onsdag 7 april 2010

Glacial Cycles and Eccentricity

Lorraine Lisiecki reports in Nature Geoscience on the connection between glacial cycles and
the eccentricity of the Earth orbit around the Sun both showing 100.000 year periodicity. The main idea is that the intensity of the insolation during the Northern Hemisphere Summer influences the growths of ice caps: It seems that large eccentricity with short strong NH Summer insolation (coupled with long weak NH Winter insolation ) keeps the Ice Age away. In particular, the tilting of the Earth axis enhances the NH Summer heating. Moreover, one can expect more heat to flow North from the Equator during the long Winter of an eccentric orbit.

A simple argument based on Kepler's 2nd Law shows that the total insolation during a year is independent of the eccentricty, which indicates that the yearly dynamics determines glaciation rather than any varying total insolation.

Of course the heat exchange between the equatorial and pole regions is important; in particular during the long NH Winter. It thus seems that the yearly dynamics of the climate driven by the eccentricity is of importance, rather than a gross global mean value temperature. It may be possible Sweden gets covered by ice while the global temperature is increasing. Maybe this is what we have seen a sign of this long NH Winter?

Compare A New and Effective Climate Model by Stephen Wilde:
  • ...virtually all climate variability is a result of internal system variability and additionally the system not only sets up a large amount of variability internally but also provides mechanisms to limit and then reduce that internal variability...The current models neither recognise the presence of that internal system variability nor the processes that ultimately stabilise it.
This connects nicely to the features of turbulent flow we have identified, namely large
local variability coupled with stable mean value outputs. The basic example is bluff body flow
in a slightly viscous fluid such as air and water, which is turbulent, highly variable and unpredictable locally (in the wake behind the body) while the drag as global mean-value is
highly predictable. This is a result of cancellation mechanisms in turbulent flow, which
comes down to saying that after up there will be down, or after rain there will be sunshine...

This gives hope of improved climate models on grids with millions of mesh points which is feasible today.

Compare with Con: Earth is never in equilibrium by Richard Lindzen.

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