After the release of the IPCC Working Group I report in September there has been a lot of attention to the issue of the GHG emissions budget that corresponds to staying below the internationally agreed global mean temperature increase of not more than 2°C. Unfortunately, the numbers being discussed are a bit confusing, since they mix “carbon” ,”carbon dioxide” and “ CO2equivalent” (i.e. all greenhouse gas together) and do often not account for the errata that IPCC issued in November regarding its Summary for Policy Makers of the recent Working Group I report.
- To go from C to CO2, numbers have to be multiplied by 3.67.
- CO2equivalent is the weighted sum of all greenhouse gases, using their respective Global Warming Potential for a 100 year period (CO2=1, Methane= 34, Nitrous Oxide= 298, HFC 134a= 1550) (see IPCC AR5, WG I, chapter 8)
- 1 Gigaton= 1 billion tons
The proper calculation goes as follows:
- IPCC WG I says the total GHG budget (from around 1860 till the end of this century) for a 2°C temperature increase, according to the RCP 2.6 scenario set, is 1000 Gigaton C
- Looking only at CO2 and omitting the other greenhouse gases this number is reduced to 800 Gigaton C
- CO2 already emitted: 515 Gigaton C (corrected value as indicated in errata issued November 13, 2013)
- Remaining budget for CO2 only: 285 Gigaton C, which is equivalent to 1050 GtCO2
Compare the budget of about 1000 GtCO2 with current annual CO2 emissions (about 34 GtCO2 from energy and industry and about 5 from deforestation and land use change): at current rates the budget will be finished in about 25 years.
Compare the budget with the currently known fossil fuel reserves: about 3/4 of those reserves will have to stay in the ground (see graph below from the Scientist Statement).