Estimating uncertainties in future carbon budgets

Estimating uncertainties in future carbon budgets for ambitious mitigation targets

Researchers: Nadine Mengis, Damon Matthews

Key highlights:

• limit global warming to 1.5ºC or < 2ºC (ambitious mitigation)
• carbon budgets provide information on our remaining future emissions
• key uncertainties:

1. Earth system, i.e., carbon cycle, ocean heat uptake, etc

2. Human behaviour (scenario uncertainty)

• Earth system modelling as a tool to assess these uncertainties,

The figures below provide the probabilistic estimate of the fossil fuel only carbon budget. (a) Best estimate (black line) and 95% credible interval (light grey) of cumulative fossil fuel (FF) carbon emissions calculated from the perturbed parameter ensemble with varying ocean and land carbon uptake. Observed cumulative FF emissions (red line) for comparison. (b) Probability density of simulated cumulative FF emissions until 2015 (light grey) compared to the observed value of 413 Petagrams C ±5% (red lines) given by the Global Carbon Cycle Project for the period of 1850–2015; Probability density of the 1.5 °C FF-only carbon budget (dark grey), with the best estimate value of 469 Petagram C (black line), compared to the default, i.e. un-manipulated, model simulation (blue line). (c) Estimates of the 1.5 °C FF-only carbon budget as a function of the 2015 cumulative ocean and land carbon uptake. Colours indicate the budget, while the size of the circle indicates the probability of the respective member of the perturbed parameter ensemble. Exemplary probability sizes are given for the best estimate, two standard deviations (2 stds) and the 2015 observed cumulative FF emissions (413 PG C).