On 17/08/2010 09:15, Rob Dekker wrote:

Related to your statements about the Milankovitch cycles, and the importance
of insolation at 65 N, I have a question :

Do you know which reasons as an explanation for why 65 N insolation in the
Milankovitch cycles seem to dictate the onset and demise of ice ages ?

I suspect because at 65N there is a large area of land with Canada and
Russia to provide positive feedback whereas in the southern hemisphere
there isn't much there at all there until you reach Antarctica. So until
it becomes cold enough for the coastal seas to freeze you don't get such
a significant change in local albedo.

Apart from a nibble by the Antarctic Peninsula 60S +/-5 is almost all
clear blue water all the way around the S hemisphere see for example:

http://www.lib.utexas.edu/maps/islan..._station.gif11

or for global view
http://www.worldatlas.com/webimage/t.../latslongs.htm

Why not 65 S ? Or even better : Why is the Vostok ice core temperature data
(in the Southern Hemisphere) a slave to insolation at 65 N (in the Northern
Hemisphere) ?
And even more interesting, the changes in solar irradiance numbers for 65 N
in the Milankovitch cycle (some 50 W/m^2 difference), seem to be consistent
with the swings in temperature for the planet as a whole (including
Antarctica), when we fill in such numbers into the Stephan Bolzmann equation
(10 C difference).

I wonder if that is a coincidence, and why the irradiance in the Southern
Hemisphere does not seem to matter much when it comes to determining the
overall planet's surface temperature...

Probably a quirk of how the continents are disposed at the present day.
Most of Antarctica is already cold enough to be white and there isn't a
great deal of other land to cover with snow until you get to almost 40S
touching bits of Australia and Africa as well as South America.

I wouldn't call it a coincidence so much as cause and effect.

Regards,
Martin Brown

On Tue, 17 Aug 2010 10:51:58 +0100, Martin Brown wrote:

On 17/08/2010 09:15, Rob Dekker wrote:

Related to your statements about the Milankovitch cycles, and the
importance of insolation at 65 N, I have a question :

Do you know which reasons as an explanation for why 65 N insolation in
the Milankovitch cycles seem to dictate the onset and demise of ice
ages ?

I suspect because at 65N there is a large area of land with Canada and
Russia to provide positive feedback whereas in the southern hemisphere
there isn't much there at all there until you reach Antarctica. So until
it becomes cold enough for the coastal seas to freeze you don't get such
a significant change in local albedo.

Apart from a nibble by the Antarctic Peninsula 60S +/-5 is almost all
clear blue water all the way around the S hemisphere see for example:

http://www.lib.utexas.edu/maps/islan..._station.gif11

or for global view
http://www.worldatlas.com/webimage/t.../latslongs.htm

Why not 65 S ? Or even better : Why is the Vostok ice core temperature
data (in the Southern Hemisphere) a slave to insolation at 65 N (in the
Northern Hemisphere) ?
And even more interesting, the changes in solar irradiance numbers for
65 N in the Milankovitch cycle (some 50 W/m^2 difference), seem to be
consistent with the swings in temperature for the planet as a whole
(including Antarctica), when we fill in such numbers into the Stephan
Bolzmann equation (10 C difference).

I wonder if that is a coincidence, and why the irradiance in the
Southern Hemisphere does not seem to matter much when it comes to
determining the overall planet's surface temperature...

Probably a quirk of how the continents are disposed at the present day.
Most of Antarctica is already cold enough to be white and there isn't a
great deal of other land to cover with snow until you get to almost 40S
touching bits of Australia and Africa as well as South America.

That sounds quite plausible to me, especially considering that Miskolczi's
model suggests the surface temperature should track the insolation. Ice
albedo feedback works by modulating the effective insolation.

I wouldn't call it a coincidence so much as cause and effect.

Regards,
Martin Brown