Jökull - 01.01.2005, Page 11

Global Warming: Take Action or Wait?
Figure 8. Solar irradiance as measured from satellites for the last two 11-year duration sunspot cycles (Fröhlich
and Lean, 1998). – Inngeislun sólar samkvæmt gervitunglamælingum síðastliðinna 20 ára. Neðri myndin sýnir
fjölda sólbletta fyrir sama tímabil, sem verða í 11-ára lotum.
COMING CONVEYOR SHUTDOWN?
Much attention has gone into claims that global warm-
ing will lead to a shutdown of the Atlantic’s conveyor
circulation with the consequence that Europe will be
hit with another ice age. In my estimation, we can
now say that these claims are greatly exaggerated.
True, model simulations do suggest that greenhouse-
induced excess rain onto and river runoff into the Arc-
tic and northern Atlantic could dilute the salt content
of surface waters to the point where deep water could
no longer be produced thereby bringing the conveyor
to a halt. But, rather than an abrupt switch from on
to off, akin to those thought to have brought about
the dramatic climate changes of glacial time, the mod-
els suggest a gradual slowdown spread over an entire
century (Stocker et al., 2001). Further, to produce
a large enough input of extra fresh water, the mod-
els call on a 4 to 6◦C warming of the planet; some-
thing that would require a tripling of the pre-industrial
CO2 level! Were the planet to warm to this extent,
there would be no chance that the conveyor shutdown
would allow sea ice forming in the northern Atlantic.
It would be too warm. In the absence of the large
amplification induced by ice cover, such a shutdown
would lose much of its punch. Instead of initiating an-
other European ice age, it would likely only partly off-
set the greenhouse warming in this part of the world.
If a future shutdown is predicted to occur gradu-
ally, how could it be that those of glacial time (and
also that which occurred 8200 years ago) happened
suddenly? The answer is that several of these shut-
downs have been documented to have been triggered
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