Early-warning indicators of Atlantic Meridional Overturning Circulation (AMOC) collapse. a, SST-based AMOC indices and fitted fixed point of a conceptual AMOC model. b, Salinity-based AMOC indices and fitted fixed point of a conceptual AMOC model. b,c, The restoring rate λ of the SST-based AMOC indices, estimated under the assumption of autocorrelated noise. d, The restoring rate λ of the salinity-based AMOC indices, estimated under the assumption of autocorrelated noise. e, Same as (c) but for the variance. f, Same as (d) but for the variance. g, Same as (c) but for the AC1. h, Same as (d) but for the AC1. The dashed lines indicate the linear trends of the three early-warning indicators, with p-values given in the legends. Values for each sliding window are plotted at the centre point of that window. Data for the first and the last w/2 = 35 yr are omitted because no full time windows to estimate the different early-warning indicators are available there. Graphic: Boers, 2021 / Nature Climate Change
Early-warning indicators of Atlantic Meridional Overturning Circulation (AMOC) collapse. a, SST-based AMOC indices and fitted fixed point of a conceptual AMOC model. b, Salinity-based AMOC indices and fitted fixed point of a conceptual AMOC model. b,c, The restoring rate λ of the SST-based AMOC indices, estimated under the assumption of autocorrelated noise. d, The restoring rate λ of the salinity-based AMOC indices, estimated under the assumption of autocorrelated noise. e, Same as (c) but for the variance. f, Same as (d) but for the variance. g, Same as (c) but for the AC1. h, Same as (d) but for the AC1. The dashed lines indicate the linear trends of the three early-warning indicators, with p-values given in the legends. Values for each sliding window are plotted at the centre point of that window. Data for the first and the last w/2 = 35 yr are omitted because no full time windows to estimate the different early-warning indicators are available there. Graphic: Boers, 2021 / Nature Climate Change

By Ben Turner
6 August 2021

(LiveScience) – One of the most crucial ocean current systems for regulating the Northern Hemisphere’s climate could be on the verge of total collapse due to climate change, a new study has revealed.

The Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream and is responsible for moderating large parts of the world’s climate, has undergone “an almost complete loss of stability over the last century”, according to a new analysis. The currents work like a conveyor belt to transport warm, salty water northward from the tropics and cold water back south along the seafloor. This giant conveyor belt had already been shown to be at its weakest in more than a thousand years, but now it could be veering toward a total breakdown.

Such a collapse would have a disastrous impact on global weather systems, leading to sea-level rises in the Atlantic; greater cooling and more powerful storms across the Northern Hemisphere; and severe disruption to the rain that billions of people rely upon to grow crops in Africa, South America and India, according to the U.K.’s meteorological office.

The study author believes that this transition could occur rapidly after the ocean current system is driven beyond a critical tipping point.

“The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse,” author Niklas Boers, a researcher at the Potsdam Institute for Climate Impact Research in Germany, said in a statement.

Map showing trends of early-warning indicators of Atlantic Meridional Overturning Circulation (AMOC) collapse. a, Linear trends of the corrected restoring rate λ estimated from the HadISST dataset assuming autocorrelated noise. b, Same as (a) but for the EN4 salinity dataset. c, Linear trends of the variance estimated from the HadISST dataset. d, Same as (c) but for the EN4 salinity dataset. e, Linear trends of the AC1 estimated from the HadISST dataset. f, Same as (e) but for the EN4 salinity dataset. Note the high positive values in the northern Atlantic and the subpolar gyre region in particular for λ and AC1, but also in the southern Atlantic ocean where a salinity pileup has recently been associated with an AMOC slowdown. Graphic: Boers, 2021 / Nature Climate Change
Map showing trends of early-warning indicators of Atlantic Meridional Overturning Circulation (AMOC) collapse. a, Linear trends of the corrected restoring rate λ estimated from the HadISST dataset assuming autocorrelated noise. b, Same as (a) but for the EN4 salinity dataset. c, Linear trends of the variance estimated from the HadISST dataset. d, Same as (c) but for the EN4 salinity dataset. e, Linear trends of the AC1 estimated from the HadISST dataset. f, Same as (e) but for the EN4 salinity dataset. Note the high positive values in the northern Atlantic and the subpolar gyre region in particular for λ and AC1, but also in the southern Atlantic ocean where a salinity pileup has recently been associated with an AMOC slowdown. Graphic: Boers, 2021 / Nature Climate Change

Data taken from past analyses of Atlantic ice cores and other geological records show that the AMOC can exist in two stable states: a stronger, faster state that humans rely upon today and another that is much slower and weaker. According to Boers, the existence of the two states implies that “abrupt transitions between the two circulation modes are in principle possible.” 

This means that the current as we know it could weaken until it reaches a tipping point, flipping from the stronger state to the weaker one. This will rapidly transform climates across the Northern Hemisphere to be much less temperate than they are right now. […]

The critical threshold after which the collapse occurs could be reached much sooner than scientists first thought, according to Boers.

“I wouldn’t have expected that the excessive amounts of freshwater added in the course of the last century would already produce such a response in the overturning circulation,” Boers said in the statement. “We urgently need to reconcile our models with the presented observational evidence to assess how far from or how close to its critical threshold the AMOC really is.” [more]

Gulf Stream could be veering toward irreversible collapse, a new analysis warns


Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation

ABSTRACT: The Atlantic Meridional Overturning Circulation (AMOC), a major ocean current system transporting warm surface waters toward the northern Atlantic, has been suggested to exhibit two distinct modes of operation. A collapse from the currently attained strong to the weak mode would have severe impacts on the global climate system and further multi-stable Earth system components. Observations and recently suggested fingerprints of AMOC variability indicate a gradual weakening during the last decades, but estimates of the critical transition point remain uncertain. Here, a robust and general early-warning indicator for forthcoming critical transitions is introduced. Significant early-warning signals are found in eight independent AMOC indices, based on observational sea-surface temperature and salinity data from across the Atlantic Ocean basin. These results reveal spatially consistent empirical evidence that, in the course of the last century, the AMOC may have evolved from relatively stable conditions to a point close to a critical transition.

Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation