Reference
Martrat, B., Grimalt, J.O., Lopez-Martinez, C., Cacho, I., Sierro, F.J., Flores, J.A., Zhan, R., Canals, M., Curtis, J.H. and Hodell, D.A. 2004. Abrupt temperature changes in the western Mediterranean over the past 250,000 years. Science 306: 1762-1765.
Background
Ruddiman et al. (2005) contend that "ice-core evidence from previous interglaciations indicates that forcing by orbital-scale changes in solar radiation and greenhouse-gas concentrations should have driven earth's climate significantly toward glacial conditions during the last several thousand years," and that "the hypothesized reason most of this cooling did not occur is that humans intervened in the natural operation of the climate system by adding significant amounts of CO2 and CH4 to the atmosphere, thereby offsetting most of the natural cooling [that otherwise would have occurred] and fortuitously producing the climatic stability of the last several thousand years."
What was done
In a study of a continuous high-resolution Western Mediterranean sea surface temperature (SST) alkenone record spanning the past 250,000 years, Martrat et al. uncovered evidence that could be interpreted to suggest much the same thing.
What was learned
Throughout marine isotope stages 5 and 7 encompassing the prior two interglacial periods, the pattern has been for rapid warmings to occur, followed by gradual and then more rapid coolings, which are abruptly terminated by rapid warmings that set the stage for the cycle to be repeated.
What it means
The fact that Martrat et al. discovered that the entire Holocene to date "shows a stable SST trend similar to those in previous interstadial stages, tending toward progressively cooler climate conditions," suggests, in their words, that "the next bifurcation of the climate system may appear as an extremely intense cooling if the future natural climate is going to develop as an analog of some of the preceding warm periods." Since this has indeed been the pattern in the past, there is no reason not to expect it to recur in the future, unless the overdue-glaciation hypothesis of Ruddiman et al. is correct, which suggests that even if the world's climate alarmists are correct in imputing great strength to the greenhouse effect of CO2, we ought not interfere with that phenomenon, as it may be what is keeping us from experiencing a far worse fate than anything the world's climate alarmists have yet imagined.
Reference
Ruddiman, W.F., Vavrus, S.J. and Kutzbach, J.E. 2005. A test of the overdue-glaciation hypothesis. Quaternary Science Reviews 24: 1-10.