How do climate scientists calculate climate mitigation probabilities… and what are the implications?

Understanding the Carbon Budget and its statistics

The normal distribution

The normal distribution is an idea in statistics that is more recognizable in its graphical form, as the bell curve

Standard deviations 

Standard deviation is a number used to tell how measurements for a group are spread out from the average (mean or expected value). A low standard deviation means that most of the numbers are close to the average, while a high standard deviation means that the numbers are more spread out.

Graphically, this means that if you divide the graph into equal ‘slices’ or rectangular sections, the dividers between those sections will correspond to one standard deviation each. 

The IPCC defines probability ranges according to:

Virtually certain (99–100% probability), extremely likely (95–100%), very likely (90–100%), likely (66–100%), more likely than not (50–100%), about as likely as not (33–66%), unlikely (0–33%), very unlikely (0–10%), extremely unlikely (0–5%), exceptionally unlikely (0–1%).

Also 66% represents the range which lies within one standard deviation of the mean (given a normal distribution).

So, in terms of the clause in the Climate and Ecology Bill:

To ensure that the United Kingdom reduces its overall contribution to global greenhouse gas emissions (‘emissions’) at a rate consistent with a 66% chance of limiting the global mean temperature increase to 1.5 degrees Celsius compared to pre-industrial levels, subject to the latest scientific evidence (but in any event at least a 66% chance), of limiting the global mean temperature increase to 1.5 degrees Celsius compared to pre-industrial levels, fulfilling its obligations set out in the UNFCCC and the Paris Agreement (‘the climate target’).

The larger the sample size, the more likely it is that the data will fit the normal distribution.

The UN IPCC calibrated uncertainty language”. They use specific terms to denote different levels of likelihood. If the IPCC says “likely” than they mean “with greater than 66% probability”

‘’Virtually certain’’- 99-100% probability

‘’Very likely’’  – 90-100% probability 

‘’Likely’’- 66-100% probability

“More greater than 50% probability”- as likely as not

‘’More likely than not”-33 to 66% probability 

‘’Unlikely’’ – 0-33% probability 

‘’Very unlikely’’- 0-10% probability 

‘’Exceptionally unlikely’’- 0-1% probability

‘’Exceptionally unlikely’’- 0-1% probability

IPCC AR6 WG1 2021 Report

The inclusion of the 83% top percentile comes together with the inclusion of a 17% bottom percentile. Together, these percentiles give an estimate of the 66% range surrounding the central median value. 

The intention was therefore to accurately reflect both what we understand and know about estimates of the remaining carbon budget as well as the uncertainties surrounding their estimates.

Reframing the measure of limiting the global mean temperature to 1.5C as a global carbon budget

The remaining carbon budget represents the total amount of CO2 that can still be emitted in the future while limiting global warming to a given temperature target (Matthews. H; Torkaska K; Nicholls Z; Rogelj J et al.2020)

Current emissions trend suggests that global society is only 10 years away until we exceed the 1.5C budget.

According to the UN IPCC’s latest estimate, the remaining carbon budget is 500 billion tonnes of carbon dioxide emissions from 2020 onward for a greater than 50% likelihood of limiting mean global warming to 1.5C. 

We will have emitted close to 80 billion tonnes during 2020-21, leaving 420 billion tonnes of carbon dioxide emissions in the budget after 2021

The year that we emit the last of this remaining carbon budget is expected to also be the year that global temperatures reach 1.5 C.

Whilst the global carbon budget consistent with > 67% of 1.5C chance from 2020 is estimated at 400 Gt CO2  the divvying-up of the global carbon budget among UNFCCC Paris Agreement parties/country signatories remains elusive and highly problematic in the light of the subjectivity of CBDR-RC. Ultimately, the sum of nations’ NDCs must not exceed the > 67% of 1.5C chance from 2020 is estimated at 400 Gt CO2 as a best-case scenario for human civilization and the safeguarding of the biosphere.

If we can manage to drive global carbon dioxide emissions to net-zero within the next two decades, we have a good chance of not reaching 1.5 C at all.

Few countries, however, have adopted this level of ambition: only a handful, including Uruguay, Finland, Iceland and Austria, have proposed net-zero emission pledges with a target year of 2040 or earlier (Matthews, H; Peters,G 2021) .

**Still, any proposals need to be enshrined in law