Download Conveyor Belt Circulation

Conveyor Belt Circulation
Entry for the “Encyclopedia of Climate and Weather” (2nd Edition)
Submitted for publication by Oxford University Press
Author: Klaus Keller
The “conveyor belt circulation” is a highly simplified conceptual model of a global ocean
circulation system consisting of surface and deepwater currents connecting the world oceans that
is driven by (and affects) patterns of water-temperatures and -salinities and the atmospheric
circulation. As the name conveyor belt circulation (coined by Wally Broecker) implies, this is a
closed loop (Broecker, 1987; Richardson, 2008). In this simplified conceptual model, a
convenient choice of starting point is the flow of surface waters in the North Atlantic via the
Gulf Stream and the North Atlantic Drift towards high latitudes (see, for example, Broecker
(1991), Lumpkin and Speer (2007), or Gordon (1996)). There, surface waters cool through heat
exchange with the colder overlying atmosphere. In addition, sea-ice formation increases the
salinity (the water salt content) due to brine rejection. The temperature decrease and salinity
increase raise the density of the surface waters. Surface waters that are denser than the
underlying water masses sink and form deep waters. The deep waters flow southwards to the
Southern Ocean (where they mix with other deep waters formed close to the Antarctic
continent). A fraction of the deep waters in the Southern Ocean flow into the Pacific Ocean,
where they return to the surface. The surface waters then flow through the Indian Ocean, around
Africa, and back to the tropical Atlantic, thus closing the loop of the conveyor belt. The ocean
conveyor circulation contains (but is not equivalent to) the Gulf Stream (Wunsch, 2004).
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Because the conveyor belt circulation is driven to a large extent by the density differences that
are a result of differences in temperature and salinity, it is sometimes also referred to as the
thermohaline circulation (THC). The ocean conveyor circulation in the North Atlantic is referred
to as the Atlantic Meridional Overturning Circulation (AMOC).
The conveyor belt circulation affects the Earth system through the transports of water,
heat, and chemical components. For example, the oceanic meridional heat transport in the
Atlantic at 25 o N is around one petawatt (10 15 watts) (Trenberth and Caron, 2001). Changes in
the conveyor belt circulation can hence change global patterns of sea-level, surface air
temperature and precipitation (Higgins and Vellinga, 2004; Knutti and Stocker, 2000; Vellinga
and Wood, 2002). The changes in surface air temperature and precipitation patterns can, in turn,
affect terrestrial and marine ecosystems (Higgins and Vellinga, 2004), global biogeochemical
cycles of elements such as carbon, oxygen, or nitrogen (Schmittner, 2005; Schmittner et al.,
2008; Zickfeld et al., 2008), the El-Niño – Southern Oscillation (Timmermann et al., 2005), and
the African monsoon (Chang et al., 2008).
The conveyor belt circulation has changed in the past in response to climate forcings.
One example of such past changes occurred during the Younger Dryas, a period of abrupt and
global-scale climate change that started approximately 13, 000 years ago and lasted for more
than a millenium (Alley, 2000). The Younger Dryas is associated with an AMOC collapse
(McManus et al., 2004) and a strong cooling in the North Atlantic region (Alley, 2007). The
conveyor belt circulation is projected in many climate models to change in response to
anthropogenic climate forcing (Meehl et al., 2007). One particular concern is a potential abrupt
and persistent threshold response due to positive feedbacks that amplify anthropogenic forcings
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(Alley et al., 2002; Marotzke, 2000; Stocker et al., 2001; Stommel, 1961). The
Intergovernmental Panel on Climate Change (IPCC) concludes that “it is very likely that the
meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st
century” and that “it is very unlikely that the MOC will undergo a large abrupt transition during
the 21st century” (Alley et al., 2007). “Very likely” and “very unlikely” in this assessment mean
probabilities of more than 90% and less than 10%, respectively. Note that this assessment
addresses the probability of experiencing an AMOC threshold response during the 21st century,
but is silent on the probability of triggering such an event during this century.
The probability of an anthropogenic conveyor belt circulation threshold response is
projected to increase with increasing anthropogenic forcing (Rahmstorf and Zickfeld, 2005;
Stocker and Schmittner, 1997; Zickfeld et al., 2007). The current assessments of the probability
of an AMOC threshold response are deeply uncertain (Keller et al., 2008; Rahmstorf and
Zickfeld, 2005; Wood et al., 2006; Zickfeld et al., 2007). One key driver for this deep uncertainty
is that the current AMOC observations are sparse and very uncertain and that the AMOC show
considerable inter- and intra-annual variability (Bryden et al., 2005; Cunningham et al., 2007).
As a result, “there is insufficient evidence to determine whether trends exist in the meridional
overturning circulation […] of the global ocean” (Alley et al., 2007). In addition, AMOC
projections hinge on divergent subjective expert judgements about model parameters such as the
equilibrium climate sensitivity (Knutti and Hegerl, 2008; Morgan and Keith, 1995; Urban and
Keller, 2008a). Last, but not least, current AMOC projections are typically based on simulations
that poorly sample the tails of the underlying probability density functions (Keller et al., 2007b;
Ramanathan and Feng, 2008; Urban and Keller, 2008b). Neglecting to sample the tails of the
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underlying probability density functions results in a downwards bias in estimates of the
probability of low-probability events (see, for example, Ramanathan and Feng (2008) and Keller
et al. (2004)).
The potential for a threshold response of the ocean conveyor circulation has important
implications for climate change policies (Schneider et al., 2007). Projections of the impacts of
an ocean conveyor circulation threshold response on ecosystem services and human welfare are
deeply uncertain, but suggest potentially large negative effects (Keller et al., 2007a; Keller et al.,
2000; Link and Tol, 2004; Schneider et al., 2007). Triggering a threshold response of the
conveyor belt circulation has been interpreted as causing a “dangerous anthropogenic
interference with the climate system”, a violation of the objective of the United Nations
Framework convention on climate change (Keller et al., 2005; Keller et al., 2000; McInerney and
Keller, 2008; Oppenheimer, 2005; Ramanathan and Feng, 2008; UNFCCC, 1992). The potential
for abrupt and persistent response of the conveyor circulation to anthropogenic forcing can
considerably increase estimates of the economically optimal greenhouse gas abatement as well as
the economic value of climate observation systems (Baehr et al., 2008; Keller et al., 2004; Keller
et al., 2007a). A sizeable reduction of the probability of triggering an anthropogenic AMOC
threshold response within the next few centuries requires, by most estimates, a strong reduction
of anthropogenic climate forcing (McInerney and Keller, 2008; Stocker and Schmittner, 1997;
Zickfeld and Bruckner, 2008).
.
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