Explainer: How the rise and fall of CO2 levels influenced the ice ages

The Earth’s climate has been quite stable over the past 11,000 years, playing an important role in the development of human civilisation. 

Prior to that, the Earth experienced an ice age lasting for tens of thousands of years. The past million years of the Earth’s history has been characterised by a series of ice ages broken up by relatively short periods of warmer temperatures.

These ice ages are triggered and ended by slow changes in the Earth’s orbit. But changing atmospheric concentrations of CO2 also plays a key role in driving both cooling during the onset of ice ages and warming at their end. 

The global average temperature was around 4C cooler during the last ice age than it is today. There is a real risk that, if emissions continue to rise, the world warms more this century than it did between the middle of the last ice age 20,000 years ago and today. 

In this explainer, Carbon Brief explores how the last ice age provides strong evidence of the role CO2 plays as a “control knob” for the Earth’s climate. It also acts as a cautionary tale of how the climate can experience large changes from relatively small outside “forcings”.

Milankovitch cycles

The Earth has experienced a number of periods over the past million years in which large continental ice sheets have covered much of the northern hemisphere. These ice ages are associated with a large drop in global temperatures – 4C or more below today’s levels – with much larger changes over land and in the high latitudes. 

These ice ages are punctuated by “interglacial” periods where temperatures rise to around current levels. The most recent ice age occurred between 120,000 and 11,500 years ago, while the current interglacial period – the Holocene – is expected to last for additional tens of thousands of years (and human activity may inadvertently delay the start of the next ice age even further).

Ice-age cycles are primarily driven by periodic changes in the Earth’s orbit. Three distinct orbital cycles – called Milankovitch cycles after their discoverer, Serbian scientist Dr Milutin Milankovitch – interact to change the distribution of incoming solar energy in ways that can dramatically affect the Earth’s climate.

Illustration of the three Milankovitch cycles from the COMET Program at the University Center for Atmospheric Research. These include:

Precession – a 26,000-year shift in the orientation of Earth’s axis of rotation that affects how much summer sun is received at high latitudes (and shifting how much reaches the north vs south).
Obliquity – a 41,000-year change in the tilt of the Earth’s axis relative to the sun that changes how much sun is received during a year at the poles versus the equator.
Eccentricity – a 100,000-400,000 change in the shape of the Earth’s orbit around the sun that alters the length of the seasons and affects the...