Caption: Plot of the Earth's axial tilt (AKA obliquity) versus time for 8,000 BCE--12,000 CE (based on J. Laskar, 1986, A&A, 157, 59 "Secular Terms of Classical Planetary Theories Using the Results of General Relativity").

The displayed time variation is the long-term axial tilt oscillation.

Features:

1. The zero point is set to year 2000: more exactly, to year 2000 Jan01, 12:00 pm UT yours truly believes. This epoch is called J2000.0. For a lot of purposes, J2000.0 as a zero point makes a lot of sense. Yours truly pities anyone who has to call their year 2331.

2. The plot shows the results of a modern theoretical calculation and is probably quite accurate.

3. At the epoch J2000.0, the Earth's axial tilt was 23°26'21.4119'' = 23.43928108° ≅ 23.4° (see Earth Orientation Center: Useful Constants). Currently, the Earth's axial tilt is decreasing by ∼ 47'' per century (see Wikipedia: Axial tilt: Earth). This means the Earth's axial tilt will be ∼ 23.2° in year 4000 (or year 2000 relative to J2000.0) as the plot shows.

4. The time variation of the Earth's axial tilt is due to astronomical perturbations (mainly gravitational perturbations).

5. The long-term axial tilt oscillation is one of the astronomical Milankovich cycles which are key factors for the Quarternary glaciation (2.58 Myr BP--ongoing, AKA Quarternary Ice Age) which started circa 2.58 Myr ago---and is still going on!!!! The 3 main Milankovich cycles:
   1. Wikipedia: Milankovich cycles: Orbital eccentricity.
   2. Wikipedia: Milankovich cycles: Axial tilt (obliquity).
   3. Wikipedia: Milankovich cycles: Axial precession.

6. Quarternary glaciation has alternating relatively long glacial periods and relatively short interglacials. The cycle of glacial periods and interglacials in the last megayear has a period of ∼ 100,000 years (see Wikipedia: Quaternary glaciation: Description). The lengths of the interglacials are various, but last six have range ∼ 12,000---50,000 years.

7. The causes of the Quarternary glaciation have NOT been fully explained (see Wikipedia: Quarternary glaciation: Causes). However, the Milankovich cycles are probably NOT a main cause in themselves. They are the main drivers of the alternation of glacial periods and interglacials.

8. The main cause may be the gradual decline in the atmospheric carbon dioxide (CO_2) abundance over the geological time scale---the volcanic outgassing of CO_2 has been slowly losing to the locking up of CO_2 in calcium carbonate (CaCO_3) rock. The long-term decrease in CO_2 decreases the greenhouse effect in atmosphere.

   The anthropogenic greenhouse gas emission is a relatively short-term modern process that causes global warming on a relatively short time scale.

9. We do NOT know when the next glacial period will start. Global warming and other anthropogenic effects may well affect the next start date. One hypothesis suggests 50,000 years from now, but much smaller and larger times have been hypothesized (see Wikipedia: Quarternary glaciation: Next glacial period).