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''The melting ice sheets inject so much low-density fresh water into the North
Atlantic that they weaken or entirely shut down the normal sinking of dense
water that fuels the ocean circulation
...
. The loss of this circulation allows sea
ice to cover the North Atlantic in winter, preventing ocean heat from warming
the air and leading to extremely cold winters in Europe and Eurasia,
...
.''
This statement does not explain what made the ice sheets melt in the first
place, nor does it explain how this putative shutdown of the MOC contributes to
warming when it seems to be a cooling process. To answer this, he invoked the
following hypothesis:
''Is there something about an 'off' MOC that helps to destroy an ice sheet?
Cheng et al.'s timing data provide support for the hypothesis that an 'off' MOC
forces CO 2 out of the Southern Ocean, warming the globe by its greenhouse
effect, which in turn causes more melting of the ice sheets, ensuring that the MOC
stays in its 'off' position in a positive-feedback loop.''
Severinghaus claimed that ''an 'off' MOC forces CO 2 out of the Southern
Ocean, warming the globe by its greenhouse effect.'' He went on to say: ''an alter-
native hypothesis is that massive ice sheets are inherently vulnerable and cannot
survive the combined onslaught of Milankovitch and CO 2 .'' Severinghaus seems
to have been determined to make rising CO 2 concentration a cause of termination
rather than an effect; yet it is not clear why the CO 2 concentration should rise due
to the putative self-inflicted shutoff of the MOC.
Like Severinghaus, there are a number of proponents of the hypothesis that
the MOC can be turned on and off, with consequences for climate change.
Broecker originated this concept in the 1980s and 1990s. Rahmstorf (2002)
discussed the role of the MOC in climate change (see Section 8.6.1). The impacts
on climate of the putative on/off behavior of the MOC have been interpreted in
various ways, often with contradictory conclusions. It seems likely that, if this
process occurs at all, it probably accounts for some sudden climate fluctuations,
but it is not clear if it is related to longer term climate change.
Knorr and Lohmann (2003), citing the results of Sowers and Bender (1995)
and Petit et al. (1999), asserted: ''during the two most recent deglaciations, the
Southern Hemisphere warmed before Greenland.'' This is one possible interpreta-
tion. Another interpretation is that both polar areas warmed over roughly the
same time period but there was a qualitative difference. In the south, the warming
was steady and continuous. In the north, the warming caused ice sheets to melt,
which periodically plunged the regional climate back to cold for a period, resulting
in jagged ups and downs in regional climate, superimposed on the secular upward
trend in temperature. In contrast to Severinghaus (2009), Knorr and Lohmann
assert that the MOC switches off during ice ages and is turned on when the
south warms before the north does so. This warms the north with a time lag.
This hypothesis seems very speculative and appears to be only one of many
interpretations.
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