Quantifying climate change mitigation urgency seems to have three aspects:

(1) Green House Gas reduction targets required to meet a specific goal such as 2C.
The first guesstimate seems to be zero net emissions by 2050. **
**Until we actually have fully scalable carbon reduction technology we must assume zero emissions.

(2) Consequences to human society and the rest of life on Earth of exceeding the 2C goal.
The IPCC estimated impacts to crop yields seems quite significant.

(3) Estimates of the point of no return (tipping point) from feed back loops:
How much longer can we wait to begin climate change mitigation action before things such as the melting permafrost make climate change mitigation action infeasible?

Hypothetical answers to make the question more clear:

(1) If there is a consensus of agreement that we must achieve zero GHG emissions by 2050 this precisely quantifies one key aspect of the degree of urgency.

(2) If there is a consensus of agreement that business as usual for 80 years would probably cause 5C of global heating this is another quantified measure of urgency.

(3) If there is a consensus of agreement that 5C of global heating would very likely cause very significantly increased risk to things such as crop production yet another quantified measure of urgency.

(4) If we are 50 PPM of CO2 away from a key tipping point feedback loop such that the melting of the permafrost cannot be stopped this would provide another quantified measure of urgency.

  • $\begingroup$ A premise of the question "How to best quantify urgency?" is that urgency can be quantified in some way. Can you provide some good examples of urgency quantification in similar contexts to demonstrate that this is possible and to get a better idea how this is done? Thanks! $\endgroup$
    – uhoh
    Feb 18, 2020 at 8:51
  • $\begingroup$ @uhoh Here is a hypothetical answer: We have 30 years to eliminate GHG emissions and we must reduce GHG emissions by 50% in the first ten years or we hit key tipping points such as the melting of the permafrost that will eventually cause so much global heating that many very bad things will happen such as the infeasibility of crop production. $\endgroup$
    – polcott
    Feb 18, 2020 at 15:41
  • 1
    $\begingroup$ @polcott food production is hit as it is now,heat waves are getting stronger and episodes of heavy precipitation are getting more common,both are very damaging to crops and livestock. $\endgroup$ Feb 22, 2020 at 11:38
  • $\begingroup$ An interesting question .. there is a lot of ambiguity around the way in which phrases like "urgent" and "emergency" are used. There is, I was amazed to discover a SMUG (Seriousness, Manageability, Urgency and Growth) tool for risk analysis which relies on quantifying urgency. SMUG was discussed in this 2002 document: civildefence.govt.nz/assets/Uploads/publications/… -- but may not be in current use. There is appears to relate to how soon action is needed --- and it is assumed you are talking about a time-limited action. $\endgroup$
    – M Juckes
    Feb 29, 2020 at 13:30