We all know Obliquity (tilt) wanders in a nice little range from about 22 degrees to 24.5 right?
Well, seems it’s not all that stable if you look further back in time.
In astronomy, axial tilt, also known as obliquity, is the angle between an object’s rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination.
At an obliquity of zero, the two axes point in the same direction; i.e., the rotational axis is perpendicular to the orbital plane. Earth’s obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle; the earth’s mean obliquity is currently 23°26′12.9″ (or 23.43693°) and decreasing.
Over the course of an orbit, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background stars. This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side—the cause of the seasons on the Earth.
But it wasn’t always like that…
History of the earth’s obliquity
The evolution of the obliquity of the ecliptic (ε), the Earth’s axial tilt of 23.5°, may have greatly influenced the Earth’s dynamical, climatic and biotic development. For ε > 54°, climatic zonation and zonal surface winds would be reversed, low to equatorial latitudes would be glaciated in preference to high latitudes, and the global seasonal cycle would be greatly amplified. Phanerozoic palaeoclimates were essentially uniformitarian in regard to obliquity, with normal climatic zonation and zonal surface winds, circum-polar glaciation and little seasonal change in low latitudes. Milankovitch-band periodicity in early Palaeozoic evaporites implies ≈ 26.4 ± 2.1°at ∼ 430 Ma, suggesting that the obliquity during most of Phanerozoic time was comparable to the present value. By contrast, the paradoxical Late Proterozoic (∼ 800−600Ma) glacial environment— frigid, strongly seasonal climates, with permafrost and grounded ice-sheets near sea level preferentially in low to equatorial palaeolatitudes—implies glaciation with ε > 54° (assuming a geocentric axial dipolar magnetic field). Palaeotidal data accord with a large obliquity in Late Proterozoic time. Indeed, Proterozoic palaeoclimates in general appear non-uniformitarian with respect to climatic zonation, consistent with ε > 54°.
So it was a couple of degrees more than present max at 26.4 degrees around 439 million years ago, and prior to about 700 million years ago it was in flopped over nearly chaotic range. Golly.
It is postulated here that the primordial Earth acquired an obliquity of ∼ 70° (54° < ε < 90°) from the Moon-producing single giant impact at ∼ 4500Ma (approach velocity ≈ 5–20km/s, impactor/Earth mass-ratio ≈ 0.08−0.14). Secular decrease in ε¯subsequently occurred under the dominant influence of dissipative core-mantle torques. From 4500-650 Ma, ε¯slowly decreased to ≈ 60° (〈ε⋅〉=−0.0009"/cy), ε¯then decreased relatively rapidly from ∼ 60° to ∼ 26° between 650 and 430 Ma ((〈ε⋅〉=−0.0556"/cy)); climatic zonation changed from reverse to normal when ε¯∼ 610 Ma, and 〈ε˙〉and the rate of amelioration of global seasonality were maxima for ε¯= 45°at∼ 550Ma (the precessional rate Ω is maximum when ε= 45°, and ε⋅pvaries as Ω^2 ). Since 430 Ma, 〈ε˙〉has been ≲ −0.0025″/cy and ε¯has remained near its Quaternary range.
The postulated relatively rapid decrease in ε¯between 650 and 430 Ma may partly reflect special conditions at the CMB which caused significant increase in dissipative core-mantle torques at that time. This inflection in the curve of ε¯versus time centred at = ε¯45°also may be partly explained by the function ε⋅p∞ (Ω^2 /ω)(sin2ε), where ω is the Earth's rate of rotation, and other dynamical effects on ε⋅p.
The Proterozoic-Phanerozoic transition may record profound change in global state caused by reduction in ε¯through the critical values of 54° and 45°. The postulated flip-over of climatic zonation at ∼ 610 Ma (ε¯= 54°) coincides with the widespread appearance of the Ediacaran metazoans at ∼ 620−590Ma, and the interval of most rapid reduction of obliquity and seasonality at ∼ 550Ma (ε¯= 45°) with the “Cambrian explosion” of biota at 550 ± 20Ma. These two most spectacular radiations in the history of life thus may mark the passage from an inhospitable global state of reverse climatic zonation and extreme seasonality (the Earth’s Precambrian “Uranian” obliquity state) to a relatively benign state of normal climatic zonation and moderate seasonality.
Further geological, palaeomagnetic and geochronological studies of Precambrian glaciogenic and aeolian deposits can test the predictions of a large obliquity (ε > 54°) and reverse climatic zonation and zonal surface winds during the pre-Ediacaran Precambrian.
Well, doesn’t that make CO2 look like a piker…