What was done
The authors derived a high-temporal-resolution record of sea surface temperature from the western tropical North Atlantic Ocean, spanning the past 29,000 years, from measurements of temperature-sensitive alkenone unsaturation in organic matter contained in a sediment core obtained from a water depth of 1300 m southeast of the island of Grenada.  They compared this temperature record with other temperature records from around the world and with a high-resolution record of Cd/Ca ratios in benthic foraminifera from the Bermuda rise, which provides a measure of the strength of the thermohaline circulation there.

What was learned
The data from the Bermuda rise indicate that during two dramatic cooling events of this period - Heinrich event H1 (16,900 to 15,400 years ago) and the Younger Dryas event (12,900 to 11,600 years ago) - there was a significant slowdown of North Atlantic Deep Water formation, which is believed to have been caused by the injection of large volumes of fresh water into the northern North Atlantic that resulted in decreased sea surface salinity there and, hence, diminished deep convective overturning of the local ocean waters.  Without a strong driving force behind it, the planet's thermohaline circulation was thereby also significantly reduced, leading to the above-named cooling events in the region of the northern North Atlantic, when Gulf Stream-derived warmth failed to reach that part of the world, as well as cooling off northwest Africa, in response to southward advection of cold surface water with the Canary Current.

In the western tropical Atlantic, on the other hand, the authors' high-resolution temperature record indicated significant contemporaneous warming, which would also be expected to occur as a consequence of reduced thermohaline flow; and they noted that Antarctic temperature records showed warming during the same time periods that the Arctic regions of the planet cooled.  Hence, they concluded that the out-of-phase millennial-scale warmings and coolings of these Northern and Southern Hemispheric portions of the planet are controlled by the world-circling thermohaline circulation, rapid changes in the strength of which would seem to have the ability to induce rapid shifts in climate.

What it means
Since, in the words of the authors, "atmospheric greenhouse forcing should cause roughly synchronous global temperature changes," their observations of "asynchronous coupling of Arctic and Antarctic temperatures as deduced from the Greenland and Antarctic ice-core records" would suggest that variations in the planet's thermohaline circulatory system have been the major force for hemispheric climate change on millennial and shorter time scales, as opposed to changes in atmospheric greenhouse gases.  Hence, the world still awaits evidence that changes in the CO2 concentration of the atmosphere of the degree experienced over the past century or so have any influence at all on the climate of the earth or any parts of it.