Carbon budget and declared reserves
The pink cube is the actual volume of carbon dioxide gas we can emit and still have a chance of keeping global warming below 2 °C. It is 81 km high (51 miles). The other volumes are the emissions from the proven reserves of fossil fuels (according to Global Energy Outlook).
(The value for the 'remaining budget' adopts a >66% probability and accounts for non-CO2 forcings. The total budget is 2,900 GtCO2 and the amount emitted by 2011 was 1,890 GtCO2 leaving 1,010 GtCO2)
Refs:
IPCC, Climate Change 2013: The Physical Science Basis - Summary for Policymakers, p 25: www.climatechange2013.org/images/uploads/WGI_AR5_SPM_broc....
Global Energy Outlook: www.iea.org/publications/freepublications/publication/Eng...
See also:
Carbon Tracker's Q&A about carbon budgets: www.carbontracker.org/ipcc_carbon_budgets