香港六合彩

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 including聽Dr David Thornalley (香港六合彩 Geography), with Woods Hole Oceanographic Institution, in Massachusetts US.

In the first comprehensive study of ocean-based records, published in聽Nature聽on 12 April, in which David is the lead author, scientists observe a marked weakening of Atlantic circulation over the past 150 years. This correlates with the end of the Little Ice Age around 1850 AD, and the onset of the effects of the industrial revolution, when glaciers and sea ice began melting to cause an influx of freshwater. It is believed that this has caused significant disturbance to ocean currents, and could have a dramatic impact on climates across North America and Western Europe.

As David comments,聽鈥淕iven 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?鈥�.

The Atlantic circulation is scientifically called the聽Atlantic Meridional Overturning Circulation (AMOC), and acts as a powerful conveyor belt carrying warm water north from the equator to the Arctic and Nordic seas. It is responsible for warming 聽Western Europe and regulating water patterns important for marine life.

An abrupt slowdown in the AMOC could trigger various global disruptions, including a sudden rise in sea levels, changes in the distribution of major rainfall, arid climate zones, and freezing winters across Western Europe. It is also important for the ocean鈥檚 absorption of carbon dioxide, and a slowdown could lead to more CO_2听accumulating in the atmosphere, where it causes global warming.

Variations in the AMOC were investigated through changes in the size of sediment grains deposited by a major deep-sea current, allowing past changes in the strength of circulation to be inferred. The study also worked out when the AMOC was weak or strong through the abundance of types of marine organisms that prefer warm and cold water.

The results are supported by other research, reported in the same issue of聽Nature, led by Levke Ceasar and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, in Germany. This study reveals that the AMOC has been weakening more rapidly since 1950 in response to recent global warming. The two new studies thus show that the present-day AMOC is exceptionally weak.

David concludes,聽鈥淒etermining 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.鈥�