Coral reefs and other marine corals are very sensitive to changes in ocean temperature and water content. This information is preserved in the corals even after they die.
Recent examination of deep sea coralsby biogeochemists Moritz Lehmann and Carsten Schubert demonstrates that the influence of the Labrador Current has been declining since the 1970s. As a result, the North Atlantic has switched suddenly to a warm water phase. It is the only change of this sort which has occurred in the past 2,000 years.
The North Atlantic Oscillation (NAO) is a weather system which influences ocean currents in the North Atlantic Ocean and determines the kind of winter that Europe will have. When the NAO is in its positive phase, the Gulf Stream dominates the North Atlantic Ocean. In the NAO's positive phase, the Gulf Stream creates a mass of salty, nutrient-rich water in the North Atlantic, off the continental shelf of Nova Scotia, at a temperature of 50 degrees Fahrenheit.
In the NAO's negative phase, the cold Labrador Current dominates. This cold subpolar water drops the temperature to 43 degrees Fahrenheit. It contains less salt and fewer nutrients than the Gulf Stream water. Its nitrates also carry a different ratio of nitrogen isotopes than the Gulf Stream. Nitrates in the Labrador Current have a higher ratio of 15N to 14N than nitrates which come from the Gulf Stream.
By measuring the ratio of the stable nitrogen isotopes 14N and 15N in deep sea corals of different ages, it is possible to determine which ocean current was dominant over the lifespan of each coral. By comparing the ratios in corals of known ages, it is possible to obtain exact dates. The ages of living corals can be obtained by counting annual growth rings, just the same as trees. (Pictures are at the bottom of this link.) The ages of dead corals can be obtained by carbon dating, and also by comparing their trapped 15N/14N ratio with younger corals.
These corals live hundreds of yards below the surface of the sea, so their only source of nitrates is the organic particles which sink down from higher oceanic waters. This means that their nitrates are influenced mostly by ocean currents and not by land conditions. The researchers also eliminated alternative bio-ecological and geochemical events which could have influenced the nitrogen ratio by measuring the 15N/14N ratio of specific amino acids in individual growth rings.
Lehman and Schubert found that deep sea corals younger than 40 years have a lower 15N/14N ratio than older deep sea corals, which means that the Gulf Stream has been dominating the region since the 1970s. The late 1970s coincides with the beginning of the recent rise in global temperatures. Prior to that time, the nitrogen isotope ratios in deep sea corals had remained steady for 2,000 years.