Ned Nikolov, Ph.D.
Dec 30, 2021
There has been a long-standing belief in Paleoclimatology that orbital variations (a.k.a. Milankovitch cycles) have been responsible for the initiation and/or duration of glacial cycles (Ice Ages) over the past 800 Ky. Milankovitch cycles are often referred to as a pacemaker of the Ice Ages. This myth dates back to 1970s, when sediment cores revealed a weak correlation in the frequency domain between Earth’s 41-ky obliquity (axial-tilt) cycle and the periodicity of Ice Ages during the early Pleistocene (Quaternary). However, in the late Pleistocene, the frequency of glacial cycles better match the Earth’s 100-ky eccentricity cycle, which further fueled the confusion. Yet, no one has been able to demonstrate a meaningful relationship between glacial cycles and any of the Earth’s 3 orbital parameters obliquity, eccentricity and precession or combination thereof on a linear time scale. A physical causation requires a strong correlation between parameters in the time domain, not the frequency domain!
Using recent data describing the dynamics of global surface air temperature inferred from geological proxies and variations of Earth’s orbital parameters computed by the best available orbital models, we show here the lack of a physically meaningful relationship in the time domain between Milankovitch cycles and Ice Ages for the past 784 ky. Orbital data came from the state-of-the-art model by Laskar et al. (2004 and 2011) and were downloaded from a page on the official website of the Virtual Observatory Paris Data Center in France. The top-of-the-atmosphere (TOA) solar insolation on summer solstice at 65o N latitude was obtained from the Milankovitch Orbital Data Viewer of Colorado State University. A time series of global surface air temperature for the late Pleistocene (past 784 ky) was constructed from published reconstructions by Snyder (2016) and Friedrich et al. (2016). All data series used in the analysis share the same temporal resolution of 1,000 years. Standard Score (a.k.a. Z-Score) is used in some graphs to plot time series having different measurement units on the same axis.
Changes in Earth’s mean annual distance to the Sun (measured in Astronomical Units, AU) resulting from variations of planet’s orbital eccentricity have been minuscule for the past 800 ky (Fig.1) causing only a ± 0.05 W m-2 variation in the Earth’s baseline TOA Total Solar Irradiance (TSI) (Fig. 2). According to recent solar reconstructions (e.g., Egorova et al. 2018), this variation is 50 – 100 smaller than the TSI fluctuations caused by Sun’s magnetic activity on centennial to millennial time scales.
Hence, orbitally induced TSI changes are bound to have an immeasurably small effect on Earth’s global surface temperature. The latter shows practically no correlation to TSI variations induced by the Milankovitch cycles (Fig. 3a and 3b).
Earth’s obliquity (axial tilt) varied narrowly between 22.3o and 24.5o for the past 800 ky (Fig. 4) while showing no relationship to global surface temperature over this time period (Fig. 5a and 5b). The linear correlation between obliquity and the global surface temperature is R2 = 0.053 (Fig. 5b).
Earth’s orbital eccentricity also varied over a narrow range (from 0.004 to 0.05) during the past 800 ky (Fig. 6). Note a pronounced 400-ky cycle in the eccentricity time series, which is not found in the global temperature record (Fig. 7a). The correlation between global temperature and eccentricity is rather weak (R2 = 0.235) (Fig. 7b) albeit a bit better than the correlation between temperature and TSI or temperature and obliquity discussed earlier.
Figures 8a and 8b depict the relationship between rates of change of global temperature and Earth’s orbital eccentricity. These time-derivatives are correlated with a coefficient R2 = 0.341, which is the strongest relationship found between global temperature and any orbital parameter of Earth! However, this correlation does not imply a physical causation, because eccentricity changes have a negligibly small effect on TOA TSI and surface temperature.
Paleoclimatologists claim that Milankovitch cycles affect Earth’s climate chiefly through changes in the solar flux reaching the top of the atmosphere at 65o N latitude on the day of June summer solstice. According to this concept, a decreased summer isolation at 65o N due to a specific configuration of obliquity, precession, and eccentricity impedes the summer melting of snow at high latitudes and promotes ice accumulation, which over time allows glaciers to grow, thus initiating an Ice Age. The reverse process is believed to take place, when the summer isolation anomaly at 65o N is above its long-term mean (baseline value). During such periods, more snow/ice is expected to melt in the summer, which is thought to initiate deglaciation… However, the data reveal a complete lack of a relationship between summer insolation at 65o N and the global surface temperature for the past 784 Ky (see Figs. 9a and 9b). The correlation coefficient between these time series is a meager R2 = 0.031 (Fig 9b).
Roe (2006) claims to have found a strong correlation between the rate of change of Earth’s ice volume and the TOA June insolation anomaly at 65o N latitude. However, in his study, the ice volume (measured in “arbitrary units”) was estimated by models relying on oxygen isotopes. Noticeably one of the models (SPECMAP) dating back to 1984 assumed a-priori that ice volume and orbital forcing were related. Hence, the ice volume estimates utilized by Roe (2006) appear to be biased toward orbital cycles and much less reliable than modern proxy-based reconstructions of global temperature. Also, planetary-level climate change is physically much better defined through variations of the absolute global surface temperature rather than fluctuations of some unitless ice-volume estimates. This is because temperature is a fundamental metric controlling the dynamics of icesheets and sea ice. In an effort to “mimic” Roe’s approach as closely as possible, we compared the rate of change of global surface temperature to the TOA June insolation anomaly at 65o N latitude for the past 784 Ky (Fig. 10a & 10b). The correlation coefficient between these time series is R2 = 0.254 (Fig. 10b), which suggests an effective lack of control by the orbitally driven high-latitude summer insolation changes on the global surface temperature of Earth.
CONCLUSION: The available data indicate that, in the time domain, the Milankovitch orbital cycles are poorly correlated (if at all) to changes of global surface temperature inferred from sediment- and ice-core proxies for the past 784 Ky. Hence, the geological record provides no evidence that Ice Ages of the past one million years were controlled or even influenced by known variations of Earth’s orbital parameters. Putting to rest the Milankovitch orbital hypothesis of climate change as an unsupported conjecture seems to be an important and necessary step toward developing a new and physically robust Paradigm of paleoclimate drivers as discussed in this video:
via Tallbloke’s Talkshop
January 3, 2022