Download Plate Tectonic and climate - Atmospheric and Oceanic Sciences

Orbital Change and climate
Summary
We have now finished the second part of the course on Orbital Change and its effect on
Climate. To summarize:
We have discussed the Milankovich theory describing changes in orbital parameters on
various time scales, and how these can be related to climate change. In particular,
changes in eccentricity (orbit’s shape more or less circular) vary on a 100 and 400 ka
time scale will affect the amount of solar radiation reaching the top of the atmosphere by
modulating the tilt and precession signal. It will also have a small effect on the
seasonality. Changes in Obliquity (changes in the earth’s tilt) vary on a 41 ka time scale
and will have a large impact on the seasonality of the solar fluxes reaching the earth
surface in the winter hemisphere. Finally changes in the precession of the earth orbit (the
precession of the equinox) vary on a 21 ka time scale and will also impact the summer
hemisphere seasonality (see Fig 8.16 Ruddiman).
Several climate records have a strong signal at those frequencies. For instance the delta
O18 ratio, CO2 and methane trapped in air bubbles, and dust have a strong 100 ka. Both
methane and CO2 also have a strong precession signal superimposed on the 100 ka cycle.
The change of CO2 and methane with the precession cycle are believed to be linked with
warmer or colder summer which leads to wetter summer and the decay of organic matter
producing these two gases. The 100 ka signal in those two greenhouse gas on the other
hand is not well understood. Looking at higher resolution climate data, it seems that the
changes in CO2 and methane follow changes in ice volume, and so they would not be
causing it, but rather they would serve to amplify the weak 100 ka change in seasonality
associated with changes in eccentricity. Note that the 100 ka is weakest signal of the 3
orbitally driven changes in insolation, yet it is the ice volume on earth is responding
strongest to this forcing. Several theories have been proposed to explain this link between
eccentricity and delta O18 signal (e.g. non linear oscillator, climate switch mechanism).
We then discussed Dansgaard Oeschger Oscillations and Heinrich Events. DO and
Heinrich Events are rapid climate change that occurred during the last glacial period. The
change in climate during those event was extremely rapid (decades) and lasted between 2
and 8 thousand years. DO event are associated with rapid warming followed by a gradual
cooling, whereas Heinrich event are associated with sudden discharge of iceberg in the
North Atlantic. Heinrich Event typically happens during the cool phase of every 4 DO
cycles. A series of DO cycle terminated by an Heinrich Event is referred to as a Bond
cycle. In this part of the course, we reviewed modeling studies that attempted to reconcile
the seemingly contradictory climate record, some involving the ocean alone, some
involving the ice itself and its interaction with the bedrock (e.g. Ganapolsky and
Rahmstorf, MacAyeal, Hunt, Clemet et al.).
In the last part of the course we will concentrate on recent climate change (8 ka ago to
present), as well future climate change.