Renewable Revolution

Mid Permian levels were higher at around 1000 to 2000 ppm, but early Permian CO2 levels were at today's levels or below (there was a significant ice age then).

We report that greenhouse [CO2]atm have been significantly overestimated because previously assumed soil CO2 concentrations during carbonate formation are too high.

More accurate [CO2]atm, resulting from better constraints on soil CO2, indicate that large (1,000s of ppmV) fluctuations in [CO2]atm did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A.D. 2100.

CO2 concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100

The biggest individual flow and pipe degassing events are considered by workers in the field to have taken place over a 1-100 year timeframe.

By showing that the Permian emissions occurred faster than the slow compensation mechanisms (weathering etc.) and at rates in the ballpark of modern emissions, from CO2 levels not far off modern values (geologically speaking), Burgess et al have shown that we can't rule out a Permian-like (or Triassic-like, etc) event at the extreme end, with business-as-usual emissions continuing.
http://www.skepticalscience.com/Lee-commentary-on-Burgess-et-al-PNAS-Permian-Dating.html

http://www.smithsonianchannel.com/videos/mass-extinction-life-at-the-brink/33962

If the above is not as near as our science can get to the certainty that there is a high probability of N.T.H.E., I don't know what is.

Incremental measures = BUSINESS AS USUAL = N.T.H.E. This is not hard to understand.