Stuart Jenkins

Abstract:

The relationship between cumulative CO2 emissions and CO2-induced warming is determined by the Transient Climate Response to Emissions (TCRE), but total anthropogenic warming also depends on non-CO2 forcing, complicating the interpretation of emissions budgets based on CO2 alone. An alternative is to frame emissions budgets in terms of CO2-forcing-equivalent (CO2-fe) emissions – the CO2 emissions that would yield a given total anthropogenic radiative forcing pathway. Unlike conventional ‘CO2-equivalent’ emissions, these are directly related to warming by the TCRE and need to fall to zero to stabilise warming: hence CO2-fe emissions generalise the concept of a cumulative carbon budget to multi-gas scenarios. Cumulative CO2-fe emissions from 1870-2015 inclusive are found to be 2900 ± 600GtCO2-fe, increasing at a rate of 67 ± 9.5GtCO2-fe/year. A TCRE range of 0.8–2.5° Cper 1,000 GtC implies a total budget for 0.6° C of additional warming above the present decade of 880–2,750 GtCO2-fe, with 1,290 GtCO2-fe implied by the CMIP5 median response, corresponding to 19 years' CO2-fe emissions at the current rate.

Abstract:

Anthropogenic climate change is well established, with historical emissions of greenhouse gases and aerosols causing 1.1C warming today, and rising at +0.2C/decade. At this rate the Paris Agreement’s 1.5C warming threshold is reached within two decades. For policymakers this proximity means that the anthropogenic warming rate is one of the most important variables to constrain today. Several methodologies exist to determine warming contributions utilizing Earth observations and climate models of varying complexity. However, many of these approaches do not establish the impact of recent emissions trends in short-lived aerosol pollutants, and consequently may underestimate the present-day rate of warming. This thesis begins by investigating the CO2, non-CO2 greenhouse gas, and aerosol contributions to anthropogenic global warming today, focusing on evidence presented by trends in satellite and in-situ observations. These trends are attributed to anthropogenic and natural sources using simplified climate models, informed by full-complexity general circulation models, to assess the observational evidence for an aerosol-induced warming acceleration. The simplified modelling approach is then pursued to evaluate physical constraints on future policy. For CO2, the use of a ‘remaining carbon budget’ — the emissions compatible with a warming threshold — simplifies policy objectives until net zero. However, the combined physical requirements of CO2 and non-CO2 mitigation ‘budgets’ is complicated by the varied lifetimes and efficacies displayed by non-CO2 pollutants, resulting in a non-trivial relationship between emissions and warming. In this thesis, I show how the forcing-equivalent metric can produce physically-coherent estimates of individual pollutants’ contribution to the remaining budget. Finally, I explore the physical conditions required for warming stabilisation, based on a mathematical framework describing the long-term properties of the carbon and thermal cycles. As a whole, my work demonstrates how models and observational constraints can be combined to provide the physical guide rails over which climate policy can be optimised.