September 2025 | Oceanography
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X. Chen and Tung, 2023). Paleoceanographic records could play an
important role in determining the spatial signature of AMOC vari-
ability by examining times with large apparent changes in AMOC
strength (e.g., HS1). This effort could then inform the modern
observations and evaluations of model results.
High temporal resolution and multiproxy paleoceanographic
records could help determine leads and lags between different cli-
mate variables during times when there were large fluctuations
in the AMOC, and thus constrain whether AMOC decline is the
cause or the effect of climate variability. It is challenging to defin-
itively calculate leads and lags using marine sediment records,
because chronology is often uncertain (especially between dif-
ferent locations), but chronological uncertainties can be circum-
vented by making proxy measurements of the AMOC and surface
climate change on the same sediment core. Using this approach,
Barker et al. (2015) showed that North Atlantic cold intervals typ-
ically precede ice rafting events during glacial times over the past
~450 kyr. In addition, as the paleoceanographic community con-
tinues to generate, compile, and synthesize data, a more complete
and nuanced view of past changes in the AMOC will likely emerge.
For example, while there is evidence for a dramatically weakened
AMOC during HS1, some sediment cores record traces of North
Atlantic-sourced water in the deep sea (Repschläger et al., 2021).
Going much further back in time, a mean climate state closer
to the modern climate may have occurred in the mid-Pliocene
(~3 million years ago), when proxy data estimate that temperatures
were ~3°C warmer, sea level was higher, and atmospheric CO2
concentration was ~400 ppm (Haywood et al., 2016; McClymont
et al., 2020). Draut et al. (2003) suggested that mid-Pliocene cli-
mate conditions were relatively stable, but the difficulty in recov-
ering marine archives that extend back >3 million years and have
high enough resolution to record centennial to millennial cli-
mate variations makes it challenging to assess the stability of the
AMOC during the Pliocene. Model simulations of the Pliocene
suggest that the AMOC was similar or slightly stronger than the
pre-industrial, but there is spread between model simulations in
the amplitude and sign of the change (Weiffenbach et al., 2023).
As a result, it is difficult to conclusively constrain AMOC stability
under warm future climate conditions, and it is important to con-
sider that current climate conditions are changing at a rate that is
likely faster than the rate of changes during the mid-Pliocene.
CONCLUSIONS
The fate of the AMOC under future anthropogenic warming is of
great interest due to the wide-ranging impacts thought to be asso-
ciated with past AMOC changes, including large and abrupt tem-
perature changes and shifts in large-scale precipitation patterns.
Paleoclimate data from the most recent glacial-interglacial transi-
tion are consistent with (but do not generally require, given their
limitations) a large and abrupt decrease in AMOC strength during
HS1 and the YD. Therefore, these time intervals could be used to
determine the mechanisms responsible for large changes in the
AMOC. Climate model simulations of the deglaciation can be
tuned to reproduce the timing of the AMOC changes inferred from
paleoclimate records, but only by applying freshwater fluxes that are
unrealistic in timing and magnitude according to sea level records
and ice sheet reconstructions (Snoll et al., 2024, and references
therein). Meltwater from icebergs, rather than liquid fresh water
introduced into the ocean from ice sheet collapse, may have driven
deglacial AMOC changes, given the correspondence between pur-
ported intervals of weak AMOC and intervals of IRD accumula-
tion in North Atlantic sediments. In addition, small-scale oceanic
processes that are not well represented in coarse resolution climate
models may have influenced the AMOC response to freshwater
fluxes from disintegrating ice caps. While the Laurentide Ice Sheet
does not exist today, some quantitative estimates of ice discharge
during past Heinrich Events are similar in magnitude to current ice
loss from the GIS (Zhou and McManus, 2024). However, it is not
known how long this freshwater flux would need to be applied in
order to significantly perturb the AMOC, or whether such a pertur-
bation depends on the background climate state.
Modern observations may be too short to resolve with high
confidence decadal trends in AMOC strength. Paleoclimate recon-
structions for the Common Era (the past 2,000 yr) give a longer
timescale context, but they do not always provide a clear picture
of AMOC history, because the relationship between each proxy
and the AMOC is complex, and because AMOC changes might
have been relatively small during this period. Longer paleoceano-
graphic records may shed light onto other aspects of the AMOC,
however. By examining deglacial intervals characterized by large
climatic changes (such as HS1 and the YD), paleoceanographic
records of surface ocean properties could be used to more clearly
estimate the fingerprints of AMOC change, which could then be
applied to modern observations. Further investigation of the 8.2 ka
climate event in the early Holocene and the mid-Pliocene may pro-
vide mechanistic insight into future changes in the AMOC, given
the similar background climate state. Thus, paleoceanography can
play a valuable role, not only in elucidating the mechanisms that
may drive changes in the AMOC but also for addressing other
open questions in the study of modern AMOC.
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