Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Chuixiang Yi

Committee Members

George Hendrey

Cecilia McHugh

Gary Hemming

Nir Krakauer

Subject Categories

Atmospheric Sciences | Climate | Other Earth Sciences

Keywords

Paleoclimatology, Glacial Interglacial Cycles, Climate Feedbacks, Tipping Points

Abstract

The Earth’s climate system displays a long history of nonlinear abrupt transitions which have resulted in significant ecosystem disruption and are recorded in the geologic data. Today significant anthropogenic changes are occurring in many Earth systems that seem to be pushing these toward critical thresholds. Thus, increasing the possibility of a transition to alternative states which can have unfavorable consequences. Therefore, it becomes compelling to forecast when and how these transitions will occur so that decision-makers can devise appropriate strategies to avoid or cope with the effects of a changeover to a new alternative state. However, due to the highly nonlinear nature of critical transitions, current understanding of knowing when thresholds have been crossed, or predictions of the rate, extent and nature of a climate transition have remained a challenge with considerable uncertainty. This dissertation aims to improve understanding of abrupt climate transitions by examining published literature and paleoclimate data of the nonlinear entrances and exits to and from interglacial conditions.

In the first part of this study, a conceptual model of two negative sea ice feedbacks that may have caused slower glaciations is proposed to explain the asymmetrical shape observed in the glacial-interglacial cycles. This proposal implies that negative feedbacks play an important role in moderating rate of climate transitions and should be considered in future studies estimating the speed of critical transitions.

Second, this dissertation examines multiple paleotemperature datasets, for changes in temporal variance before an entrance or exit from an interglacial. Increased variance has been shown to announce the imminence of critical thresholds in earlier theoretical and modeling studies of dynamic systems with different underlying gradually changing forcing. Here, using empirical data and two statistical methods, the moving variance and the Ansari-Bradley tests, the results indicate, increases in paleotemperature variance announces a glacial termination. Additionally, this study found evidence suggesting that the size of the variance increase forecasting an interglacial may predict its maximum. Thus, implying that the magnitude of the preceding variance increase has the potential of being developed as a novel tool to estimate the interglacial maximum following it.

Finally, this study examined δ13C isotopic records covering the last five termination events from both shallow and deep Atlantic Ocean sources and found a decrease in values coinciding with glacial terminations. This provides an added line of evidence supporting the hypothesis of a previous study that suggested thermal destabilization of methane clathrates occurred during the last glacial termination. Additionally, the results show that the decrease in δ13C observed following the start of a warming event, implies the temperature increase might have caused the methane clathrate dissociation. The main implication of this finding is that current global warming can potentially destabilize the existing large methane clathrate reservoir buried under permafrost as sea ice continues to melt and resulting in amplified warming due to the climate-carbon feedback. For future work, a simple climate-carbon model is assembled along with detailed derivation of the equations and is presented here with the intent of further analyzing the mathematical phenomenon of bifurcation and critical thresholds in the climate system.

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