An examination of deep sea corals has revealed drastic changes to ocean currents in the western North Atlantic since the 1970s.
The influence of the cold water Labrador Current, which is in periodic interchange with the warm Gulf Stream, has been decreasing continually in the last forty years.
The North Atlantic Oscillation (NAO), the periodic variation of atmospheric pressure difference between the Azores and Iceland, is
One of the oldest known weather systems in the world.
It not only dictates whether the winters in Europe will be cold and dry or wet and warm, but also influences the oceanic currents in the North Atlantic. On the continental shelf off Nova Scotia, it seems to control the interaction between different water masses.
Now, using new geochemical methods, an international team led by Moritz Lehmann of the University of Basel has shown that a drastic change to a warm water mode occurred in the western North Atlantic in the early 1970s. This change – the timing of which coincides with global warming – is unique in the last 2000 years.
The researchers made use of the fact that water masses carry different nitrogen isotopic signatures, depending on their origins. These signatures are recorded in the biomass of the deep-sea corals that feed on sinking organic particles from above.
It’s thus possible to reconstruct the oceanic current ratios over the last few decades by examining the corals’ annual growth rings.
The researchers were able to show a clear fall in the 15N/14N ratio since 1970, indicating that the role of the cold Labrador Current is becoming less important.
They say they can show that this hasn’t been caused by changes in the food chain by using component-specific nitrogen analyses of the corals.
An examination of fossil deep sea corals from the same region confirms that the nitrogen isotope ratios – and thus the oceanic current situation – have remained practically unchanged over the past 2000 years – until the 1970s.
The researchers suspect there is a direct connection between the changes in the oceanic currents in the North Atlantic and manmade global warming.