The IPCC is now looking to the future based on Representative Concentration Pathways (RCPs). These are essentially assumptions on watts per square meters of forcing. (They were developed as inputs to climate models and are now being mistakenly used as predictions or projects, neither of which they are.)
They then tell us that some things may result from this level of forcing. But that doesn”t necessarily follow. We don’t know how the earth will respond to forcings. We don’t know if more watts per square meter over the ocean will cause more clouds to develop that will reflect sunlight back out of the atmosphere. We don’t know how sensitive the atmosphere is to a doubling of concentrations of CO2.
Similarly, it is popular now to demand that anyone proposing an alternative to panicked running around like headless chickens produce a carbon budget to gain a seat at the table.
But whatever number they develop, from 1,000 petatons to a spoonfull of coal, it is completely dependent on what sensitivity is. Without knowing sensitivity you cannot create a carbon budget.
Do you budget for family expenses without considering the cost of food or gas? Do countries budget for military expenditures without knowing what they will pay their soldiers?
The rush to forget sensitivity is understandable, as successive efforts to calculate it keep coming back with lower and lower values for it. Climate warriors now want to pretend it has been settled. At a higher value than current observations.
But it hasn’t. And until we have a good range of potential values for sensitivity it is not only ludicrous to talk about carbon budgets–it is impossible.