Scientists have demonstrated that a huge circulation pattern in the Atlantic Ocean has slowed dramatically over the past decade, and there has been concern among oceanographers about the long-term stability of this system.
A new study led by the University of Washington (UW) has found that this transition is not a result of climate change, but is part of a regular cycle that will affect temperatures in the coming decades.
Study co-author Ka-Kit Tung is a UW professor of Applied Mathematics.
“Climate scientists have expected the Atlantic overturning circulation to decline long-term under global warming, but we only have direct measurements of its strength since April 2004. And the decline measured since then is 10 times larger than expected,” said Professor Tung.
“Many have focused on the fact that it’s declining very rapidly, and that if the trend continues it will go past a tipping point, bringing a catastrophe such as an ice age. It turns out that none of that is going to happen in the near future. The fast response may instead be part of a natural cycle and there are signs that the decline is already ending.”
The speed of the current determines how much surface heat is transferred deep into the ocean, and faster circulation means that more heat will be sent below. On the other hand, if the current slows, less heat will be stored and air temperatures will rise at a faster rate.
“The global climate models can project what’s going to happen long-term if carbon dioxide increases by a certain amount, but they currently lack the capability to predict surface warming in the next few decades, which requires a knowledge of how much the excess heat trapped by greenhouse gases is being absorbed by the oceans,” explained Professor Tung.
The new study argues that the Atlantic Meridional Overturning Circulation (AMOC) is not collapsing, but is simply transitioning from its fast phase to its slower phase, which has implications for heating at the ocean’s surface.
“We have about one cycle of observations at depth, so we do not know if it’s periodic, but based on the surface phenomena we think it’s very likely that it’s periodic.”
According to Professor Tung, recent measurements of density in the Labrador Sea suggest the cycle is beginning to shift. This means that the AMOC will not continue to transfer atmospheric heat into the North Atlantic in the years to come.
“The good news is the indicators show that this slowdown of the Atlantic overturning circulation is ending, and so we shouldn’t be alarmed that this current will collapse any time soon,” said Professor Tung. “The bad news is that surface temperatures are likely to start rising more quickly in the coming decades.”
The research is published in the journal Nature.