1.5 \xb0C carbon budget dependent on carbon cycle uncertainty and future non-CO2 forcing

Nadine Mengis,Jonathan Jalbert,Antti-Ilari Partanen,H Damon Matthews

doi:10.1038/s41598-018-24241-1

Nadine Mengis, Jonathan Jalbert + Show 2 more

Open Access

https://doi.org/10.1038/s41598-018-24241-1

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Abstract

Estimates of the 1.5 °C carbon budget vary widely among recent studies, emphasizing the need to better understand and quantify key sources of uncertainty. Here we quantify the impact of carbon cycle uncertainty and non-CO2 forcing on the 1.5 °C carbon budget in the context of a prescribed 1.5 °C temperature stabilization scenario. We use Bayes theorem to weight members of a perturbed parameter ensemble with varying land and ocean carbon uptake, to derive an estimate for the fossil fuel (FF) carbon budget of 469 PgC since 1850, with a 95% likelihood range of (411,528) PgC. CO2 emissions from land-use change (LUC) add about 230 PgC. Our best estimate of the total (FF + LUC) carbon budget for 1.5 °C is therefore 699 PgC, which corresponds to about 11 years of current emissions. Non-CO2 greenhouse gas and aerosol emissions represent equivalent cumulative CO2 emissions of about 510 PgC and −180 PgC for 1.5 °C, respectively. The increased LUC, high non-CO2 emissions and decreased aerosols in our scenario, cause the long-term FF carbon budget to decrease following temperature stabilization. In this scenario, negative emissions would be required to compensate not only for the increasing non-CO2 climate forcing, but also for the declining natural carbon sinks.

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, recognizing that this would significantly reduce the risks and impacts of climate change” (Article 2 1.(a) of the Paris Agreement[1])
We have provided a new best-estimate and likelihood range for the fossil fuel (FF) carbon budget that is constrained by cumulative historical FF emissions, and accounts for prior knowledge about the historical ocean and land carbon fluxes
Including CO2 emissions from land-use change increases the best-estimate budget to 699 GtC at 2055 and 625 PgC at 2200

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, recognizing that this would significantly reduce the risks and impacts of climate change” (Article 2 1.(a) of the Paris Agreement[1]). We present a probabilistic estimate of the 1.5 °C fossil fuel threshold avoidance carbon budget, from 1850 until the year at which 1.5 °C is reached, as well as for the 150 years of stable global mean temperature afterwards. In deriving this new budget estimate, we quantify the contribution of two key sources of uncertainty. We estimate the equivalent cumulative CO2 emissions represented by both historical and future non-CO2 forcing, which allows us to quantify their contribution to the historical budget and in addition highlights the considerable scenario uncertainty associated with future decisions about non-CO2 greenhouse gas mitigation

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