The oceanic origin of path-independent carbon budgets

Andrew H Macdougall

doi:10.1038/s41598-017-10557-x

Abstract

Virtually all Earth system models (ESM) show a near proportional relationship between cumulative emissions of CO2 and change in global mean temperature, a relationship which is independent of the emissions pathway taken to reach a cumulative emissions total. The relationship, which has been named the Transient Climate Response to Cumulative CO2 Emissions (TCRE), gives rise to the concept of a ‘carbon budget’. That is, a finite amount of carbon that can be burnt whilst remaining below some chosen global temperature change threshold, such as the 2.0 °C target set by the Paris Agreement. Here we show that the path-independence of TCRE arises from the partitioning ratio of anthropogenic carbon between the ocean and the atmosphere being almost the same as the partitioning ratio of enhanced radiative forcing between the ocean and space. That these ratios are so close in value is a coincidence unique to CO2. The simple model used here is underlain by many assumptions and simplifications but does reproduce key aspects of the climate system relevant to the path-independence of carbon budgets. Our results place TCRE and carbon budgets on firm physical foundations and therefore help validate the use of these metrics for climate policy.

Highlights

Simulations with Earth system models consistently show a near-linear relationship between cumulative emissions of CO2 and change in global mean temperature[1,2,3], a relationship which has been shown to be consistent with observations[3]
The near path-independence of carbon budgets is evident in the full spectrum of Earth System Models (ESMs) from simple box models[11, 15], to intermediate complexity climate models[10, 16] to full ESMs17
The first step in our analysis is to confirm that the path-independence of temperature versus cumulative emission curves arises from modulation of CO2 concentration by the simulated carbon cycle

Summary

Introduction

Simulations with Earth system models consistently show a near-linear relationship between cumulative emissions of CO2 and change in global mean temperature[1,2,3], a relationship which has been shown to be consistent with observations[3]. The second feature of TCRE, its path-independence, is arguably more important for climate policy as one can compute carbon budgets for non-linear temperature versus cumulative emission curves but only if the relationship remains path-independent. We hypothesize that the path-independence of carbon budgets must arise from some fundamental aspect of carbon cycle dynamics. To explore this hypothesis we will derive a simple model of the Earth system that exhibits path-independent carbon budgets and examine why the model exhibits path-independence. By stripping an Earth system model down to its most fundamental components we hope to derive a model which can be more understood

Methods

Results

Conclusion