There are a few apples and oranges issues to watch out for when talking about the carbon budget and understanding the budget numbers calculated by The Carbon Tracker Initiative and the London School of Economics, the International Energy Agency, and the UN’s Intergovernmental Panel on Climate Change.

The first comparison disparity to watch out for is the difference between carbon and carbon dioxide, or C vs CO2.  The new AR5 report published by the IPCC has its budget numbers calculated in carbon, while the Carbon Tracker Initiative published their calculations in carbon dioxide. The difference between the two is substantial, especially when talking about billions of tons (Gt).  Essentially you need to calculate the weight of the two atoms of oxygen in the chemical compound carbon dioxide. Here’s the equation: One ton of carbon equals 44/12 = 11/3 = 3.67 tons of carbon dioxide. So, if you wanted to compare Carbon Tracker’s budget of 565 GtCO2 to the budget found in the IPCC report of 269 GtC, you could multiply 269 x 3.67 = 987 GtCO2.

We have now landed at the second comparison issue.  These two numbers (565 and 987) are at different levels of probability.  Probability is measured in percentages of certainty. Probability is a funny metric and from 350’s perspective, it seems wild to build discourse around anything but the highest probability of safety and human wellbeing.  The 269 GtC* found in the IPCC report is only a 66% chance of staying below 2 degrees Celsius of global warming (the IPCC’s high end of probability).  The 565 GtCO2 found in the Carbon Tracker report is a more conservative 80% chance of staying below the target 2 degrees. This is all based on probabilistic climate change modeling to calculate the total volume of emissions.

The third point of comparison to watch out for is the inclusion or exclusion of “non-CO2 forcings” or levels of non-CO2 greenhouse gasses.  The IPCC budget number is 1000GtC or 800GtC when accounting for non-CO2 forcings. The Carbon Tracker budget number is 565GtCO2 or 900GtCO2 when assuming greater reductions in non-CO2 emissions.

The easiest way to look at the budget is as a range – from 565 GtCO2 to 987GtCO2. That range has a host of factors including probability (or level of confidence) and future mitigation scenarios.  One thing is unanimously described in all of the research; human produced emissions need to be drastically reduced in a short period of time.  The other realization that is evident from the research is that the budget is a fraction of the reserves of coal, oil, and gas when converted to CO2 equivalent.

Here are the budget quotes from the reports mentioned:

IPCC: “Limiting the warming caused by anthropogenic CO2 emissions alone with a probability of  >33%, >50%, and >66% to less than 2°C since the period 1861–188022, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1560 GtC, 0 and about 1210 GtC, and 0 and about 1000 GtC since that period respectively.”

Carbon Tracker: “Minus emissions from the first decade of this century, this leaves a budget of 565 GtCO2 for the remaining 40 years to 2050.”

Carbon Tracker (2013): “This analysis estimates that the available budget is 900GtCO2 for an 80% probability to stay below 2°C. This CO2 budget is higher as it assumes greater reductions in non-CO2 emissions, such as methane, which have a higher global warming potential.”

*1000GtC (66% probability) – nonCO2 = 800GtC (66% probability)
800GtC – 531GtC (carbon burned between 1861 through 2011) = 269GtC