Carbon dioxide dip propelled longer glaciation cycles

A long-term decline in atmospheric carbon dioxide (CO2) levels and the removal of rocky soils overlying bedrock together drove a crucial shift in the pattern of the Earth’s glacial-interglacial cycles around a million years ago, new German research suggests.

A team led by Matteo Willeit from Potsdam Institute for Climate Impact Research used an Earth system model to demonstrate that the gradual decrease in CO2 initiated northern hemisphere glaciation and was likely responsible for increasingly large temperature swings between glacial and interglacial periods during the Quaternary period, which stretches from 2.6 million years ago to now. 

This, they suggest, when combined with the removal of loose rock and soil, collectively called called the regolith, by scouring glaciers, “best explains the transition from a more muted 41,000-year cycle to the much larger 100,000-year glacial cycle that has dominated the last one million years”.{%recommended 7805%} 

Both hypotheses for what is known as the mid-Pleistocene transition (MPT) have been considered before, but this is the first study to suggest that in fact both were in play.

Together they are “essential to reproduce the realistic evolution of climate variability during the Quaternary, and their combination controls the timing of regime changes of climate variability,” the researchers write in a paper published in the journal Science Advances.

Of perhaps more immediate concern, they say their results also confirm that today’s concentration of atmospheric CO2 is unprecedented over the past three million years and that global temperatures during the Quaternary were never more than two degrees Celsius above preindustrial temperatures – the period up until, roughly, 1750. 

Human activity has warmed the planet by about one degree since then.

“In the context of future climate change, this implies that a failure in substantially reducing CO2 emissions to comply with the Paris Agreement target of limiting global warming well below two degrees Celsius will not only bring Earth’s climate away from Holocene-like conditions but also push it beyond climatic conditions experienced during the entire current geological period,” the researchers write.

For the study, Willeit and colleagues performed a large set of transient simulations with an Earth system model called CLIMBER-2, which includes atmosphere, ocean, vegetation, global carbon and dust models, and the three-dimensional thermomechanical ice sheet model SICOPOLIS.

They note that CLIMBER-2 is only of “intermediate complexity”, requiring “a rather coarse spatial resolution and considerable simplifications in the description of individual processes, in particular atmospheric dynamics”.

The study did not seek to find the reason for the gradual CO2 decline.