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Simple rule predicts when an ice age ends

Chronis Tzedakis and colleagues unlock mysteries of the ice ages

Simple rule predicts when an ice age ends

A new study published in Nature on 22 February, led by Professor Chronis Tzedakis (UCL Geography, Environmental Change Research Centre), with colleagues from the Universities of Cambridge and Louvain, combines existing ideas into a simple model which predicts which solar energy peaks during the last 2.6 million years led to the melting of the ice sheets and the start of warm periods.

It has long been realised that the cycles of Earth’s climate, between cold (glacial) and warm (interglacial) periods, have been influenced by changes in the planet’s orbit around the Sun and the tilt of its axis. These affect the solar energy available in summer to melt ice at high northern latitudes.

The study of such fluctuations since the previous deglaciation shows that the succession of 110 solar energy peaks, about every 21,000 years, led to only 50 complete ice sheet meltings, every 41,000 years,

Chronis explains why this might be the case:

“The basic idea is that there is a threshold for the amount of energy reaching high northern latitudes in summer. Above that threshold, the ice retreats completely and we enter an interglacial.”

From 2.6 to 1 million years ago, this sequence predicted almost perfectly when interglacials started and the ice sheets disappeared. About a million years ago, however, the threshold rose, so that the ice sheets kept growing for longer than 41,000 years, and became larger and more unstable.

The research combined these observations into a simple model, using only solar energy and waiting time since the previous interglacial, to predict all the interglacial onsets of the last million years, about every 100,000 years. The reasons the energy threshold rose around a million years ago require more research, but may be due to a decline in the concentration of CO2.

Nevertheless, as Chronis comments, “Finding order among what can look like unpredictable swings in climate is aesthetically rather pleasing”.

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Arctic Ice

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