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.

What was done
Kerr (1998) discusses the work of Oppo et al. (1998), who 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 delta ¹⁸O and delta ¹³C from benthic foraminifera and delta ¹⁸O from planktonic foraminifera to develop histories of deep water circulation and sea surface temperature.

What was learned
Oppo et al. 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 times alike.  They also cited evidence from other studies that have revealed similar 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).

What it means
This study reveals that earth's climate oscillates in a fairly well-defined and repeatable manner on a number of different time scales.  Of particular importance to the planet's current residents is the fact that large climate excursions during interglacial times, such as the one in which we are now living, basically never happen.  This well-established observational fact must weigh heavily against the unverified theoretical predictions of imminent major CO2-induced global warming.