Non-CO2 forcing changes will likely decrease the remaining carbon budget for 1.5\u2009\xb0C

Nadine Mengis,H Damon Matthews

doi:10.1038/s41612-020-0123-3

Abstract

One key contribution to the wide range of 1.5 °C carbon budgets among recent studies is the non-CO2 climate forcing scenario uncertainty. Based on a partitioning of historical non-CO2 forcing, we show that currently there is a net negative non-CO2 forcing from fossil fuel combustion (FFC), and a net positive non-CO2 climate forcing from land-use change (LUC) and agricultural activities. We perform a set of future simulations in which we prescribed a 1.5 °C temperature stabilisation trajectory, and diagnosed the resulting 1.5 °C carbon budgets. Using the historical partitioning, we then prescribed adjusted non-CO2 forcing scenarios consistent with our model’s simulated decrease in FFC CO2 emissions. We compared the diagnosed carbon budgets from these adjusted scenarios to those resulting from the default RCP scenario’s non-CO2 forcing, and to a scenario in which proportionality between future CO2 and non-CO2 forcing is assumed. We find a wide range of carbon budget estimates across scenarios, with the largest budget emerging from the scenario with assumed proportionality of CO2 and non-CO2 forcing. Furthermore, our adjusted-RCP scenarios produce carbon budgets that are smaller than the corresponding default RCP scenarios. Our results suggest that ambitious mitigation scenarios will likely be characterised by an increasing contribution of non-CO2 forcing, and that an assumption of continued proportionality between CO2 and non-CO2 forcing would lead to an overestimate of the remaining carbon budget. Maintaining such proportionality under ambitious fossil fuel mitigation would require mitigation of non-CO2 emissions at a rate that is substantially faster than found in the standard RCP scenarios.

Highlights

In the Paris agreement, adopted on December 12th 2015, 195 parties agreed to hold “the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change” (Article 2 1.(a) of the Paris Agreement[1])
Based on the partitioning of recent emissions data from single non-CO2 forcing agents, we partition the current non-CO2 forcing twice as large as the positive forcing from co-emitted non-CO2 greenhouse gases (GHGs) (0.36 W/m2). This non-compensatory behaviour in terms of into three categories depending on the anthropogenic activities that cause the emissions: (1) fossil fuel combustion, (2) land-use non-CO2 forcing from both FFC and land-use change (LUC)+AGRIC holds important implications for future forcing pathways: if FFC is to be reduced in changes and agriculture and (3) other human activities, such as compliance with ambitious mitigation targets this will eliminate a emissions of ozone-depleting substances and other refrigerants. large part of the negative forcing from co-emitted aerosols
Diagnosed CO2 and CO2 forcing equivalent (CO2-fe) emissions For our simulations, we have prescribed a 1.5 °C temperature change scenario as the input to the UVic ESCM, and used the model to estimate scenarios in which fossil fuel combustion is to be strongly reduced, we would expect a strong decline in aerosol emissions, causing a shift towards more positive net non-CO2 climate forcing

Summary

Introduction

In the Paris agreement, adopted on December 12th 2015, 195 parties agreed to hold “the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change” (Article 2 1.(a) of the Paris Agreement[1]). Assuming proportionality of CO2 and non-CO2 forcing is itself a choice of a future scenario, but is one that is not consistent with the recent trend of increasing net non-CO2 forcing, nor the likely independent mitigation of emissions from fossil fuels vs LUC

Results

Conclusion