Consensus climate scientists have decided to err on the side of least drama and thus ignore the Precautionary Principle in order to extend the period for the 66% confidence level carbon budget from the AR5 estimated 3-year period to 10-years to be committed to a 1.5C increase in GMSTA above pre-industrial (whatever those words mean). If they are wrong, mankind will suffer what it must because it doesn't have the strength to face the reality of our current situation:
Title: "Analysis: Why the IPCC 1.5C report expanded the carbon budget"
https://www.carbonbrief.org/analysis-why-the-ipcc-1-5c-report-expanded-the-carbon-budgetExtract: "The newly published Intergovernmental Panel on Climate Change’s (IPCC) special report on 1.5C (SR15) significantly expands the budget for a 66% chance of avoiding 1.5C to the equivalent of 10 years of current emissions. This compares to the IPCC’s fifth assessment report (AR5), which put it at around three years.
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Based on estimates made in the IPCC’s fifth assessment report (AR5), there would be around 120 gigatonnes of CO2 (GtCO2) remaining from the beginning of 2018 – or around three years of current emissions – for a 66% chance of avoiding 1.5C warming. For a 50/50 chance of exceeding 1.5C, the remaining budget was a modestly larger 268GtCO2 – or around seven years of current emissions.
The IPCC’s new SR15 significantly revises these numbers. It raises the budget for a 66% of avoiding 1.5C to 420GtCO2 – or 10 years of current emissions. Similarly, the budget for a 50/50 chance of exceeding 1.5C is increased to 580GtCO2 – 14 years of current emissions.
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Even with the revised 1.5C carbon budget is unlikely to be the end of the debate, however, given a number of large remaining uncertainties. These include:
• The precise meaning of the 1.5C target.
• Disagreement about what “surface temperature” actually refers to.
• The definition of the “pre-industrial” period.
• What observational temperature datasets should be used.
• What happens to non-CO2 factors that influencing the climate.
• Whether Earth-system feedbacks like melting permafrost are taken into account.
Finally, the emission scenarios considered in the new SR15 also tend to emit far more than the budget would allow, but make up for it with the large-scale use of negative emissions in the future.
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The carbon budgets featured in the IPCC AR5 were based on this subset of 20 climate models that could calculate both past temperature change and emissions, rather than on actual observations of temperature and emissions. Because some of these models had emissions and temperature changes that diverged significantly from observations, it caused a number of problems in calculating the carbon budget.
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The main change in the way carbon budgets are calculated in the new IPCC SR15 report is the use of observations – rather than values from ESMs – to determine the amount of warming and emissions between the mid-1800s and present. The relationship between cumulative emissions and temperatures – based on ESMs and observational constraints from the IPCC AR5 – is then used to calculate the remaining budget from present.
This approach, originally proposed by Millar and colleagues in a 2017 paper, effectively eliminates the problems associated with ESMs underestimating historical cumulative CO2 emissions and projecting temperatures warmer than have been observed.
Correcting both of these issues is broadly accepted by the scientific community. There is no doubt that using observations rather than model estimates leads to a more accurate estimate of the remaining budget for the 1.5C and 2C targets."
See also:
http://www.nature.com/articles/ngeo3031