The Weekend Wonk: Stefan Rahmstorf – Is the North Atlantic Near a Tipping Point?

Is the Atlantic Ocean circulation hashtag#AMOC approaching a tipping point? This lecture video is globally respected researcher Stefan Rahmstorf’s take after researching this topic for over 30 years, and receiving the Alfred Wegener Medal of the European Geosciences Union.

Science – Physics Based Early Warning Signal Shows that AMOC is on Tipping Course:

One of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC), which can potentially collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have so far not been investigated. Here, we show results of the first tipping event in the Community Earth System Model, including the large climate impacts of the collapse. Using these results, we develop a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. Reanalysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping.

Rahmstorf’s talk is based on this recent paper in Oceanography.

Stefan Rahmstorf – Is the Atlantic Overturning Circulation Approaching a Turning Point?

In 1751, the captain of an English slave-​trading ship made a historic discovery. While sailing at 25°N in the subtropical North Atlantic Ocean, Captain Henry Ellis lowered a “bucket sea-gauge,” devised and provided to him by the British clergyman Reverend Stephen Hales, through the warm surface waters into the deep. By means of a long rope and a system of valves, water from various depths could be brought up to the deck where its temperature was read from a built-in thermometer. To his surprise, Captain Ellis found that the deep water was icy cold.

He reported his findings to Reverend Hales in a letter: “The cold increased regularly, in proportion to the depths, till it descended to 3900 feet: from whence the mercury in the thermometer came up at 53 degrees (Fahrenheit); and tho’ I afterwards sunk it to the depth of 5346 feet, that is a mile and 66 feet, it came up no lower.”

These were the first ever recorded temperature measurements of the deep ocean. They revealed what is now known to be a fundamental and striking physical feature of the world ocean: deep water is always cold. The warm waters of the tropics and subtropics are confined to a thin layer at the surface; the heat of the sun does not slowly warm the depths during centuries or millennia as might be expected.

Ellis’s letter to Hales suggests he had no inkling of the far-reaching significance of his discovery. He wrote: “This experiment, which seem’d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate” (Ellis, 1751).

In fact, Ellis had struck upon the first indication of the ocean’s overturning circulation, the system of deep ocean currents that circulates cold waters of polar origin around the planet.

But it was not until several decades later, in 1797, that another Englishman, Count Rumford, published a correct explanation for Ellis’s “useful” discovery: “It appears to be extremely difficult, if not quite impossible, to account for this degree of cold at the bottom of the sea in the torrid zone, on any other supposition than that of cold currents from the poles; and the utility of these currents in tempering the excessive heats of these climates is too evident to require any illustration” (Thompson, 1797).

Now, over 200 years later, we have a reasonable understanding of the complex system of deep ocean circulation and, what Rumford found so evident, the role it plays in climate. However, some major puzzles remain that may be of fundamental importance to our future.