Date of Award

Summer 2025

Language

English

Embargo Period

8-14-2025

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Oliver Timm

Committee Members

Brian Rose

Keywords

Equilibrium Climate Sensitivity, Emerging Constraint, Two-Layer Energy Balance Model, Scenario Projections, Transient Climate Response (TCR ), CMIP6 Models

Subject Categories

Atmospheric Sciences

Abstract

ABSTRACT

Uncertainty in how much the Earth will warm in response to rising atmospheric CO₂ concentrations remains a major challenge for climate science and policy. In particular, estimates of equilibrium climate sensitivity (ECS) and transient climate response (TCR) directly affect projections of global mean surface temperature (GMST) and the allowable carbon budget for limiting warming to 2°C by 2100.

This study assesses the performance of a two-layer energy balance model (EBM) in reproducing TCR and ECS values under CMIP6 forcing scenarios, with a focus on SSP2-4.5. EBM outputs are compared against General Circulation Models (GCMs), and agreement is evaluated using root mean square error (RMSE) and correlation analysis, validating the application of the EBM parameterization for the global mean climate analysis. The study further examines the application of the emerging constraint framework and performs a sensitivity analysis showing that radiative forcing (s′) exerts the greatest influence on ECS estimates. Notably, the emerging constraint method yields a higher central estimate of ECS (mean:5.15 K, 95%CI:[2.09-8.17 K]) compared to the CMIP6 EBM simulations (3.80 K, 95% CI: [2.11-5.49 K]), reflecting the influence of high forcing to temperature ratios in select models. Finally, probabilistic projections of exceeding 2°C warming by 2100 are generated using weighted SSP scenario ensembles informed by socioeconomic indicators. Results suggest that mid-range scenarios (e.g., SSP2-4.5 and SSP4-3.4) represent the most likely pathways, with probabilities of exceeding 2 °C of global warming reaching 70-80% by late century. These findings highlight the need for caution in applying emerging constraints to policy contexts and support the use of weighted scenario frameworks for robust climate risk assessment.

Comments

This thesis investigates Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR) using CMIP6 Earth system models and a two-layer energy balance framework. By applying emergent constraint techniques to historical warming, the analysis narrows uncertainty in climate sensitivity estimates and improves probabilistic projections of global mean surface temperature. Results inform policy-relevant scenarios, including the likelihood of exceeding 2 °C of warming by 2100 under different SSP pathways.

License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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