Paleoclimate - Pacemaker of Quaternary Climate Change

Review the role of orbital forcing as the “Pacemaker of the Ice Ages” (Hays et al., 1976). Are there elements of the Quaternary climate record that orbital forcing alone cannot explain?

Trinity Hall, Fiona Fang, 03/2024, 1992 words

Changes in the earth’s orbital geometry are the fundamental cause of the succession of Quaternary ice ages which began about 2.58 Ma (Hays et al., 1976). There are three orbital parameters, namely eccentricity, obliquity, and precession. The Earth’s obliquity varies every 41 kyr from 21.5° to 24.5°, with shallower tilt results in a more even distribution of heat between summer and winter, vice versa. Precession, Earth’s axial wobble, alters the timing of seasons with a cycle of 23 kyr. Eccentricity, changes the orbit’s shape from less to more elliptical every 100 kyr due to planetary gravitational forces, has less impact on solar radiation but significantly affects climate by modulating precession’s influence. When eccentricity is high, axial precession has a greater impact on seasonality (Woodward, 2014). These parameters collectively govern Quaternary glaciation by changing summer insolation at high latitudes. However, events like the initiation of Northern Hemisphere Glaciation (iNHG) and the Mid-Pleistocene Transition (MPT) are not solely explained by orbital factors. Millennial-scale climate variations during glaciations also exhibit independence from orbital theories, though they respond to the broader climate state shaped by insolation. The following essay will introduce these climate events, ultimately concluding that although orbital forces are deemed the “Pacemaker of the Ice Ages” (Hays et al., 1976), the role of internal dynamics in shaping Quaternary climate is equally significant and should not be overlooked.

Astronomical parameters drive Quaternary glacier cycles by altering the summer insolation in the mid to high latitudes of the Northern Hemisphere (NH), as the NH hosts a significant portion of the Earth’s terrestrial surfaces and hence are susceptible to changes in solar radiation. Cooler summers, during which winter snow does not fully melt, are conducive to the growth of ice sheets, vice versa. When using traditional method of using the insolation data of June, July, and August (JJA) to represent summer insolation, the precession signal has the most significant impact on summer insolation in the Northern Hemisphere compared the other two parameters, as illustrated in Figure 1. However, both glacial cycles of the Quaternary, the 40-kyr cycle of the Early Pleistocene and the 100-kyr cycle of the Late Pleistocene, lack a precession signal. For the 40-kyr cycle of the Early Pleistocene, such mismatch may be attribute to the calculation method, as according to Kepler’s second law, the large variations in the amplitude of insolation changes occur at the 23-kyr period during summer are cancelled by shortened summer durations. A more appropriate indicator would be Integrated Summer Insolation (ISI) that calculate the insolation data of the entire summer half year. Using ISI, the 41-kyr obliquity signal is more predominant than the 23-kyr signal (Huybers, 2006). The offsetting responses in the Northern and Southern Hemispheres may also have contributed to the absence of 23-kyr signal (Raymo et al., 2006). While attractive, such cancellation has yet to be verified by data sets (Herbert et al., 2023). The late Pleistocene 100-kyr glacial cycles are even harder to explain, as the eccentricity which produces the 100-kyr signal seems to be the least powerful parameter in impacting NH summer insolation. The nature of this 100-kyr periodicity has aroused a heated debate. Some scholars argue that quasiperiodic 100-kyr cycle does not result directly from eccentricity forcing but rather consists of multiples of obliquity beats. According to Huybers and Wunsch (2005), the null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level, whereas the corresponding ones for eccentricity and precession cannot be rejected. Huybers (2007) further argues that the late Pleistocene 100-kyr cycle is the result of “skipping” one or two obliquity maxima, which correspond to 80 or 120 kyr cycles respectively and give an average variability of 100 kyr. There are also studies highlighting the importance of internal forcings in driving this prolonged glacial cycle, such as ice volume and CO2 (Ganopolski and Calov, 2011) and sea ice variability (Gildor and Tziperman, 2001). Lastly, Wunsch (2003) suggests a more radical claim that the contribution of the Milankovitch frequencies to climate change at most represents only a small fraction of total climate variance. From the ice and marine core records dominated by red noise and random walk processes, he inferred that the Quaternary climate variation is a semi-random fluctuation without true periodicity.

Figure 1 Orbital signals and insolation variation over the past 2 Ma (Berends et al., 2019)

While orbital theory can, at least partially, explain the timing of Quaternary glacial cycles, it struggles to clarify certain climatic events. The first is the iNHG at the start of the Pleistocene, evidenced by the cooling trend in benthic δ18O data and the presence of “untouched” organic tundra soils in deepest portion of present Greenland Ice Sheet (Fedorov et al., 2013; Bierman et al., 2014). While some have attempted to attribute iNHG to orbital changes, such as an increased amplitude in obliquity cycles from 3.2 Ma leading to heightened NH seasonality and a consequent global cooling trend (Maslin et al., 1998), these orbital changes alone are insufficient to explain the phenomenon, especially since similar orbital extremes at 5 and 7.5 million years ago did not result in comparable glaciation (Walker, 2014). More convincing explanations emphasis the importance of internal forcing, including tectonic change and greenhouse gases. For instance, the narrowing of the Indonesian Seaway between 4 and 3 million years ago likely reduced heat transfer to northern latitudes (Cane & Molnar, 2001), while the uplift of Tibetan-Himalayan and Sierran-Coloradan ranges strengthened monsoonal patterns and rainfall, promoting global cooling through increased chemical weathering and CO2 removal from the atmosphere (Raymo and Ruddiman, 1992). Additionally, the closure of the Panama gateway bolstered the Gulf Stream, bringing warm saline waters to the north and reinforcing deep-water formation in the Labrador Sea, subsequently increasing moisture in the high northern latitudes (Haug and Tiedemann, 1998). Lunt et al. (2008) suggest CO2 decrease to be the main driver of iNHG while other mechanisms above playing only a minor role, as their model shows that an increase in snowfall alone over Greenland is not sufficient for Greenland glaciation, while the prevalence of cooler summers is a necessary condition for substantial ice sheet growth. They suggested a dramatic decrease in the CO2 from the Maximal Pliocene values of 410 μatm to early Pleistocene values of 300 μatm at 2.0 Ma. Tan et al.’s model (2018) also shows that orbital forces by themselves are inadequate for the iNHG, which only ensues under CO2 concentrations less than 320 ppm.

The MPT is another significant climatic shift that can’t be explained by orbital theories. It occurred at about 1 Ma and marked the change in glacial cycles from 41-kyr to 100-kyr, without corresponding changes in Earth’s orbital configuration. This shift likely resulted from internal climatic forcings, notably within ice sheets and oceanographic storage. Firstly, marine benthic δ18O records indicate a transformation of the Northern Hemisphere’s ice sheets during the MPT from thin but extensive to much taller ice caps (Portier et al. 2021). This change can be explained by the ice-bedrock interaction. Earlier glaciations in NH formed ice sheets over thick layers of soil and sediment (10-50 m) that has been unglaciated for millions of years (Berends et al., 2021). The weight of the ice melted the bottom ice layers, causing water to trickle into the sediments, reducing friction and facilitating slippage (Ruddiman, 2013). MPT marked the time when this regolith was entirely eroded away, exposing the bedrock (Clark and Pollard, 1998). This change made the base less prone to slipping, allowing ice sheets to grow thicker and survive through weaker insolation maxima (Ruddiman, 2013). The dust produced by sediments erosion also increased the albedo and facilitated ice sheet growth (McGee et al., 2010). The sedimentary records in the US show that ancient soils and sedimentary rocks were the main type of debris eroded prior to the last 1.5 to 1 million years, while freshly pulverized debris from hard bedrock has predominated since that time, providing evidence for this hypothesis (Ruddiman, 2013). Willeit et al.’s model (2019) also gives the best fit result to the observed δ18O record with a prescribed gradual removal of regolith during the Pleistocene, model gives the best fit result to the observed δ18O record. However, they still find a maximum ice-sheet extent that is substantially smaller pre-MPT than post-MPT, at odds with the (sparse) geomorphological evidence. Recent studies also suggest a significant impact of global deep ocean temperature changes on the benthic δ18O curve, possibly diminishing the previously assumed dominance of ice sheet dynamics in driving global climate cycles (Köhler & Van de Wal, 2020). Beyond ice feedbacks, carbon’s role in driving MPT is also gaining attention. As articulated by Farmer et al. (2019), during the MPT, the southern-sourced carbon-rich water might flood the Atlantic, bolstering the deep ocean’s carbon reservoir (Figure 2). Concurrently, increased iron dust supply to the Southern Ocean, particularly during ice ages, boosted biological productivity and CO2 drawdown, potentially supporting ice sheet growth through insolation maxima (Martínez-Garcia et al., 2011; Chalk et al., 2017). This contributed to the self-sustained growth of ice sheets and helped it survive insolation maxima. However, the exact causes of these shifts in Southern Ocean circulation and biology, and whether they are causes or results of MPT, remain uncertain (Ford and Chalk, 2020; Menviel, 2019).

Figure 2 Oceanic circulation during Pleistocene (Ford and Chalk, 2020)

The abrupt millennial-scale oscillations during glacial periods, known as Dansgaard-Oeschger (D-O) cycles, are also not readily explained by orbital theories. DO cycles are marked by swift warming phases followed by more gradual cooling, as evidenced in the last glacial period’s Greenland ice cores (Ruddiman, 2013). A leading theory, the “salt oscillator hypothesis” (Broecker et al., 1990), suggests that these rapid climatic fluctuations are initiated by the Atlantic Meridional Overturning Circulation (AMOC). During warmer interstadials, the northward movement of oceanic heat warms northern Atlantic, while melting ice sheets and the influx of freshwater lower surface salinity, consequently weakening the AMOC, leading to colder stadial periods. During the stadials, a reduction in meltwater input increases salinity, strengthening AMOC and prompting a return to interstadials. Such shift in AMOC also impact meridional heat transport. When a surge of freshwater in the North Atlantic suppresses AMOC, the northern hemisphere cools, while the southern hemisphere and the tropics warm up. Once deep-water formation is reinitiated, the meridional heat transfer resumes, reversing the temperature trends, known as “bipolar seesaw” (Knutti et al., 2004). However, this explanation is complicated by the fact that changes in Atlantic heat transport may be offset or even overwhelmed by larger-scale heat transfers in the global atmosphere and variations in Pacific Ocean heat transport (Wunsch, 2006; Pedro, 2018). Meanwhile, it has been posited that AMOC variability could be a consequence of DO events rather than their cause (Kleppin et al., 2015). Sea ice coverage is hypothesized to be another trigger of DO cycles. During cold stadial periods, the sea ice layers prevent warm, salty layers beneath to loss heat into the atmosphere. At the onset of an interstadial, a loss of sea ice reduces stratification, facilitating the upward mix and release of this stored heat, creating a positive feedback (Dokken et al., 2013). Notably, while the phase and structure of millennial events are mainly dominated by internal forces, insolation can indirectly control their timing and frequency by regulating the total ice volume (Sun et al., 2021). Statistical analysis illustrates that millennial events are more likely to occur during the intermediate glacial periods during the past 1.5 Ma, because both too high and too low insolation suppress the free oscillations of thermohaline circulation (Rial and Yang, 2007).

In summary, although orbital forces have played a crucial role as the pacemaker of Quaternary glaciations, their influence is not exclusive. The initiation of the Northern Hemisphere Glaciation (iNHG) highlights the significance of tectonic shifts and greenhouse gas concentrations, while the Mid-Pleistocene Transition (MPT) underscores the ice sheet dynamics and variations in oceanic carbon storage. Dansgaard-Oeschger (DO) cycles point to the dynamic nature of the Atlantic Meridional Overturning Circulation (AMOC) and potential sea ice feedback mechanisms. These internal drivers are also pertinent to current concerns regarding CO2 levels and ice melt amid anthropogenic climate change.

Supervisor feedback: This is an excellent essay Fiona, well done! You have demonstrated an excellent level of knowledge and understanding, and you have clearly worked very hard on this essay. I am impressed with the level of detail you managed to include, with every point backed up with sufficient explanation and evidence. I liked that you included figures and the bibliography is very extensive. I do not have much to critique in this essay. All I can say is well done and keep up the good work!

 

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