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My understanding is that the root cause of the ice age cycles is changes is insolation (i.e., effective heating from the sun) due to changes in the orbit/tilt-axis of the earth, but that this alone would not cause the temperature changes, UNLESS you also take into account the consequent melting of ice (which darkens those areas formerly covered with ice), and also causes the oceans and land to release greenhouse gasses, including carbon dioxide.

The point is that if we release CO2 as an independent forcing, then the ice melting and CO2 release that amplify the natural ice age cycles, will also amplify our independent forcing. In other words, they are the SAME feedbacks as the ones arising from warming due to insolation - although it might be reasonable to expect the GHG feedback mechanism would be slightly weaker since the initial GHC concentration is higher. Here is a quote from Hansen:

Natural Climate Changes over Millennia. The large climate changes discussed above,
occurring over millions of years, are usually slow, because they involve transfer of carbon between the Earth’s crust and the surface reservoirs (atmosphere, ocean, soils, and biosphere). But graphs of global temperature (Fig. 1) also show, superposed on these large climate swings, more rapid and regular oscillations of temperature, the familiar glacial-interglacial oscillations that occur over tens and hundreds of thousands of years.

The large glacial-interglacial climate swings are synchronous with and instigated by
small changes of the Earth’s orbit3. The orbital changes alter the seasonal and geographical distribution of sunlight on Earth. The effect of insolation variations is magnified by two strong feedback mechanisms. First, seasonal insolation changes can cause melting or buildup of high latitude ice sheets, bringing into play the powerful ice-albedo feedback. Second, when the planet warms (cools) the ocean, soil and biosphere release (absorb) CO2, CH4 and N2O, these GHGs providing another powerful amplifying feedback4. Glacial-interglacial changes of ice sheet size and CO2 amount are large (of order 100 meters of sea level and 100 ppm of CO2) and practically
coincident (Fig. 2A).

The direct forcing due to orbit changes is negligible, the annual mean perturbation of the Earth’s energy balance never exceeding 0.2 W/m2 averaged over the planet. But the ice-albedo and GHG feedbacks each cause (approximately equal) perturbations of several W/m2 (Fig. 2B). Together these two feedbacks fully account for the global temperature swings from glacial to interglacial conditions (Fig. 2C), with a climate sensitivity of 3/4°C per W/m2 of forcing, or 3°C for doubled CO2 forcing. This empirical climate sensitivity confirms the climate sensitivityestimated by most climate models.

Close examination of glacial-interglacial data reveals that temperature change usually
leads the GHG change. This is as expected, because the GHG change is a feedback to the
temperature change. The average lag is a few hundred years, the time required for CO2, which is the dominant GHG feedback, to be flushed from surface reservoirs, mainly from the ocean4.

Despite longstanding knowledge that GHGs changes are a feedback amplifying glacial-interglacial global temperature change, and thus GHG changes necessarily lag temperature change, global warming ‘contrarians’ point to this lag as proof that GHGs are not an important cause of climate change!

3 Changes of the Earth’s orbit are the eccentricity of the orbit, the day of year at which the Earth is closest to the sun,
and the tilt of the spin axis relative to the plane of the orbit. These orbital elements fluctuate due to gravitational
tugs of Jupiter, Saturn and Venus as they alternately move closer or farther from the Earth. The orbital perturbations
have negligible effect on the amount of solar energy falling on the Earth averaged over the year and planet.

4 CO2 provides most of the GHG feedback. CO2 is released by the ocean as the climate warms because of the
temperature dependence of CO2 solubility and increased ocean mixing in a warmer climate, which flushes out deep
ocean CO2 and alters ocean biological productivity.


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