Highly recommended to read in full per link below.
A number of carbon budgets for 1.5°C have been published. Three are shown in Table 1. most frequently quoted case is the third listed (Table 2.2. of AR5 WG1 Synthesis report). The figure of 400 GtCO2 from 2011 is based on a “threshold exceedance budget” or TEB. But this is an overestimate, as Carbon Brief explains:
“(the TEB) assumes that emissions stop immediately once the threshold temperature is reached, which is essentially impossible in the real world. It also assumes there is no further warming once emissions have stopped, yet recent research shows this isn’t the case, says Dr Joeri Rogelj, a research scholar at the Energy Program of the International Institute for Applied Systems Analysis (IIASA), who is lead author on the Nature Climate Change study. He explains to Carbon Brief: ‘This means these (TEB) budgets are a bit of an overestimate of the carbon we have left to burn because temperatures would continue to warm for about a decade after we stopped emitting CO2’.” (14)
In the Nature Climate Change paper, the authors say that: “The TEB approach thus leads to a consistent overestimate of the CO2 budget compatible with a given temperature limit…” (15)
In summary, the first two lines of evidence show that we already have no carbon left for 1.5°C from 2017 onwards. A sensible risk-management approach would take the more conservative end of the ranges. And the third line of evidence — showing fours years left — is an overestimation.
Coal-ition doesn’t dispute science, so tell them it’s time they acted on it.