|MadSci Network: Earth Sciences|
Dear Thom, The short answer to your question is that we don’t know. I know of no evidence in the geological record that would bear on the Earth’s angle of tilt towards the Sun or its “wobble” on its axis. The wobble is a combination of two aspects of the Earth’s orbital motion, namely the Earth’s angle of tilt on its axis with respect to its plane of orbit, and what is known as the “precession of the equinoxes”, whereby the direction in which the Earth’s axis points traces a circle in the sky, like an unstable, spinning top. At the present time the Earth’s angle of tilt with respect to the plane of its orbit is 23.4 degrees. However, the angle of tilt actually varies in a regular cycle between 21.8 degrees and 24.4 degrees. The cycle takes 40,000 years to complete, and the angle is now decreasing. So it will decrease to 21.8 degrees, then increase to 24.4, and then decrease again so that in about 40,000 years the angle of tilt will again be 23.4 degrees. So on the short geological timescale of 40,000 years the angle of tilt does change. What we do not know is whether the range within which the tilt rocks back and forth has ever been more extreme in the distant geological past. The precession of the equinoxes is yet a shorter cycle. The direction of tilt traces a full circle about every 22,000 years, so in 11,000 years from now the direction of tilt will be in the opposite direction to what it is today. These two components of the Earth’s orbital motion are known as Milankovich Cycles, named after the Serbian astronomer Milutin Milankovich, who first proposed them. There is also a third Milankovich Cycle, namely the eccentricity of the Earth’s orbit, which changes from a near circle to an elipse and back again with a period of about 110,000 years. This eccentricity cycle is not part of the “wobble” – I mention it for completeness! A website with some useful diagrams is: http://www-spof.gsfc.nasa.gov/stargaze/Sprecess.htm The rhythmic change in the angle of tilt does affect climate, not by changing the amount of solar radiation received on the Earth, but by changing its distribution over the surface. As the angle of tilt increases to 24.4 degrees the Tropic of Cancer, where the Sun is directly overhead on the summer solstice, moves north to 24.4 degrees North; and likewise, the Tropic of Capricorn moves south to 24.4 degrees South. This is a slight poleward shift of the tropics. The effect is to increase the seasonal difference between summer and winter slightly, because during the northern summer the Northern Hemisphere would be a little warmer, and at the same time the corresponding winter in the Southern Hemisphere would be a little cooler, than they are at present. Likewise at the minimum tilt of 21.8 degrees the seasonal differences between summer and winter would be slightly reduced. There are other climatic effects associated with the precession of the equinoxes in combination with the 110,000 year eccentricity cycle, which I won’t go into here. Suffice it to say that on a timescale of tens of thousands of years the Earth’s wobble does have an effect on climate. However, your question is about the distant geological past, and specifically the Mesozoic Era. We can trace the climatic effects, and indeed the orbital cycles themselves, going back about 800,000 years because they have left a record in continental ice sheets in Greenland and Antarctica. But there is no ice as old as the Mesozoic, which lasted from 240,000,000 to 65,000,000 years ago. And the rock record does not record the orbital cycles in any other way that I know. So, it is possible that the range of tilt angles of the Earth’s axis, or the speed with which the precession of the equinoxes took place, may have been different in the distant geological past, but we have no way of knowing. Any climatic effects would have been along the lines I have described above, increasing and decreasing the differences between the seasons over short periods of tens of thousands of years. However, I would draw your attention to two points. The first is that the mass of continental crust today is a mere 0.7% of the total mass of the Earth. In the Mesozoic it would have been about the same or slightly less. I doubt, therefore, whether the existence of Pangea would have had more than a negligible effect on the orbital cycles. The second point is that the orbital motions of the planets were undoubtedly affected early in the history of the Solar System by impacts with large bodies such as smaller planets, moons, very large asteroids, etc. Uranus, for example, is tipped over on its side, so to speak, so that its angle of tilt is about 90 degrees to the plane of its orbit and one pole always points towards the Sun, the other away. This extreme tilt is thought to have been caused by collision with a massive object early in Uranus’s history. But these events took place perhaps 4.5 billion years ago. It is almost inconceivable that a body massive enough to change the angle of tilt of the Earth on its axis could still have been roaming loose in the Solar System on an Earth-crossing orbit as late as the Mesozoic. The Chicxulub meteorite which some geologists credit with the mass extinction at the end of the Cretaceous would have been far too small to affect the Earth’s tilt. So, I would predict that if we ever do discover a way to estimate the Earth’s angle of tilt and the rate of precession of the equinoxes during the Mesozoic, we will find that the range of angles of tilt and the rate of precession were not very different, if at all, from those of today. Best wishes, David Scarboro
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