by E. W. Wolf, November 4, 2019 in WUWT

From Nature

An analysis of air up to 2 million years old, trapped in Antarctic ice, shows that a major shift in the periodicity of glacial cycles was probably not caused by a long-term decline in atmospheric levels of carbon dioxide.

Eric W. Wolff

During the past 2.6 million years, Earth’s climate has alternated between warm periods known as interglacials, when conditions were similar to those of today, and cold glacials, when ice sheets spread across North America and northern Europe. Before about 1 million years ago, the warm periods recurred every 40,000 years, but after that, the return period lengthened to an average of about 100,000 years. It has often been suggested that a decline in the atmospheric concentration of carbon dioxide was responsible for this fundamental change. Writing in Nature, Yan et al.¹ report the first direct measurements of atmospheric CO₂ concentrations from more than 1 million years ago. Their data show that, although CO₂levels during glacials stayed well above the lows that occurred during the deep glacials of the past 800,000 years, the maximum CO₂ concentrations during interglacials did not decline. The explanation for the change must therefore lie elsewhere.

Understanding what caused the shift in periodicity, known as the mid-Pleistocene transition (MPT), is one of the great challenges of palaeoclimate science. The 40,000-year periodicity that dominated until about 1 million years ago is easily explained, because the tilt of Earth’s spin axis relative to its orbit around the Sun varies between 22.1° and 24.5° with the same period. In other words, before the MPT, low tilts led to cooler summers that promoted the growth and preservation of ice sheets.

But after the MPT, glacial cycles lasted for two to three tilt cycles. Because the pattern of variation in Earth’s orbit and tilt remained unchanged, this implies that the energy needed to lose ice sheets² had increased. One prominent explanation³ is that atmospheric levels of CO₂ were declining, and eventually crossed a threshold value below which the net cooling effect of the decline allowed ice sheets to persist and grow larger.

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