Milankovitch Cycles

Cycles play key roles in Earth’s short-term weather and long-term climate. A century ago, Serbian scientist Milutin [blank_start]Milankovitch[blank_end] hypothesized the long-term, collective effects of changes in [blank_start]Earth’s[blank_end] position relative to the [blank_start]Sun[blank_end] are a strong driver of Earth’s long-term [blank_start]climate[blank_end], and are responsible for triggering the beginning and end of glaciation periods ([blank_start]Ice Ages[blank_end]).

The Milankovitch cycles include: The [blank_start]shape[blank_end] of Earth’s [blank_start]orbit[blank_end], known as [blank_start]eccentricity[blank_end]; The [blank_start]angle[blank_end] Earth’s [blank_start]axis[blank_end] is [blank_start]tilted[blank_end] with respect to Earth’s orbital plane, known as [blank_start]obliquity[blank_end]; and The [blank_start]direction[blank_end] Earth’s [blank_start]axis[blank_end] of rotation is [blank_start]pointed[blank_end], known as [blank_start]precession[blank_end].

Eccentricity – Earth’s annual pilgrimage around the Sun isn’t perfectly circular, but it’s pretty close. Over time, the pull of gravity from our solar system’s two largest gas giant planets, Jupiter and Saturn, causes the shape of Earth’s orbit to vary from nearly [blank_start]circular[blank_end] to slightly [blank_start]elliptical[blank_end]. [blank_start]Eccentricity[blank_end] measures how much the shape of Earth’s orbit departs from a perfect [blank_start]circle[blank_end]. These variations affect the distance between [blank_start]Earth[blank_end] and the [blank_start]Sun[blank_end].

When Earth’s orbit is at its [blank_start]most elliptic[blank_end], about [blank_start]23[blank_end] percent more incoming solar radiation reaches Earth at our planet’s [blank_start]closest[blank_end] approach to the [blank_start]Sun[blank_end] each year than does at its farthest departure from the Sun. [blank_start]Currently[blank_end], Earth’s eccentricity is near its least elliptic ([blank_start]most circular[blank_end]) and is very slowly decreasing, in a cycle that spans about [blank_start]100,000[blank_end] years.

Obliquity – The angle Earth’s axis of rotation is [blank_start]tilted[blank_end] as it travels around the Sun is known as obliquity. [blank_start]Obliquity[blank_end] is why Earth has [blank_start]seasons[blank_end]. Over the last million years, it has varied between [blank_start]22.1[blank_end] and [blank_start]24.5[blank_end] degrees perpendicular to Earth’s orbital plane. The greater Earth’s axial tilt angle, the more [blank_start]extreme[blank_end] our seasons are, as each hemisphere receives more solar radiation during its [blank_start]summer[blank_end], when the hemisphere is tilted toward the Sun, and less during [blank_start]winter[blank_end], when it is tilted away. [blank_start]Larger tilt[blank_end] angles favor periods of deglaciation (the [blank_start]melting[blank_end] and retreat of [blank_start]glaciers[blank_end] and [blank_start]ice[blank_end] sheets). These effects aren’t uniform globally -- higher latitudes receive a larger change in total solar radiation than areas closer to the equator.

Earth’s axis is [blank_start]currently[blank_end] tilted [blank_start]23.4[blank_end] degrees, or about half way between its extremes, and this angle is very slowly decreasing in a cycle that spans about [blank_start]41,000[blank_end] years. It was last at its maximum tilt about 10,700 years ago and will reach its minimum tilt about 9,800 years from now. As [blank_start]obliquity decreases[blank_end], it gradually helps make our seasons [blank_start]milder[blank_end], resulting in increasingly warmer winters, and [blank_start]cooler[blank_end] summers that gradually, over time, allow [blank_start]snow[blank_end] and [blank_start]ice[blank_end] at high latitudes to [blank_start]build up[blank_end] into large ice sheets. As [blank_start]ice[blank_end] cover [blank_start]increases[blank_end], it [blank_start]reflects[blank_end] more of the Sun’s [blank_start]energy[blank_end] back into [blank_start]space[blank_end], promoting even further [blank_start]cooling[blank_end].

Precession – As Earth [blank_start]rotates[blank_end], it [blank_start]wobbles[blank_end] slightly upon its [blank_start]axis[blank_end], like a slightly off-center spinning toy top. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation. The trend in the direction of this wobble relative to the fixed positions of stars is known as axial [blank_start]precession[blank_end]. The [blank_start]cycle[blank_end] of axial [blank_start]precession[blank_end] spans about [blank_start]25,771.5[blank_end] years.