Documentation for COP21-Blocks-2C graph:

From the “Advance Version” of the UNFCCC Report, issued 30 Oct. 2015, Synthesis report on the aggregate effect of the intended nationally determined contributions.

Paragraph 33 says: “Global cumulative CO2 emissions after 201110 are expected to reach 541.7 (523.6– 555.8) Gt CO2 in 2025 and 748.2 (722.8– 771.7) Gt CO2 in 2030.”

Paragraph 190 says the same:”Global cumulative CO2 emissions resulting from the implementation of the communicated INDCs after 2011 are expected to reach 541.7 (523.6 to 555.8) Gt CO2 in 2025 and 748.2 (722.8 to 771.7) Gt CO2 in 2030.”

1. Note that the lowest value for cumulative CO2 emissions resulting from the INDCs as of 2030 is the bottom of the range: (722.8 to 771.7), or 722.8. (In the write-up I rounded this to 723.)
2. That leaves 1000 – 722.8 remaining before the 2C emission limit of 1000 Gt, or 277.2.
3. Note that in 2025 the corresponding low value is 523.6.
4. So the increase in the 5 years between 2025 and 2030 is 722.8 – 523.6 = 199.2.
5. Dividing this by 5 years gives the annual rate of CO2 emission, 39.84, which I rounded to 40 in the write up.
   • There was no rounding in the spreadsheet that made the graph.
6. If we emitted at 39.84 until we used up the remaining 277.2 Gt, that would take 277.2 / 39.84 = 6.96 years (slightly less than 7).
7. If we cut back to zero CO2 emissions by an equal amount each year, we will average half of the maximum emissions, so we can take twice as long, or 14 year. That is the line shown on the graph. It ends in 2044.

Documentation for the 1000 Gt limit for 2C Scenarios:

Paragraph 42 of the same report says: “According to the AR5, the total global cumulative emissions since 2011 that are consistent with a global average temperature rise of less than 2 °C above pre-industrial levels at a likely (>66 per cent) probability is 1,000 Gt CO2.”

This is more thoroughly documented in the UNEP’s Emissions Gap Report, 2014. See Figure ES.1, and Chapter 1 (page 1).

UNFCCC web page for INDC Synthesis Report

UNEP web pages for Emission-Gap documents: Main page 2015, Press 2015, Report 2014