What would happen if the rotation axis of the Earth changed ?

How the ding dang would something like this ever happen! The spin axis of Earth, and its spin energy are extremely robust and it would require a close encounter with another planet-sized body to torque the Earth into a substantially different orientation. Such and encounter would last a week for a deep encounter, and the tectonic upheavals would be tremendous, horrible, etc etc. The surface of the Earth would become molten as the tremendous tidal energies are dissipated. NO ONE would possibly survive it and the biosphere+atmosphere would be transfigured.

Currently, the Earth is spinning about 1000 miles per hour at the equator. A LOT less towards the poles. An appreciable change in rotation direction would produce winds with speeds of thousands of miles per hour EVERYWHERE. Anything not anchored to bedrock would be torn out of the soil and mixed with a world-enveloping hurricane of activity. The Earth would remain engulfed in thick clouds of dirt, dust and debris for decades. I cannot imagine anything surviving such a catastrophy, because the solid Earth would be wracked by earthquakes the likes of which have never been experienced by living organisms in over 3 billion years or more. There would be no spot on the Earth not affected by 100 - 500 mile per hour winds and earthquakes of magnitude +7 and MUCH HIGHER.

Any scenario that includes such a catastrophy would have to presume the complete destruction of the biosphere as a given. The only survivors would be bacteria. The tidal forced exerted upon the Earth would be enough to raise the surface of the earth several miles or more, and the tidal energy would probably be enough in a close encounter to liquify large portions of the crust.

The polar ice caps would be melted by any change in the mean global temperature, and the changing wind patterns and tectonic activity would provide a more than adequate source of heating. After they melted, and the sea level rose by 30 feet, they would eventually reform when the Earth settled down to its new spin orientation assuming the temperature of the atmosphere returned to current levels. I don't really see that that would happen for many centuries afterwards because it takes time for the Earth to cool after such a cataclysmic change. Perhaps millions of years would be needed to quell the vulcanism that must surely result from such a realignment. The carbon dioxide level in the atmosphere and the consequent Greenhouse Heating would probably stay with the Earth for millennia.

If the collision was a direct one with a body, say, as large as the asteroid Ceres ( 450 kilometers) this would produce a crater larger than the continent of North America, and probably be enough of an impulse to cause the axis of the Earth to precess with a very large amplitude over thousands of years. I can't be too specific because the details depend on the geometry and energy of the impact. Most solar system bodies are 1) orbiting the Sun in the same sense as the Earth is rotation, so such impacts probably would not greatly affect the spin direction, but might increase the spin rate so that a day might be several hours shorter than the standard 24 hours. If the impact is a grazing collision, portions of the Earth's mass would be ejected into space to orbit the Earth, be lost from the Earth's gravitational field, or eventually rain back upon the Earth as a lethal meteor storm lasting hundreds or thousands of years. I can't imagine what it would be like living through such times when death could fall out of the sky at any moment.

Sorry to be so gloomy. It's all pretty hypothetical anyway. There are lots of other things of greater likelihood that are bound to happen to the Earth in the next hundred thousand years that would be more local, but still lethal.

A change in the rotation axis of the Earth, or its spin rate would be catastrophic. The number of the seasons would change and their duration. If the rotation axis became parallel to the orbital plane, as for Uranus, we could have winter in the Northern hemisphere for 6 months followed by summer. The Sun would set on the entire Northern hemisphere and not rise again for 6 months. Less extreme axial tilts would produce a different pattern of seasons at each earth latitude.

Growing seasons and all of the natural cycles of the biosphere would be catastrophically affected, and if the change happened in a year or so, no living things, especially plants, would survive unmolested. There would not be enough time for plants to genetically alter their growing cycles. Large diebacks of plant ecosystems would occur followed by the animals that feed on the plants, and then the carnivores...etc etc.

If the length of the day changed appreciably, we would still operate on the basis of the 24 hour cycle because that is genetically programmed into us, but for organisms that rely on the 24 hour day to set their reproductive cycles and feeding cycles, they may be adversely affected.

Still, to seriously affect the Earth's rotation axis and spin, you would need a catastrophic force ( collision etc) whose physical affect would in itself be enough to exterminate much of the biosphere. Any changes in the seasons would be the least of the concerns of the biosphere compared to surviving the cause of the change itself!