Will Earth's Temperature Decrease in the Next 20,000 Years?

At present, our planet is experiencing a notable rise in global average temperatures, a trend observed over the past 140 years thanks to reliable temperature records. It is generally accepted that this increase is primarily driven by human-induced emissions of greenhouse gases, particularly CO2, which have surged by approximately 50% since the pre-industrial period of the early 1700s. However, natural phenomena also influence Earth’s climate. Ian Graham poses an intriguing question regarding whether these natural factors might lead to a cooling of Earth’s temperature in the near future.

Graham queries, “I’m trying to understand the implications of Earth’s axial tilt, which currently varies by 23.5 degrees, and how it relates to Milankovitch’s theory. With the perihelion increasing and Earth warming as a result, what happens with both the increase in perihelion and Earth moving away from the Sun? I suspect the global temperature may decline over the next 20,000 years.”

To address this question, we must first explore Milankovitch's ideas.

In the early 1900s, Serbian astrophysicist Milutin Milankovitch sought to connect the physics of the Solar System with Earth’s climate theory. While annual changes in Earth's orbit around the Sun are minor, they play a significant role over longer timescales. Factors such as the Earth's axial rotation, the elliptical nature of its orbit, the gravitational influence of the Moon, and the pull from other celestial bodies all contribute to its climatic evolution.

These interactions give rise to three primary long-term variations:

1. Precession: The gradual shift in the orientation of Earth's rotational axis.
2. Axial Tilt: Variations in the angle of Earth's tilt, currently at 23.5°.
3. Eccentricity: The degree of elongation of Earth's orbit, fluctuating between circular and elliptical shapes.

1. Precession This effect is straightforward: Earth's axis, tilted at 23.5° to its orbital path, experiences a slow rotation over time. This process leads to changes in the timing of equinoxes and solstices but does not significantly impact overall temperature. The axial precession, influenced mainly by the Moon and Sun, operates on a cycle of roughly 25,771 years, while apsidal precession, caused by Jupiter and Saturn, contributes additional cycles approximately every 112,000 years.

2. Axial Tilt Currently, Earth’s axial tilt is 23.5°, which is crucial for determining seasonal variations. Over long periods, this angle oscillates between 22.1° and 24.5° in cycles of about 41,000 years. The Moon's gravitational presence stabilizes this tilt, preventing more significant fluctuations observed on Mars.

While the total energy received by Earth remains relatively stable, the distribution of energy across latitudes is highly sensitive to axial tilt. A higher tilt typically favors melting glaciers, whereas a lower tilt supports their growth. With our current tilt decreasing, the next 20,000 years may favor ice sheet growth, potentially leading to cooling.

3. Eccentricity Eccentricity refers to the shape of Earth's orbit, which varies due to gravitational interactions primarily from the gas giants. This variation influences the total solar radiation Earth receives. Currently, Earth's orbit is relatively circular, but over 100,000-year cycles, it can shift towards a more elongated shape, affecting seasonal temperature distributions.

Ultimately, the combined effects of precession, axial tilt, and eccentricity illustrate the complexity of Earth's climatic system. However, the overwhelming influence on today's climate is due to human-driven greenhouse gas emissions, which have raised Earth's average temperature by about 0.98°C since 1880. This human impact far exceeds the minor variations caused by natural cycles.

The current trajectory highlights a pressing need to address human contributions to climate change, as natural variations alone are insufficient to counteract the significant warming we are experiencing.

Send your questions to Ask Ethan at startswithabang at gmail dot com!

Starts With A Bang is authored by Ethan Siegel, Ph.D., and is based on his works, including Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.