The right carbon budget

The IPCC Fifth Assessment Report (AR5) in 2013 and 2014 has estimated the amount of CO2 the world can emit while still having holding global warming below 2°C relative to pre-industrial levels. These global ‘carbon budget’ estimates range from 590 to 2390 billion tonnes of carbon dioxide. The large differences between these estimates are because some of these estimates do not account for non-CO2 emissions, such as methane, which leads to significant warming in addition to CO2, and are not consistent with the 2°C limit.

The Paris Agreement contains global goals of holding warming well below 2°C and pursue efforts to limit the temperature increase to 1.5 °C. For the 2°C limit, the recent IPCC Fifth Assessment report contains useful estimates for the associated limit to global total cumulative CO2 emissions, but these estimates require further explanation and interpretation.

A new paper in Nature Climate Change, co-authored by Dr. Michiel Schaeffer of Climate Analytics, assesses the methodologies behind a number of these carbon budget estimates. The researchers determined that the estimate most in line with the 2°C warming limit is in the range of 590-1240 billions of tonnes CO2.

“We find that allowed CO2 budgets differ depending on the policy or research question asked, among others because of the climatic effect of non-CO2 emissions, like methane” says Michiel Schaeffer. “Of the various estimates in IPCC AR5 and other literature, we find one estimate in the AR5 Synthesis Report, equivalent with a range of 590-1240 billions of tonnes CO2 from 2015 onwards, is the most appropriate budget to be used for holding warming below 2°C with a probability of 66%, or higher.”

Different estimates of the carbon budget either ignored the climate effects of non-CO2 gases, ignored co-emissions, were based on a smaller sample of emissions scenarios, or used a different threshold of probability for holding warming below 2°C.

For instance, another estimate in IPCC AR5 points to a budget of up to 2390 billion tonnes of CO2, but this excludes the warming effects of non-CO2 emissions and is only relevant for a mere 50% chance of holding warming below 2°C. This carbon budget estimate is therefore irrelevant for climate policy in a world with “realistic” emissions of CO2 and non-CO2 and was never intended for such use.

Instead, this estimate was simply a necessary step in demonstrating the basic theoretical concept of carbon budgets, namely that warming is directly related to cumulative global CO2 emissions.