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Tying celestial mechanics to Earth’s ice ages

Mark Maslin examines the impacts of human intervention on long-term climate cycles

Tying celestial mechanics to Earth’s ice ages

In a paper in the American Institute of Physics journal, Physics Today, Professor Mark Maslin has returned to the classic hypothesis of Milutin Milanković, the Serbian mathematician and climatologist, who postulated in 1941 that variations in Earth’s orbit could push the planet’s climate in or out of an ice age. He argued that the amount of solar radiation at 65°N can vary seasonally by 25%, and that reductions in summer insolation would allow some winter ice to survive and accumulate over thousands of years, eventually forming sheets large enough to trigger an ice age.

Thirty years later, three scientists, James Hays, Nicholas Shackleton and John Imbrie, joined forces to test and verify Milanković’s theory using the international Ocean Drilling Program’s deep-sea sediment cores. Their climate record contained the same temporal cycles seen in Earth’s orbit, based on its eccentricity, obliquity, and precession. Over the past 2.5 million years, these were associated with some 50 major ice ages, each substantially changing the planet’s climate.

Mark’s paper traces detailed evidence for the current extended interglacial period and persistently high greenhouse gas (GHG) emissions during the past 7,000 years, supporting unusually stable climates, and possibly associated with the emergence of human empires.

Such effects, however, have been small compared to what humans have emitted since the start of the industrial revolution. Atmospheric carbon dioxide has increased by 47% to more than 410 ppm and methane by some 250% to over 1860 ppb. Global temperatures are thus likely to continue rising by between 1.5 to 5.6 °C during the next century. Such GHG emissions could delay the next ice age by 60 000 years, and even the next glaciation by half a million years if they reached the highest predicted level. Human fossil-fuel use may thus be creating a super-interglacial period, overriding the effect of orbital forcing on Earth’s climate.

The Quaternary may still be an appropriate term for the current geologic epoch if humans have simply delayed the next ice age. If glacial processes have been permanently changed, however, the current period may justifiably be renamed, as proposed by some, as the Anthropocene.  

See: Physics Today 73, 5, 48 (2020); doi: 10.1063/PT.3.4474


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