We now know how much global warming has delayed the next ice age

Without human-induced climate change, Earth may have been on track to plunge into another ice age within 11,000 years. This long-term forecast of the planet’s “natural” climate is based on a new analysis of how wobbles in the shape of its orbit and tilt of its axis combine to change the amount of solar energy reaching the planet.

For millions of years, these orbital oscillations – known as Milankovitch cycles – brought the planet in and out of glacial periods about every 41,000 years. But the past 800,000 years have seen ice ages occur only every 100,000 years or so. Ambiguities in the record of when ice sheets advanced and retreated meant it wasn’t possible to explain how orbital changes were involved in driving this longer cycle, a mystery known to palaeoclimatologists as the “100 thousand year problem”.

Where previous studies tried to link changes in orbit to specific periods, such as the onset of an ice age, Stephen Barker at Cardiff University, UK and his colleagues took a new tack. They looked at the overall patterns of how glacial periods, also called ice ages, fade and return during the intervening “interglacials”. This enabled them to link changes in orbit with changes in ice – despite ambiguities in the ice record over the past million years.

They found these 100,000-year cycles appear to follow a straightforward rule. For the past 900,000 years, every interglacial has occurred after Earth’s axis wobbled at its furthest point towards the Sun at the same time it was tilting closer to the sun, following the most circular phase of Earth’s orbit.

This suggests that all three of these aspects of Earth’s orbit – known as precession, obliquity and eccentricity – combine to create the 100,000-year glacial cycle, says Barker. “Since 900,000 years ago this simple rule predicts every one of those major glacial termination events. This tells us that it’s really quite easy to predict,” he says.

Based on that rule, and absent the warming influence of our greenhouse gas emissions, the next interglacial period following the one we are living in – known as the Holocene – could be expected to begin around 66,000 years from now. But that “could only start if there was a glacial period before then”, says Barker.

The phasing of obliquity and precession that preceded the Holocene suggests that ice would be likely to be well underway between 4300 and 11,100 years from now. We might even be currently living at what would have been the onset of this next ice age. “Of course, that’s only in a natural scenario,” says Barker.

The more than 1.5 trillion tonnes of carbon dioxide humans have emitted into the atmosphere since the industrial revolution are expected to cause enough warming to disrupt this long-term glacial cycle.

“The amount we’ve already put into the atmosphere is so great that it will take hundreds to thousands of years to pull that out via natural processes,” says Barker. However, he says more research is needed to define Earth’s future natural climate in more detail.

This is in line with earlier modelling that suggests rising CO2 levels due to anthropogenic emissions will prevent the onset of the next ice age for tens to hundreds of thousands of years, says Andrey Ganopolski at the Potsdam Institute for Climate Impact Research in Germany.

However, he says even pre-industrial levels of CO2 in the atmosphere may have been high enough to delay the onset of the next ice age by 50,000 years. That’s due to the unusually minor orbital variations expected in coming millennia and the unpredictable way Earth responds to those changes.