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«From Greenhouse to Icehouse: Understanding Earth’s Climate Extremes Through Models and Proxies Clay Richard Tabor A dissertation submitted in ...»

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From Greenhouse to Icehouse: Understanding Earth’s Climate

Extremes Through Models and Proxies

Clay Richard Tabor

A dissertation submitted in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy


in the University of Michigan

Doctoral Committee:

Professor Chris J. Poulsen, Chair

Associate Professor Jeremy N. Bassis

Associate Professor Brian K. Arbic

Senior Scientist David Pollard


Foremost, I would like to thank my advisor Chris Poulsen for his exceptional mentoring throughout my time as a graduate student. My accomplishments over the past five and a half years were the result of his continual assistance, understanding, and friendship. I look forward to our continuing and future collaborations.

I would also like to thank my collaborator and committee member Dave Pollard. His impressive body of work and guidance were the foundation for my research; my dissertation would not have been possible without his help. To my other committee members, Brian Arbic and Jeremy Bassis, who graciously gave their time to many fruitful conversations, I would like to thank you for your help improving this dissertation. Thanks to my other collaborators, Bette Otto-Bleisner, Nan Rosenbloom, and Dan Lunt, for sharing your experience and research with me; I am excited about our ongoing collaboration. I would like to thank the past and current members of the Climate Change Research Lab for their numerous discussions and friendship over the years. Ran Feng, Rich Fiorella, Louise Jeffery, Dan Horton, Jing Zhou, Adam Herrington, Nadja Insel, Sierra Petersen, Chris Skinner, Chana Tilevitz, Phoebe Aron, Alex Thompson, Hong Shen, and Dan Lowry, you all have kept the lab both entertaining and productive. Further I would like to thank Mike Messina for tolerating my Unix ignorance time and time again.

Beyond research, I am grateful to my friends. They have been a constant source of relaxation, entertainment, and support throughout my graduate career. I would not have made it this far without them. Finally, I would like to thank my family, especially my parents, who have always supported and believed in me, even when I clearly have no idea what I am doing. Thank you for reading!

ii Table of contents Acknowledgements

List of figures

List of tables

Chapter 1 Introduction

1.1 Motivation

1.2 Methods

1.3 Background

1.3.1 Quaternary ice ages

1.3.2 Cretaceous Greenhouse

1.4 Outline

1.5 Publications and abstracts resulting from this dissertation

Chapter 2

Mending Milankovitch Theory: Obliquity Amplification by Surface Feedbacks................. 11



2.2 Introduction

2.3 Methods

2.4 Results

2.4.1 Climate-only experiments

2.4.2 Climate-ice sheet experiments

2.5 Discussion and conclusion

2.6 Caveats

2.7 Acknowledgements

2.8 Appendix A

iii Chapter 3

How Obliquity Cycles Powered Early Pleistocene Global Ice-Volume Variability.............. 34

3.1 Abstract

3.2 Introduction

3.3 Methods

3.3.1 Earth system model

3.3.2 Experiment design

3.4 Results

3.4.1 Ice-volume spectral power

3.4.2 Climate signal decomposition

3.4.3 Summer season feedbacks

3.4.4 Precession seasonal insolation offset

3.4.5 Cycle frequencies and nonequilibrium

3.5 Discussion

3.5.1 Orbital bias

3.5.2 Ice-volume hemispheric offset

3.5.3 GHG fluctuations

3.5.4 Response changes after the mid-Pleistocene transition

3.6 Conclusion

3.7 Acknowledgements

3.8 Appendix B

Chapter 4

Simulating the Mid-Pleistocene Transition Through Regolith Removal

4.1 Abstract

4.2 Introduction

4.3 Methods

4.3.1 Model and Coupling

4.3.2 Experiment Design

4.4 Results

4.4.1 Ice-Volume and Area

iv 4.4.2 Spectral Power

4.4.3 Ice Dynamics

4.4.4 Climate feedbacks

4.5 Discussion

4.5.1 Role of CO2

4.5.2 Model/Proxy Discrepancies

4.5.3 Model Comparison

4.5.4 Limitations and Justifications

4.6 Conclusions

4.7 Acknowledgements

4.8 Appendix C

Chapter 5

The Contributions of Paleogeography and CO2 to Late Cretaceous Cooling

5.1 Abstract

5.2 Introduction

5.3 Methods

5.3.1 Climate simulations

5.3.2 Proxy records

5.4 Results

5.4.1 Response to paleogeography

5.4.2 Response to atmospheric ρCO2

5.5 Discussion

5.5.1 Proxy comparison

5.5.2 Amount of Cooling

5.6 Limitations

5.7 Conclusions and outlook

5.8 Acknowledgements

5.9 Appendix D

5.9.1 Model descriptions

5.9.2 Model setup

5.9.3 Energy balance calculations

–  –  –

Chapter 6 Conclusions and Future Work

6.1 Summary of results

6.2 Key findings

6.3 Continuing and future work

vi List of figures

Figure 1.1: Oxygen isotope record of glacial cycles

Figure 2.1: High-latitude climate feedbacks to orbital forcing

Figure 2.2: High-latitude temperature sensitivity to obliquity and precession

Figure 2.3: Ice volume responses to obliquity and precession

Figure 2.4: Ice volume responses to standardized orbital forcing

Figure 3.1: The 41 kyr problem and the ice sheet responses to orbital forcing

Figure 3.2: Decomposition of high-latitude climate feedbacks to orbital forcing

Figure 3.3: Comparison of ice-volume spectral power under different scenarios

Figure 4.1: Experiment forcings and ice responses

Figure 4.2: Comparison of select glacial cycles and simulated ice-volume

Figure 4.3: Proxy and model ice-volume spectral power

Figure 4.4: Ice ablation and basal sliding during two local maxima in ice-volume

Figure 4.5: Summer surface temperature and low level winds over ice sheets

Figure 4.6: Summer snowfall and low level winds over ice sheets

Figure 4.7: Comparison of the climate over the ice sheets through time

Figure 5.1: Late Cretaceous paleogeography

Figure 5.2: Late Cretaceous mean annual surface temperatures

Figure 5.3: Zonal average SSTs from proxies and models

vii List of tables

Table 2.1: Orbital configurations

Table 5.1: Model configurations and statistics

–  –  –

Within the last century, global temperatures have risen from nearly the coldest to warmest levels of the past 11,300 years (Marcott et al., 2013). Studies suggest that this warming, due in large part to anthropogenic greenhouse gas (GHG) emissions, will have profound impacts on all aspects of life over the next century regardless of realized GHG emission scenario (5th IPCC report). However, there remains uncertainty about Earth system sensitivity to changes in GHG concentrations (Rohling et al., 2012). Feedbacks and decadal variability within the exceptionally complex Earth system lead to a large range of model predictions and hinder our ability to accurately forecast the amplitude and heterogeneity of climate change into the next century and beyond (Knutti et al., 2010b).

One of the greatest difficulties about modeling future climate change is the unique nature of the problem. Civilization has not experienced this magnitude of CO2 increase (Petit et al., 1999) and many of the climate responses will take centuries to unfold (Hansen et al., 1984).

Observations only extend back a few hundred years, with relatively poor spatial coverage before the satellite era beginning in the 1970s. As a result, they provide limited data to benchmark climate models and little insight into how climate responds to large GHG forcing. Further, without sufficient records for calibration, climate models run the risk of being over-tuned to the short observation period, which might prevent them from accurately simulating the future climate state.

Paleoclimate research is a powerful method for testing climate model capability and understanding key periods in Earth’s history (Taylor et al., 2012). Studying past greenhouse climates, such as the Cretaceous or Eocene, gives us the ability to better understand high CO2 conditions (e.g. Poulsen et al., 1999; Lunt et al., 2012) while studying past glacial cycles provides insight into ice sheet stability (e.g. Pollard and DeConto, 2009; Abe-Ouchi et al., 2013).

Specifically, modeling paleoclimates and comparing the outputs against proxy records provides a powerful tool for improving climate models and increasing confidence in climate projections

–  –  –

This dissertation utilizes General Circulation Models (GCMs) in combination with proxy reconstructions to explore extreme climates in Earth’s past. All original data contributions come from GCM experiments. Here, I use the following Earth system models to explore the Quaternary (2.588 – 0.005 Ma) and Late Cretaceous (100 - 66 Ma): Global ENvironmental and Ecological Simulation of Interactive Systems (GENESIS) and Community Earth System Model (CESM) developed by the National Center for Atmospheric Research, and Hadley Centre Model (HadCM3L) developed by the UK Met Office. They are considered Earth system models and simulate the interactions of the atmosphere, ocean, land, ice, and biosphere. Models with the same underlying architecture have been used in IPCC reports and many other paleoclimate studies (e.g. Lunt et al., 2010; Zhou et al., 2012; Rosenbloom et al., 2013). The chosen model configurations are on the upper end of complexity commonly used for paleoclimate research. My results and those of previous studies (e.g. Horton et al., 2010) demonstrate the value of added complexity for solving the enigmas of paleoclimatology.

1.3 Background

1.3.1 Quaternary ice ages Proxy evidence suggests that the past 2.7 Myr of Earth’s history were some of the most climatically variable, cycling from interglacial conditions similar to the present-day to glacial conditions with massive Northern Hemisphere ice sheets. These glacial-interglacial cycles display quasi-cyclic variations on timescales of 104 to 106 years that loosely fluctuate with changes in Earth’s orbit (Hays et al., 1976), which varies with degree of axial tilt (obliquity), direction of axial tilt (precession), and circularity (eccentricity). The most robust evidence for the cyclicity and trends in ice-volume through the Quaternary come from the combination of many benthic δ18O records (Lisiecki and Raymo, 2005). These “stacked” records clearly show δ18O variability at periods of eccentricity and obliquity, and provide details about Northern 2 Hemisphere ice sheet initiation, changes in ice age cycles through time, and global temperature trends. Yet despite the significance of these records, there remain uncertainties as to what the changes in δ18O represent. δ18O is the relative abundance of 18O to 16O. The δ18O value recorded in foraminifera is a function of temperature and δ18O of the water, which is dependent on δ18O of the water source region and ice-volume. Most attempts to interpret the long-term δ18O records agree that the ice-volume/temperature relationship changes through time (Bintanja et al., 2005) and could vary in response to different orbital forcings (Lisiecki et al., 2008). However, the spatial variability and physical mechanisms responsible for these differences remain poorly resolved.


–  –  –

Even if we assume δ18O records responds linearly to changes in ice-volume, our understandings of the glacial-interglacial cycles remain incomplete. The most widely held theory for the relationship between ice-volume and Earth’s orbit comes from the calculations of Milutin Milankovitch who proposed that high-latitude summer insolation determines the amount of snow cover that can survive summer melt and consequently, the amount of ice-sheet growth or retreat (Milankovitch, 1941). However, there are many intriguing inconsistencies between Milankovitch’s theory and proxy records of the Plio-Pleistocene. For instance, early Pleistocene (2.6 - 0.8 Ma) ice-volume proxy records vary almost exclusively at the frequency of the obliquity, despite the fact that changes in precession account for most of the variability in highlatitude summer insolation (Raymo and Nisancioglu, 2003; Figure 1.1). Further, a transition 3 from 41 kyr to 100 kyr ice-volume cycles occurs between ~1.2 - 0.7 Ma with seemingly little change in orbital forcing or CO2 (Clark et al., 2006). This transition, known as the midPleistocene transition (MPT), is particularly perplexing since the 100 kyr ice-volume cycles of the late Pleistocene (0.8 – 0.117 Ma) suggest eccentricity forcing, which has almost no direct influence on insolation. Here, I use a complex Earth system model with a 3-dimensional thermomechanical ice sheet to help resolve these apparent paradoxes between Milankovitch theory and the Pleistocene ice-volume proxy records.

1.3.2 Cretaceous Greenhouse Proxy temperature reconstructions define the Cretaceous as one of the warmest periods of the past 140 Ma. Throughout much of the Late Cretaceous (100-66 Ma), climate was characterized by a reduced equator-to-pole temperature gradient (Huber et al., 2002) and dense vegetation extending into the polar regions (Upchurch et al., 1999). Warmth peaked during the Cenomanian-Turonian (100 - 90 Ma) when evidence suggests Earth was in a greenhouse state, free of polar ice sheets (MacLeod et al., 2013). Temperatures then cooled considerably into the Maastrichtian (72 – 66 Ma) (Friedrich et al., 2012; Linnert et al., 2014) with the possible appearance of land-ice (Miller et al., 2005); however, reconstructions suggest that temperatures remained significantly warmer than present-day (Upchurch et al., 2015).

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