Lecture 5: Lecture Topic: Milankovitch Theory and the Ice Ages

Milankovitch Theory and the Ice Ages

I. Introduction

Overview of Earth’s Climate Variability

The Earth's climate system is characterized by substantial changes across various timescales, with glacial-interglacial cycles playing a critical role in shaping long-term climate trends. These cycles have a profound impact on ecological systems, sea levels, and global climate patterns, making it essential to understand the mechanisms underlying these fluctuations.

Significance of Milankovitch Theory

Milutin Milankovitch (1879-1958) made pivotal contributions to the understanding of the relationship between Earth’s orbital characteristics and climate changes. He is credited with developing the orbital theory of ice ages, which posits that variations in Earth’s orbit and axial position influence glaciation and deglaciation processes. Although earlier scientists had suggested a link between orbital variations and glaciation, Milankovitch's detailed analyses and precise calculations provided a solid foundation for this theory.

Core Hypothesis of Milankovitch

Milankovitch's hypothesis can be summarized as follows:

• Orbital variations cause changes in the seasonal and latitudinal distribution of solar insolation, which then act as a “pacemaker” for glacial and interglacial cycles.

• The theory posits that astronomical forcing is responsible for setting the periodicity of these climate cycles, rather than being the direct driver of climatic oscillations.

Empirical Support for Milankovitch’s Hypothesis

The landmark study by Hays, Imbrie, and Shackleton in 1976 provided empirical evidence supporting Milankovitch's theory. Their research indicated that orbital periodicities manifest within the marine δ18O record, which serves as a reliable proxy for global ice volume. Through spectral analysis, they identified significant power at frequencies corresponding to Earth's orbital parameters. However, despite this empirical foundation, the mechanisms by which changes in insolation lead to the large climate shifts observed during glacial-interglacial cycles remained largely unexplained, particularly regarding the dominance of the 100-kyr cycle.

II. Milankovitch's Contributions and Key Concepts

Milankovitch’s foundational text, "Canon of Insolation and the Ice-Age Problem" (1941), is a detailed examination of how orbital mechanics interact with climatic conditions affecting glaciation. This comprehensive work is organized into six chapters, addressing:

1. Planetary motion and mutual perturbations

2. Earth's rotation

3. Secular wanderings of the rotational poles

4. Earth's insolation and its secular changes

5. Connections between insolation and temperature variations

6. Mechanisms, structure, and chronology of ice ages

Key Concepts and Terminologies

Caloric Summer Half-Year Insolation

Milankovitch emphasized the critical role of seasonal insolation on climate, particularly the concept of caloric summer half-year insolation. This refers to the amount of solar energy received during those months when daily insolation exceeds the average insolation received in the converse period, allowing for the preservation of snow and promotion of ice sheet growth. The measurement typically spans from the March equinox around March 21 to the September equinox around September 21.

Latitude of 65°N

Milankovitch specifically focused on the latitude of 65°N, which is strategically located near the Arctic Circle. This latitude is particularly sensitive to changes in summer insolation, strongly influencing the dynamics of ice sheets. The equilibrium line, the boundary that separates regions of net ice accumulation from those of net ablation, is found at this latitude. This line is influenced by temperature and precipitation, determining whether ice sheets can advance or retreat based on climatic conditions.

Equilibrium Line and Summer Insolation

The Equilibrium Line is a critical concept in understanding glacial dynamics. It defines the zone where ice accumulation (through snowfall and new ice formation) balances the rate of ablation (which includes melting and evaporation). The position of this line is not static; it fluctuates based on temperature and precipitation patterns, affecting the overall stability of ice masses. Milankovitch theorized that during periods characterized by lower-than-normal caloric summer insolation, cooler summer temperatures would allow for snow and ice to persist, thereby facilitating the growth of glacial bodies.

III. Orbital Periodicities and the δ18O Record

Influential Earth’s Orbital Parameters

Milankovitch's theory emphasizes several orbital parameters that influence insolation:

1. Eccentricity: This parameter refers to the shape of Earth's orbit as it varies between being more elliptical and more circular, operating over cycles approximately every 100,000 and 400,000 years.

2. Obliquity (Axial Tilt): This is defined as the tilt of Earth's axis relative to its orbital plane, varying between 22.1 and 24.5 degrees over approximately 41,000 years. Changes in axial tilt affect seasonal temperature distributions across the planet.

3. Precession (Axial and Orbital): Precession describes the slow wobble of Earth's axis and the rotation of its elliptical orbit, with periods of roughly 19,000 and 23,000 years. This results in changes in the timing of the seasons.

δ18O Record as a Proxy for Ice Volume

The ratio of the two oxygen isotopes, 18O (heavy) and 16O (light), present in marine sediments, serves as a proxy for global ice volume. During glacial periods, lighter isotopes are preferentially stored in ice sheets, which increases the concentration of heavier isotopes in the oceans and subsequently in the calcium carbonate shells of marine organisms. This methodology provides scientists with insight into historical climatic variations.

Observed Periodicities in the δ18O Record

Through spectral analysis of the δ18O record, key periodicities have been identified, including:

• ~100 kyr cycle primarily associated with eccentricity changes.

• ~41 kyr cycle corresponding to variations in obliquity.

• ~19-23 kyr cycles linked to axial and orbital precession.

The Quaternary Period

The Quaternary Period, which spans the last 2.58 million years, has been marked by continuous glacial-interglacial cycles divided into distinct intervals:

• Intensification of Northern Hemisphere Glaciation (INHG): Initiated in the late Pliocene (around 2.7 million years ago), leading to substantial growth of ice sheets in the Northern Hemisphere.

• The "41-kyr world" of the early Pleistocene: Characterized by glacial cycles with an approximate periodicity of 41,000 years. These cycles were less intense and exhibited shorter, more symmetrical characteristics.

• Middle Pleistocene Transition (MPT): Marked a significant shift from 41-kyr cycles to longer, more intense cycles, occurring approximately between 1.2 and 0.65 million years ago. This transition brought a distinct increase in ice volume and produced a saw-tooth pattern in the δ18O record.

• The "100-kyr world" of the late Pleistocene: This period is recognized for glacial cycles averaging about 100,000 years, with notable fluctuations in ice volume represented by characteristic saw-tooth patterns in the δ18O record.

Power Spectra Analysis

Power Spectra Analysis, particularly Fourier analysis, is applied to decompose the time series data of the δ18O record into constituent frequencies. Key observations include:

• Prior to 1.2 million years ago, the δ18O record exhibited a strong peak at the 41-kyr frequency related to obliquity.

• After this period, a notable increase in the power of precession (19 and 23 kyr) and eccentricity (100 kyr) was observed, even though the 41-kyr peak remained evident.

• The differing power spectra of the δ18O record and caloric summer half-year insolation reveal important differences in climatic responses, showing that the δ18O record in the late Pleistocene is dominated by the 100-kyr cycle, while insolation forcing is primarily influenced by precession with diminished power at eccentricity frequencies.

IV. Challenges to the Classical Milankovitch Theory

The 100-kyr Problem (Eccentricity Problem)

One primary challenge affiliated with the Milankovitch theory is the prominence of the 100-kyr cycle in the δ18O record during the late Pleistocene. This poses questions as the variations in orbital eccentricity exhibit relatively minor changes in radiative forcing. The observed dominance of the 100-kyr cycle requires further explanation regarding the climate response mechanisms at work.

Milankovitch Mismatches

• Observed climate responses do not always align with the expected predictions of Milankovitch's calculations. For example:

  • Stage 7: A period during which an extensive insolation change occurred, yet the climate record reflected only a minimal response, demonstrating potential nonlinearities in climatic systems.

  • Stage 11: Characterized by minor insolation changes leading to a significant climate response, illustrating the unpredictable complexities inherent within the climate system.

Causality Problem

In certain cases, analyses reveal that the observed climatic effects precede the expected insolation forcing, which raises compelling questions regarding the linearity of causation. A notable example is the documented rise in sea level during Termination II, which, when dated using Uranium/Thorium dating methods on corals from Tahiti, suggests that it occurred prior to the documented increase in boreal summer insolation.

Lack of Precession in the "41-kyr world"

The characterization of the early Pleistocene suggests dominance by obliquity with significantly less influence from precession than what is expected based on Milankovitch's insolation model, highlighting additional complexities in climatic responses based on multiple orbital parameters.

V. Possible Explanations for the 100-kyr Cycle

Amplitude Modulation (AM) of Precession

Research suggests that variations in eccentricity can modulate the amplitude of the precession signal. Thus, when eccentricity is high, the resulting climate response could be more pronounced, whereas during periods of lower eccentricity, the amplitude of precession may be dampened.

Signal Rectification (Clipping)

If the climate system exhibits non-linear responses—particularly to insolation values above certain thresholds—this clipping can effectively obscure the influence of eccentricity in power spectrum outcomes. Milankovitch proposed that ice volume dynamics respond primarily beyond specific insolation thresholds, potentially impacting the observed climate signals.

Conceptual Models and Simulations

Several simplified conceptual models have been developed to explore the dynamics of climate systems and glacial cycles:

• Calder (1974): A linear model that assumes the existence of an insolation threshold determining conditions for ice accumulation or melting. The model proposes that ice volume changes according to different constants based on whether conditions favor snow accumulation or melting

• Imbrie and Imbrie (1980): A model incorporating variable time constants for ice growth and decay, this model attempts to simulate the observed saw-tooth patterns in glacial records.

• Paillard (1998): A sophisticated tri-stable state model outlining three different climate regimes: interglacial, mild glacial, and full glacial states. It simulates transitions between these phases based on thresholds in insolation and ice volume, demonstrating nonlinear dynamics of glacial cycles.

Nonlinear Relaxation Oscillator

Another conceptual framework emphasizes viewing the climate system as an oscillator with two phases:

• A slow charging phase (ice growth) followed by a rapid discharging phase (deglaciation).

• In this analogy, ice sheets are likened to capacitors accumulating ice until a threshold is reached, triggering rapid deglaciation, with estimates indicating that the average time period for reaching this critical ice volume is approximately 100,000 years.

VI. Internal Oscillators and Ice Sheet Dynamics

Internal Processes and Ice Sheet Instability

Large ice sheets inherently possess instability due to multiple internal feedback mechanisms. Critical processes include:

• Isostatic Depression: The significant weight of ice sheets causes the underlying lithosphere to subside, establishing lower elevations where warmer temperatures further exacerbate ice melting processes and increase ablation rates.

• Pressure Melting Point: As the pressure of a thick ice sheet increases, the melting point of ice can decrease. This effect may lead to the formation of liquid water at the base of ice sheets, providing lubrication for ice flow and enhancing the potential for faster movement and calving.

Simple Ice Sheet Models

Some theoretical frameworks suggest that internal dynamics of ice sheets alone can generate 100-kyr cycles, indicating the potential contribution of these dynamics to ice sheet instability without necessitating external orbital forcing.

The 100-kyr "Myth"

It's crucial to recognize that the timing of terminations varies significantly across glacial cycles, revealing that the 100-kyr cycle may represent an average periodicity, and the distinct terminations may align with multiples of obliquity and precession cycles. Huybers and Wunsch (2005) proposed a fascinating hypothesis indicating that the observed 100-kyr cycle might be an artifact resulting from skipped obliquity cycles, suggesting occurrences of terminations at multiples of 41 kyr, which consequently reflects an average periodicity of approximately 100 kyr.

Triggering of Deglaciations

Periods of deglaciation may often be triggered when both precession and obliquity align to create strong insolation forcing events. Moreover, the occurrence of terminations often results from a complex interplay between orbital forcing and internal climatic feedback mechanisms, highlighting the multifaceted nature of climate dynamics.

VII. Mismatches and the Causality Problem

Mismatches Between Forcing and Response

The relationship between predicted insolation changes and the observed climatic responses frequently reveals inconsistencies, emphasizing that the climate system reacts in a non-linear manner to orbital forcing.

• Some terminations occur during periods of significantly strong insolation forcing, while others arise when the forcing is weak. Conversely, several instances of strong insolation do not lead to any recorded terminations, complicating the understanding of climatic responsiveness.

Revisiting the Causality Problem

An examination of causality indicates instances where observed effects appear to predate corresponding insolation forcing. Notably, the analysis on sea level rises during Termination II highlights that sea levels began to rise before the associated increases in boreal summer insolation. This points toward the idea that glacial-interglacial cycles could be paced by orbital forcing in ice growth, while the processes yielding deglaciation may be triggered by internal mechanisms when ice sheets reach critical mass.

• The mechanisms underlying this premise include isostatic depression resulting from lithospheric loading and lowering the melting point due to pressure effects from large ice sheets.

VIII. Conclusion

The Milankovitch theory provides a crucial framework for understanding glacial-interglacial cycles; however, the complexities of this dynamic climate system necessitate further research and exploration. Current conceptual models yield useful insights yet often lack sufficient physical foundations to predict climate outcomes conclusively.

• Future studies must embrace more complex interactions between orbital forcing, internal feedback processes, and ice sheet dynamics that contribute to the climate's oscillatory nature. Furthermore, a shift in focus from merely identifying climatic terminations to examining the underlying mechanisms governing ice sheet growth and decay may yield deeper insights into the causes of glacial-interglacial cycles, ultimately enhancing our understanding of both past and future climate changes.