Water masses carry different ratios of two stable isotopes of nitrogen, depending on their origins. These signatures are recorded in the skeletons of deep-sea corals, and individual samples can be dated because the corals produce clear growth rings, providing a record of the currents flowing above and around the corals over time.

The results of the present study, which appeared in the Proceedings of the National Academy of Sciences, revealed that during the early 1970s, the North Atlantic Oscillation (NAO), a periodic variation of atmospheric pressure differences between the Azores and Iceland that influences oceanic currents in the North Atlantic, entered a ‘warm water mode’. In this mode, the influence of the cold Labrador current is diminished and the warmer Gulf Stream becomes dominant.

The change coincided with the recent rise in global temperatures and the researchers who carried out the study suspect that global warming is to blame. A similar analysis of fossil deep-sea corals from the same region revealed that the ocean currents had remained practically unchanged over the past 2,000 years, suggesting that a change in currents on this scale is unprecedented in recent times.

February 2011