finding 13.2 : key-finding-13-2

The potential slowing of the Atlantic Meridional Overturning Circulation (AMOC; of which the Gulf Stream is one component)—as a result of increasing ocean heat content and freshwater driven buoyancy changes—could have dramatic climate feedbacks as the ocean absorbs less heat and CO2 from the atmosphere.4f0d16ed-f945-4947-bf38-8e6cc0bf3132 This slowing would also affect the climates of North America and Europe. Any slowing documented to date cannot be directly tied to anthropogenic forcing primarily due to lack of adequate observational data and to challenges in modeling ocean circulation changes. Under a higher scenario (RCP8.5) in CMIP5 simulations, the AMOC weakens over the 21st century by 12% to 54% (low confidence).



This finding is from chapter 13 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The increased focus on direct measurements of the AMOC should lead to a better understanding of 1) how it is changing and its variability by region, and 2) whether those changes are attributable to climate drivers through both model improvements and incorporation of those expanded observations into the models.

Description of evidence base: Investigations both through direct observations and models since 2013bc140b4c-c2d9-4d99-a684-5c054dc5134f have raised significant concerns about whether there is enough evidence to determine the existence of an overall slowdown in the AMOC. As a result, more robust international observational campaigns are underway currently to measure AMOC circulation. Direct observations have determined a statistically significant slowdown at the 95% confidence level at 26°N (off Florida; see Baringer et al. 2016fb5c7023-5a14-4a25-95f0-5426dcd725e2) but modeling studies constrained with observations cannot attribute this to anthropogenic forcing.49fdae12-6085-4009-b611-65ae5903471b The study143752b0-3899-41fe-963b-5b040305a5be which seemed to indicate broad-scale slowing has since been discounted due to its heavy reliance on sea surface temperature cooling as proxy for slowdown rather than actual direct observations. Since Rhein et al. 2013,bc140b4c-c2d9-4d99-a684-5c054dc5134f more observations have led to increased statistical confidence in the measurement of the AMOC. Current observation trends indicate the AMOC slowing down at the 95% confidence level at 26°N and 41°N but a more limited in situ estimate at 35°S, shows an increase in the AMOC.f8c888b3-0044-4515-90c5-bc2c70df8bc7 fb5c7023-5a14-4a25-95f0-5426dcd725e2 There is no one collection spot for AMOC-related data, but the U.S. Climate Variability and Predictability Program (US CLIVAR) has a U.S. AMOC priority focus area and a webpage with relevant data sites (https://usclivar.org/amoc/amoc-time-series).

The IPCC 2013 WG1 projections indicate a high likelihood of AMOC slowdown in the next 100 years, however overall understanding is limited by both a lack of direct observations (which is being remedied) and a lack of model skill to resolve deep ocean dynamics. As a result, this key finding was given an overall assessment of low confidence.

New information and remaining uncertainties: As noted, uncertainty about the overall trend of the AMOC is high given opposing trends in northern and southern ocean time series observations. Although earth system models do indicate a high likelihood of AMOC slowdown as a result of a warming, climate projections are subject to high uncertainty. This uncertainty stems from intermodel differences, internal variability that is different in each model, uncertainty in stratification changes, and most importantly uncertainty in both future freshwater input at high latitudes as well as the strength of the subpolar gyre circulation.

Provenance
This finding was derived from figure -.2: Confidence / Likelihood

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