The chart above is 'carbon-budgeting' from the latest UNEP GAP Report compared with the 'carbon-budgeting' in Plan B.

Carbon budgeting is the amount of carbon we can still burn (and perhaps to some extent offset),
to keep future temperature rise to Paris Agreement (1.5°C to 'Well Below' 2.0°C). 

Carbon budgeting is a weight measurement and that means not much (if any) . . .

• Gt C = Billions (Giga) of tonnes of Carbon
• Gt CO2 = Billions (Giga) of tonnes of Carbon Dioxide (the gas)
• That is a conversion ratio of 1 tonne carbon to 3.6667 Tonnes Carbon Dioxide

Its a story that goes back a long way (so there may be some homework here)? . . . . 

Here's the Plan B global carbon budgeting basis in full: -
http://www.gci.org.uk/CREDIT-DEBIT.html

Here is a comparion of the UK Government's Carbon Budget with Plan B Carbon Budgets for the UK.

Longer term (30 Years) context of the story here