What was done
The authors "narrow down" temporal constraints on the millennial-scale variability of climate evident in ice-core ð¹⁸O records by "determining statistically significant anomalies in the major ion series of the GISP2 ice core," after which they conduct "a process-oriented synthesis of proxy records from the Northern Hemisphere."

What was learned
With respect to the temporal relationships among various millennial-scale oscillations in Northern Hemispheric proxy climate records, the authors conclude that a "compelling case" can be made for their being virtually in-phase, based on (1) "the high degree of similarity in event sequences and structures over a very wide spatial domain," and (2) "the fact that our process-oriented synthesis highlights a consistent common theme of relative dominance shifts between winter-type and summer-type conditions, ranging all the way across the Northern Hemisphere from polar into monsoonal latitudes." These findings, they additionally note, "corroborate the in-phase relationship between climate variabilities in the high northern latitudes and the tropics suggested in Blunier et al. (1998) and Brook et al. (1999)."

On another note, the authors report that although individual cycles of the persistent climatic oscillation "appear to have different intensities and durations, a mean periodicity appears around ~1500 years (Mayewske et al., 1997; Van Kreveld et al., 2000; Alley et al., 2001)."  They further report that "this cycle seems independent from the global glaciation state (Mayewski et al., 1997; Bond et al., 1999)," and that "¹⁰Be and delta ¹⁴C records may imply a link with solar variability (Mayewski et al., 1997; Bond et al., 2001)."

What it means
The millennial-scale climatic oscillation that alternately brought the world the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age, and Modern Warm Period is also a persistent feature of glacial epochs; and being a common feature of both glacial and interglacial climates, it is clear it must be externally forced, most likely driven by cyclical variations in solar activity.  Hence, there is absolutely no need to invoke the historical rise in the air's CO2 content to explain the temperature rise of the past century or so that produced the Modern Warm Period.  That warming was simply the most recent expression of the persistent millennial-scale climatic oscillation that has been doing its thing for as far back in time as we can determine.

References
Alley, R.B., Anandakrishnan, S. and Jung, P.  2001.  Stochastic resonance in the North Atlantic.  Paleoceanography 16: 190-198.

Blunier, T., Chapellaz, J., Schwander, J., Dallenbach, A., Stauffer, B., Stocker, T.F., Raynaud, D., Jouzel, J., Clausen, H.B., Hammer, C.U. and Johnsen, S.J.  1998.  Asynchrony of Antarctic and Greenland climate change during the last glacial period.  Nature 394: 739-743.

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.

Bond, G.C., Showers, W., Elliot, M., Evans, M., Lotti, R., Hajdas, I., Bonani, G. and Johnson, S.  1999.  The North Atlantic's 1-2kyr climate rhythm: relation to Heinrich events, Dansgaard/Oeschger cycles and the little ice age. In: Clark, P.U., Webb, R.S. and Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales.  American Geophysical Union Geophysical Monographs 112: 35-58.

Brook, E.J., Harder, S., Severinghaus, J. and Bender, M.  1999.  Atmospheric methane and millennial-scale climate change. In: Clark, P.U., Webb, R.S. and Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales.  American Geophysical Union Geophysical Monographs 112: 165-175.

Van Kreveld, S., Sarnthein, M., Erlenkeuser, H. Grootes, P., Jung, S., Nadeau, M.J., Pflaumann, U. and Voelker, A.  2000.  Potential links between surging ice sheets, circulation changes, and the Dansgaard-Oeschger cycles in the Irminger Sea, 60-18 kyr.  Paleoceanography 15: 425-442.