Around 11.7 thousand years ago, the ice around the world started to recede. Across the northern hemisphere, the massive Laurentide and Finnoscandian shelves were sent into remission. The meltwater from the Laurentide became the Great Lakes in the United States. The Finnoscandian shelf was integrated into the Arctic. Life was once again able to comfortable inhabit Canada, the Northern United States, and Eurasia. The Earth was renewed - global populations surged. The long winter had ended and spring had begun.

Doubtless, it was a time for celebration. For 103,300 years the Earth had been a frozen tundra - having succumb to the previous stadial period. For nearly 1,500 generations men endured the tundra. To the early humans who emerged into the modern age, the notion of a moderate climate must have long become the stuff of legend. When the clouds finally broke and the sunshine emerged bright and clear for the first time, those weary men must have been in disbelief. They may have imaged that the warmth would be a fleeting thing and that soon enough, the monumental ice shelves would be heard grinding, crunching, and popping as they carved their way back across the barren tundra.

In a way, the skeptics were right. The warmth truly is a fleeting thing. The modern scholarly community has yet to provide us with a clear understanding of our cirumsmtances at present. There's much debate about when the last ice age began, although we do know with confidence that it "ended" in 11.7 MYE. The first problem is that there isn't a clear line of demarcation between the "ice age" and the "present age". The transition was a gradual one. It wasn't precipitated by a single event but a set of interconnected incremental shifts in the Earth's position. The best course of action may be to consider Earth's current location, relative to where it was during the ice age.

As a reminder, Earth's orbit around the Sun varies between being nearly circular to being more elliptical in cycles of 90,000 to 100,000 years. Planetary orbits have a perihelion, which is the closest point on its orbital path to the Sun, and an aphelion which is the furthest point in its orbit from the sun. When eccentricity is near circular, the difference in distance between aphelion and perihelion are negligible. A near circular orbit means the temperature difference between seasons is very low. When eccentricity is high, the difference in distance between aphelion and perihelion are substantial, causing dramatic yearly change in temperature.

Currently Earth's eccentricity is 3%: near circular. When the eccentricity is at its greatest, there's a shocking difference of 20% to 30% in sunlight exposure between seasons. Changes in eccentricity alone are thought to produce climate change which may account for periods of glaciation. However, eccentricity's overall effect is either compounded or reduced by other factors in Earth's motion.

Obliquity refers to Earth's tilt on its axis. A common misunderstanding is that the seasons are primarly caused by changes in distance from the sun throughout the course of the year. While distance does play a role, obliquity is far more impactful. The amount of heat transfer that occurs between Earth and the Sun is primarily determined by the angle of intercept of in coming light on the face of the planet. When the surface is perpendicular to the direction of light travel, heat transfer is at its greatest. On Earth, the Sun's rays are at their most potent near the equator, although not identical to it. As the angle of intercept increases beyond 90 degrees (perpendicular), more of the light is reflected off the surface instead of being absorbed. This effect occurs most strongly at the poles.

Obliquity (tilt) determines the habitable zone of Earth. Just as important however, is its role in seasonal change and glaciation. When tilt is at is greatest, each of the poles receives more sunlight than when tilt is decreased. It's easiest to conceptualize a scenario where the tilt were zero degrees. In such a case, neither pole would ever receive any direct sunlight. This would quickly cause ice to build at the poles which would cool the entire planet.

On Earth, the poles experience very irregular sunlight exposure. North and south poles each experience a period of extended darkness with zero sunlight - while the other pole experiences the opposite: a long day without night. In the same way that the length of day and night changes throughout the year in response to the seasons, the poles experience extended periods of darkness and light. They are much more strongly effected by Earth's position relative to the Sun.

Obliquity on Earth oscillates between 22.1 to 24.5 degrees and has a periodicity of 40,000 years. Today Earth's axis is tilted 23.5 degrees. Our current obliquity is near ideal. It allows each of the poles to experience enough exposure to light to prevent further freezing during its warm period, and enough of a cool period to prevent large scale deglaciation. As Earth's tilt nears it's least oblique however, sunlight reaching both poles is lessened substantially year round. Instead of Earth allowing each a period of warm and cool, both are almost always cool-ish.

As the Earth spins (daily) on its tilted axis revolving (yearly) around the Sun, it also rotates (or precesses) in a cycle that lasts 25,722 years. Precession has been one of the most important ways for humans to reckon time and to judge location and position for a very long time. Precession alters what's referred to as the "celestial dome"; The celestial dome is the "night sky" as seen from Earth. Precession is caused by the tidal forces of the Moon and Sun on Earth as it spins on its axis. In short, it's being "pulled" off center in proportion to its obliquity and distance from other gravitational bodies.

The effects of precession on climate require a working understanding of the other mechanisms at play. Earth's revolution around the Sun varies in eccentricity. Eccentricity is measured by the difference in aphelion and perihelion; the furthest and closest points from the Sun during a yearly revolution, respectively. Earth's obliquity (tilt) means that either the northern or the southern hemisphere receives more direct sunlight during certain times of year. When the northern hemisphere is tilted toward the Sun it experiences summer while the southern hemisphere (tilted away) experiences winter.

At present, the southern hemisphere is tilted toward the Sun during Earth's perihelion (closest orbital period). The southern hemisphere is tilted away from the Sun during aphelion (furthest orbital period). The result of this is that the southern hemisphere experiences summers that are warmer because they take place while Earth is closest to the Sun, and winters that are colder because they take place while Earth is furthest from the Sun. Simultaneously, the northern hemisphere experiences a dampening of the seasons. Summers begin while Earth is furthest from the Sun, and winter begins when it's closest. The overall effect is that the climate in the northern hemisphere is more moderate and invariant, while the southern hemisphere experiences more dramatic seasonal shifts.

The scenario outlined above is all a product of our current precessional period. As the precessional cycle continues, the relationship between Earth's tilt, eccentricity, and their combined effect on the hemispheres will cycle through the full gamut of possibility. 13,000 years from now, Earth will tilt in the opposite direction. The northern hemisphere will endure the more pronounced seasonal shift for a time. Similarly, there will be periods where neither hemisphere experiences pronounced seasonal changes.

The scenario outlined above is all a product of our current precessional period. As the precessional cycle continues, the relationship between Earth's tilt, eccentricity, and their combined effect on the hemispheres will cycle through the full gamut of possibility. 13,000 years from now, Earth will tilt in the opposite direction. The northern hemisphere will endure the more pronounced seasonal shift for a time. Similarly, there will be periods where neither hemisphere experiences pronounced seasonal changes.

There are a few ideas about what comes next. Milankovitch (the man who first described and measured these cycles) believed that obliquity played the most important role in Earth climate. More specifically, he believed that changes in the amount of light received in the northern hemisphere during summer months played a pivotal role in climate change - predicting a glacial period every 41,000 years. However, by taking samples of ice cores modern researchers have been able to demonstrate that glaciation is occuring regularly at 100,000 year intervals and has been doing so for the last couple million years.

There is no scientific consensus on the subject and surprisingly little research is being done to address the issue. Uncovering the truth requires a more complete data set. A more accurate picture of Earth's orbital cycles along with its relationship to other planets within the solar system may provide helpful clues. We may very well be approaching the next stadial period. It may be in the next decade or it could be 200 years from now.