Uncertainty in carbon budget estimates due to internal climate variability

Our results show that internal variability contributes approximately +/- 0.09 degrees C to the overall uncertainty range of the human-induced warming to-date, leading to a spread in the remaining carbon budgets as large as +/- 50 PgC, when using approach (i). Differences in diagnosed TCRE due to internal variability in individual models can be as large as +/- 0.1 degrees C/1000 PgC (5%-95% range). Alternatively, spread in the remaining carbon budgets calculated from (ii) using future concentration-driven simulations of large ensembles of CMIP6 and CMIP5 models is estimated at +/- 30 PgC and +/- 40 PgC (5%-95% range).

These results are important for model evaluation and imply that caution is needed when interpreting small remaining budgets in policy discussions. We do not question the validity of a carbon budget approach in determining mitigation requirements. However, due to intrinsic uncertainty arising from internal variability, it may only be possible to determine the exact year when a budget is exceeded in hindsight, highlighting the importance of a precautionary approach.