Milankovitch Cycles, Ice Ages, and Human Caused Climate Change with Steve Soter | StarTalk Explainer

Overview

In this StarTalk Explainer, Steve Soter discusses how Earth's climate has swung between ice ages and warmer interglacials due to long term orbital cycles. He outlines the three Milankovitch cycles, their mechanics, and how they modulate sunlight across seasons and latitudes to pace glaciations. The conversation connects these celestial rhythms to past climate episodes like Snowball Earth and to today’s warming driven by fossil fuel emissions.

They also cover isotopic evidence that the current CO2 increase is fossil-fuel derived, and explain why preserving the natural greenhouse is different from the rapid change caused by human activity. The result is a clearer picture of how natural cycles interact with human forces to shape future climate.]

Introduction

StarTalk explores how Earth’s climate has historically oscillated between ice ages and warm periods, guided by celestial mechanics and natural feedbacks.

Milankovitch Cycles

The core concept is Milankovitch cycles, comprising obliquity, precession, and eccentricity. Obliquity refers to the tilt of the Earth's axis, currently about 23.5 degrees, which slowly fluctuates and alters how sunlight is distributed between hemispheres. Precession is the slow wobble in the orientation of the axis, with a period of roughly 26,000 years. Eccentricity is the changing shape of Earth’s orbit around the Sun, with cycles around 100,000 and 400,000 years. When combined, these cycles change the seasonal and latitudinal distribution of sunlight and drive the pacing of ice ages and interglacials.

Ice Ages and Interglacials

Geological records show about 10 major ice age cycles in the last million years, with the Earth currently in an interglacial period that supports stable, civilization-building climates. The ice-albedo feedback, where more ice increases reflectivity and cooling, contrasts with warming that reduces ice and accelerates melting, creating complex climate dynamics.

Snowball Earth and Deglaciation

There were times in Earth’s history when ice covered nearly the entire planet, threatening a runaway cooling. Deglaciation likely required volcanic CO2 inputs to build up greenhouse forcing in an otherwise ice-covered world, enabling a thaw and a return to warmer conditions.

Human Influence and Evidence

The conversation then shifts to the present, where human activities have increased atmospheric CO2 by about 50 percent. Isotopic analysis shows the carbon in the atmosphere originates from fossil fuels, not volcanic sources, and the rate of emission exceeds natural sinks by roughly forty times, leading to rapid climate change and a disruption of the Milankovitch-driven rhythm. This new forcing could delay the next ice age for tens of thousands of years and push the climate into states not seen in millions of years, with positive feedbacks like Arctic sea ice loss amplifying warming.

Conclusion

Ultimately, Soter emphasizes the distinction between the beneficial natural greenhouse effect and the rapid, human amplified warming that reshapes Earth’s climate trajectory, highlighting the importance of understanding both celestial rhythms and anthropogenic forces in predicting future climate outcomes.