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will be forcing the climate well above what would have been expected for our current
interglacial.
One question that those with some knowledge of climate change, and Earth systems
(or biosphere) science, often ask is whether there will be changes to the ocean and/or
atmospheric circulation system. Here the short answer is very likely to be yes. Yet
specifically the question arises as to whether there will be changes in the Broecker
thermohaline circulation (Figure 4.7) and the meridional overturning circulation
(MOC) in the North Atlantic; that is, the drawdown of surface water in the North
Atlantic into the abyssal depths. The fear is that if the MOC shuts down then warm
water in the Gulf Stream (more accurately the North Atlantic Drift) will cease to travel
so far north. (It will continue to operate because the Earth will carry on rotating, so
providing a Coriolis force, but it would not be drawn to the current North Atlantic
down-welling region on the latitude of Greenland, Iceland and Norway.) If this were
to happen then north-west Europe would begin to experience harsher winters as does
Canada or other parts of continental Eurasia on the same latitude.
This question is a difficult one. The IPCC's 2007 report said that:
It is very likely that the Atlantic MOC will slow down over the course of the 21st
century. The multimodel average reduction by 2100 is 25% (range from zero to
about 50%) for SRES emission scenario A1B.
Scenario A1B is an IPCC Business-as-Usual scenario with some switching away from
fossil fuel use in the 21st century. The term 'very likely' is precise IPCC parlance
for 'more than 90% probability'. This does seem to confer some sort of certainty.
However, note the range the IPCC also cite: 'range from zero to about 50%'. In short,
there is a chance that the decline in the MOC could be close to zero. In Britain the
Natural Environment Research Council (NERC; the UK independent government-
funded agency for distribution of environmental research funds), together with other
national stakeholders (from the USA), started in 2004 to run a monitoring programme
called RAPID-WATCH, which monitored the Atlantic MOC on a daily basis. It used
an array of moored instruments deployed along latitude 26.5 N in the N. Atlantic.
The programme is currently (2012) scheduled to run until 2014, but early results
were published after just a couple of years in 2007 with Stuart Cunningham as the
lead author. Their preliminary conclusion was that the Atlantic MOC was currently
highly variable. The problem with this result is that by 2004-6 the Earth was already
warming and moving away from its late-Holocene Little Ice Age state. The issue that
consequently arises is whether this variability is 'natural' for the MOC across the
whole Holocene (past 11 700 years of our current interglacial) or is it a sign that
the Earth system is close to a 'critical transition' (a so-called tipping-point) and that
the global climate is about to cross a 'climate threshold'? (As we shall see in section
6.6.8, system variability rises when close to critical transitions.) If the system is close
to a critical transition, then the planet may not just 'gradually' warm as the IPCC's
assessment reports (1990, 1995, 2001b, and 2007) consensually conclude; 2
it may
2
This is not to say that the IPCC up to and including its 2007 assessment was wrong: far from it. the
science of critical transitions as applied to the Earth system has only been developed in recent years and
in any case the IPCC does warn us to be wary of climate 'surprises'.
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