North Atlantic circulation is weaker today than it has been for over a thousand years, and leading climate change models could be overestimating its stability, according to a team of scientists led by UCL and Woods Hole Oceanographic Institution, US.
In the first comprehensive study of ocean-based records, published in Nature, scientists have observed a marked weakening of Atlantic circulation over the past 150 years. This weakening correlates with the end of the Little Ice Age, around 1850 AD, and the onset of the industrial revolution when glaciers and sea ice melted, causing an influx of freshwater.
“Our results suggest that when simulating historical climate events, leading climate change models are either not sensitive enough to changes in the natural environment, such as the influxes of freshwater, or they are not including all the relevant processes.”
“Given that climate models do not fully capture the events that we are reporting, we have to ask: what does this mean for the future, and how does this relate to the changes expected with global warming?” said lead author, Dr David Thornalley (UCL Geography).
The Atlantic circulation, which is scientifically called the Atlantic Meridional Overturning Circulation (AMOC), is a powerful conveyor belt like system that carriers warm water north from the equator and sends cool water back down from the Arctic and Nordic seas. It is responsible for transporting warm water, and with it warm weather, to Western Europe and regulating water patterns important for marine life.
The AMOC is crucial to the world’s climate, and an abrupt slowdown could trigger various disruptions globally. These include a sudden rise in regional sea levels, changes in the position of major rainfall, arid climate zones and freezing winters across Western Europe.
The Atlantic current is also important for the ocean’s absorption of carbon dioxide, and a slowdown in its operation could lead to more carbon dioxide accumulating in the atmosphere, where it causes global warming.
To investigate variations in the AMOC, changes in the size of sediment grains deposited by a major deep-sea current were examined to infer past changes in the strength of circulation. The study also used the fact that the AMOC transports heat, to work out when the current was weak or strong by examining changes in the abundance of types of marine organisms that prefer warm and cold water.
The results are supported by another new study in the same issue of Nature, led by Levke Ceasar and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, Germany. This work looked at climate model data to confirm that sea-surface temperature patterns can be used as an indicator of AMOC strength. Then, using instrumental data of past sea-surface temperatures, they reveal that AMOC has been weakening more rapidly since 1950 in response to recent global warming.
The two new studies together provide complementary evidence to show that the present-day AMOC is exceptionally weak, offering both a longer-term perspective as well as detailed insight into recent decadal changes.
“Determining the future behaviour of the AMOC will depend on understanding just how sensitive the North Atlantic circulation is to external influences such as the influx of freshwater, and how these will vary or increase in the future.” concluded Dr Thornalley.
The research involved Cardiff University and the University of Reading, and was funded by a National Science Foundation grant, the Leverhulme Trust and the European Union’s Horizon 2020 Research and Innovation Framework Programme.
The research also forms part of a large European Union funded project, ATLAS, which is investigating how an altered AMOC may affect deep-sea ecosystems, such as cold water corals that provide important fish habitats and help recycle ocean nutrients.
Source : University College London