetskaya, A.T., Vasilenko, P.Y., Bosin, A.A., Derkachev, N.A., Goldberg, L.E. and Ignatie, A.V. 2012. Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350 kyr. Deep-Sea Research II 61-64: 73-84.

Harada, N., Sato, M., Seki, O., Timmermann, A., Moossen, H., Bendle, J., Nakamura, Y., Kimoto, K., Okazaki, Y., Nagashima, K., Gorbarenko, S.A., Ijiri, A., Nakatsuka, T., Menviel, L., Chikamoto, M.O., Abe-Ouchi, A. and Schouten, S. 2012a. Sea surface temperature changes in the Okhotsk Sea and adjacent North Pacific during the last glacial maximum and deglaciation. Deep-Sea Research II 61-64: 93-105.

Menviel, L., Timmermann, A., Timm, O.E., Mouchet, A., Abe-Ouchi, A., Chikamoto, M.O., Harada, N., Ohgaito, R. and Okazaki, Y. 2012. Removing the North Pacific halocline: effects on global climate, ocean circulation and the carbon cycle. Deep-Sea Research II 61-64: 106-113.

Seki, O., Harada, N., Sato, M., Kawamura, K., Ijiri, A. and Nakatsuka, T. 2012. Assessment for paleoclimatic utility of terrestrial biomarker records in the Okhotsk Sea sediments. Deep-Sea Research II 61-64: 85-92.