04/11/2018. The Atlantic overturning – one of Earth’s most important heat transport systems, pumping warm water northwards and cold water southwards – is weaker today than any time before in more than 1000 years. Sea surface temperature data analysis provides new evidence that this major ocean circulation has slowed down by roughly 15 percent since the middle of the 20th century, according to a study published in the highly renowned journal Nature by an international team of scientists. Human-made climate change is a prime suspect for these worrying observations.
“We detected a specific pattern of ocean cooling south of Greenland and unusual warming off the US coast – which is highly characteristic for a slowdown of the Atlantic overturning, also called the Gulf Stream System,” says lead-author Levke Caesar from the Potsdam Institute for Climate Impact Research (PIK). “It is practically like a fingerprint of a weakening of these ocean currents.” As the currents slow down, they bring less heat towards the north, causing widespread cooling of the northern Atlantic – the only ocean region that has cooled in the face of global warming. At the same time, the Gulf Stream shifts northwards and closer to shore and warms the waters along the northern half of the US Atlantic coast.
“That region has warmed faster than most other parts of the world ocean in recent decades,” says co-author Vincent Saba from the National Oceanic and Atmospheric Administration (NOAA) Laboratory in Princeton, USA. “This specific ocean temperature pattern has been projected by high-resolution computer simulations as a response to rising carbon dioxide levels in the atmosphere – now it has been confirmed by measurements.”
Measurements of sea surface temperatures confirm computer simulations
For decades, scientists have investigated the dynamics of the Atlantic overturning. Computer simulations generally predict that it will weaken in response to human-caused global warming. But whether this is already happening has so far been unclear, due to a lack of long-term direct current measurements. “The evidence we’re now able to provide is the most robust to date,” says Stefan Rahmstorf from the Potsdam Institute, who conceived the study. “We’ve analysed all the available sea surface temperature data sets, comprising data from the late 19th Century until the present.”