However, because of the omission of key feedback effects from the UKMO model behind the UK Climate Act, [something now admitted by the UKMO] this choice crucially also needs to be considered in relation to the way feedbacks are represented in the model.
So another key-choice is 'feedback-type'. Using these prompts, CBAT users are invited to investigate and make a choice between 'INTEGRATED' & 'SEGREGATED' FEEDBACK: -
The CBAT default is set to 'INTEGRATED-FEEDBACK' which simulates the UKMO's 'control-model'. In this, feedback-emissions are unspecified, mixed up with and indistinguishable from budget-emissions in deceleration patterns for emissions and concentrations. This could only be plausible if significant feedback-effects were omitted from the model [which the UKMO admitted in 2010] and the planet was cooling [which it isn't] and so CAF goes to significantly to less than 50%.
UKMO have also now  conceded that, "other Earth system feedbacks, associated for example with the cycling of carbon through natural systems andreleases of carbon from permafrost melt, will change, and are likely to increase the actual expected warming." However, these releases are still not included in the climate-models generally, let alone in the model used to generate the UKCA.
If we change the default to 'SEGREGATED-FEEDBACK' this shows feedback-emissions separated and distinguishable from budget-emissions. This approach suggests 'loss of control' and is more plausible, as feedback-emissions and concentrations will occur and will accelerate as the planet warms [which the UKMO has now acknowledged] as has been happening already over much of the last 100 years and as CAF gradually becomes more than 50%.
This is the generic and visible difference of deceleration and acceleration curves. It is the difference between Integrated-Feedback-Curves [as from UKMO and in the UK Climate Act] which are convex and the Segregated-Feedback-Curves which are concave [as essayed in CBAT].
The policy-implication for UNFCCC-compliance is choose a smaller emissions-budget triggering less feedback. If we choose a larger emissions-budget [as with the UK Climate Act for example] we may well trigger uncontrollable rates of feedback-emissions that will accelerate, outpace and overwhelm efforts at later-larger budget-emissions control.
In a nutshell, this is a risk-judgement about 'control curves' versus 'loss of control curves'.
Making this judgement recognizes that pursuing growth of the economy by running risks that we don't have to run, will end up creating feedbacks with costs that we cannot afford to pay.
Extensive evidence from GCI was presented to the 2nd UK House of Commons Environmental Audit Committee Enquiry into this matter in June 2013.
In this, GCI compared the upper band of the UKMO model 'integrated-feedback-effect' on emissions-concentrations to 516 PPMV by 2110 [as in the UKCA], with the CBAT 'segregated-feedback-effect' on emissions-concentrations to the same 516 PPMV by 2110.
The significant difference is immediately apparent.
The UKMO model is unrealistic. Next to that CBAT is much less unrealistic.
The UKMO model is unrealistic as it shows feedback-emissions jumping up very suddenly between 2010 and 2016 and then remaining roughly constantly stable at an ouput value of around 2 Gt C a year for the next Century with concentrations rising sharply at the outset and then just slowing down over the next Century. The project this whilst also projecting that the planet warms to ~ 3 degrees Celsius above pre-Industrial temperature by 2110. In this model, feedback-emissions from permafrost melt for example are simply left out.
The CBAT model next to that is realistic as it shows feedback-emission rising slowly from 2010 and just accelerating at a constant rate increasing the output value per annum [the user can select the rate] with concentrations rising slowly as a 50% function of that rate of feedback-emissions over the Century ahead.
What comes clearly into view when those two methods are compared [both arriving at 516 PPMV in 2110 as shown in this example] is that UKMO's unrealistic approach is alarmist whilst also being misleading. The UKMO curves for concentrations are consistently higher that the CBAT curves for the 100 year time-frame and the UKMO curves for emissions are consistently higher for the first 50 years.
However, UKMO's approach is seriously misleading, as it falsely suggests that 'we will be keeping 'control' over the Century ahead by simply enacting the budget-emissions control as in defined in the UK Climate Act [2016 4% low].
The greater likelihood is that we won't be keeping so much as losing control, with this CBAT MEDIUM Emissions Budget i.e. acheiving zero emissions globally by 2110 as feedback emissions-concentrations will accelerate uncontrollably as the planet warms and the permafrost melts, making it increasing probable that 'we will actually be losing control' of the rates of climate change as this recipe for doing too-little too-late is acted out.
If we are to have any chance of 'keeping control', the Emissions-Budget will need to be nearer the CBAT 'LOW' Emissions Budget, i.e. reflecting the emergency we are in and acheiving zero emissions globally by no later than 2050, led and accompanied by vigorous transition policies and measures.