IPCC: Time for a Global Carbon Budget

Political commitments on climate action, to the extent they exist, are usually pitched in terms of targets and timelines. BC, for example, has a legislated target of 33% below 2007 levels by 2020; Canada’s official target is a 17% reduction by 2020 relative to 2005 levels. Neither target will be met under status quo policy, which is, de facto, to extract as much carbon and put it in the atmosphere as possible.

In recent years, the concept of a carbon budget has come to the fore. That is, not just targets for some future date but a total amount of carbon that can be released before we cross dangerous thresholds; typically, more than 2 degrees C of temperature increase relative to pre-industrial times. In a CCPA paper released in March, we considered the concept of a carbon budget for Canada and its implications for the country’s vast reserves of fossil fuels, and concluded that most of our reserves needed to stay below ground.

The new IPCC report for the first time sets out a carbon budget. Here is the relevant paragraph from the Summary for Policy-makers:

• 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 1861–1880, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1570 GtC (5760 GtCO2), 0 and about 1210 GtC (4440 GtCO2), and 0 and about 1000 GtC (3670 GtCO2) since that period, respectively. These upper amounts are reduced to about 900 GtC (3300 GtCO2), 820 GtC (3010 GtCO2), and 790 GtC (2900 GtCO2), respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 515 [445 to 585] GtC (1890 [1630 to 2150] GtCO2), was already emitted by 2011. {12.5}

[Author’s note, 1/16/2014: IPCC stated a few errors in the above text, corrected in the final version. I have pasted in the final text, and modified the paragraph below. Changes are relatively minor, resulting from a decrease in the amount of CO2 already emitted by 2011 (from 531 GtC to 515), an increase/decrease in estimated impact of non-CO2 forcings for 66%/50% probability estimates respectively.]

So for a two-thirds probability of not exceeding 2 degrees, the world’s carbon budget is 1000 billion tonnes of carbon (gigatonnes or Gt C) less 210 Gt C for other non-CO2 emissions (like methane) and less the 515 Gt C already released as of 2011, which leaves us with 275 Gt C. Translated into CO2, that is 1009 Gt CO2. For a 50% chance (a coin-toss for exceeding 2 degrees), we can emit up to 1,119 Gt CO2 (305 GtC). For context, annual emissions globally from fossil fuel combustion are about 33 Gt CO2. So if we add emissions for 2012 and 2013, the carbon budget drops to 1,053 Gt CO2 for 50% odds, and 943 Gt CO2 for 66% odds. [From my estimates based on the draft version of the summary, the previous calculations were 1068 and 921 GtC respectively]

[Aside: Some reports on this concept make a mathematical error, confusing carbon with carbon dioxide. Most discussion in terms of carbon dioxide (CO2) because that is the main gas from the combustion of fossil fuels that is warming the planet, but many scientists still use carbon (C). The mass of carbon dioxide is 3.67 times that of carbon (see discussion here), and sometimes people fail to convert. In what follows I will stick to CO2 instead of C as that makes more intuitive sense.]

The carbon budget analysis in our carbon liabilities report was based on a Potsdam Institute study, which had tighter parameters. To be prudent and precautionary, we used an 80% chance of staying below 2 degrees, amounting to 500 Gt CO2, but noted a 50% chance would be 1,000 Gt CO2.

What’s this mean for Canada? Using the 500 Gt budget, we estimated a Canadian carbon budget of 9 Gt based on share of global GDP, and 2.4 Gt based on share of population. That said, I inflated the plausible upper limit to 20 Gt because of Canada’s role as a fossil fuel exporter. But even if we take the biggest carbon budget of 20 Gt, 78% of proven reserves and 89% of proven-plus-probable reserves would need to stay underground.

The basic problem does not really go away if we use higher numbers from the IPCC report, so if we go with the (weaker) 50-50 chance (1068 Gt), Canada would be allocated a 5.1 Gt carbon budget based on population, and 19.2 Gt based on share of GDP. Even though this is very conservative, assume Canada’s shrewd negotiators were able to secure a 40 Gt carbon budget, more than double our share of GDP, and we still have the result that 56% of proven fossil fuel reserves (which are equivalent to 91.4 Gt) and 77% of proven-plus-possible reserves need to stay below ground.

No matter how you slice it, Canada has a carbon bubble on its hands – if the world heeds warnings from bodies like the IPCC.