With Speed and Violence Fred Pearce - Global Commons Institute

fluctuation shouldn't produce temperature changes of more than 0.35°F.
But, although the global temperature change may well have been close to
that, in parts of Europe and North America the pulses produce changes ten
times as great.
Researchers have struggled to find amplifying mechanisms that might
have caused that. Sea ice, the ocean conveyor, and tropical flips like El Nino
have all been suggested, but none seems up to the task. Shindell says the
answer is his stratospheric feedback. The heart of the mechanism this time is
ultraviolet radiation. While the total solar radiation reaching Earth's surface
during Bond's pulses varies by only a tenth of a percentage point, the
amount of ultraviolet radiation reaching Earth changes by as much as 10
percent. Most of the ultraviolet radiation is absorbed by the ozone layer in
the stratosphere, so its impact at ground level is small. But the process of
absorption causes important changes in energy flows in the stratosphere.
These eventually change the stratospheric jets, and with them the Arctic
Oscillation in the Northern Hemisphere and the SAM in the South.
Shindell modeled the likely effects of the last reduction of solar radiation
at the Maunder Minimum in the depths of Europe's little ice age, 350 years
ago. The GISS model without the stratosphere was unmoved by the tiny
change in solar radiation. But with the stratosphere included, it delivered a
drop in temperatures of 1.8 to 2.6°F in Europe, but only a tenth as much
globally—results remarkably close to likely events in the real world. The
declining flows of ultraviolet radiation into the stratosphere triggered a
slowdown in the westerly winds at ground level, says Shindell. That, in turn,
caused winter cooling, particularly over land, in the higher latitudes of the
Northern Hemisphere.