There has been a lot of comment on a recent paper at nature geoscience “Emission budgets and pathways consistent with limiting warming to 1.5◦C” (hereafter Millar et. al 2017)
When making a case for public policy I believe that something akin to a process of due diligence should be carried out on the claims. That is, the justifications ought to be scrutinized to validate the claims. With Millar et al 2017, there are a number of issues with the make-up of the claims that (a) warming of 1.5◦C or greater will be achieved without policy (b) constraining the emissions
The baseline warming
The introduction states
Average temperatures for the 2010s are currently 0.87°C above 1861–80,
A similar quote from UNIPCC AR5 WG1 SPM page 5
The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available.
These figures are all from the HADCRUT4 dataset. There are three areas to account for the difference of 0.09°C. Mostly, it is the shorter baseline period. Also, the last three years have been influenced by a powerful and natural El-Nino, along with the IPCC, using an average of the last 10 years.
The warming in the pipeline
There are valid reasons for the authors differing from the IPCC’s methodology. They start with the emissions from 1870 (even though emissions estimates go back to 1850). Also, if there is no definite finish date, it is very difficult to calculate the warming impact to date. Consider first the full sentence quoted above.
Average temperatures for the 2010s are currently 0.87°C above 1861–80, which would rise to 0.93°C should they remain at 2015 levels for the remainder of the decade.
This implies that there is some warming to come through from the impact of the higher greenhouse gas levels. This seems to be a remarkably low and over a very short time period. Of course, not all the warming since the mid-nineteenth century is from anthropogenic greenhouse gas emissions. The anthropogenic element is just guesstimated. This is shown in AR5 WG1 Ch10 Page 869
More than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010 is very likely due to the observed anthropogenic increase in greenhouse gas (GHG) concentrations.
It was after 1950 when the rate of the largest increase in CO2 levels was experienced. From 1870 to 1950, CO2 levels rose from around 290ppm to 310ppm or 7%. From 1950 to 2010, CO2 levels rose from around 310ppm to 387ppm or 25%. Add in other GHG gases, and the human-caused warming should be 3-4 times greater in the later period than the earlier one, whereas the warming in the later period was just over twice the amount. Therefore if there is just over a 90% chance (very likely in IPCC speak) of over 50% of the warming post-1950 was human-caused, a statistical test relating to a period more than twice as long would have a lower human-caused element of the warming as being statistically significant. Even then, I view the greater than 50% statistic as being deeply flawed. Especially when post-2000, when the rate of rise in CO2 levels accelerated, whilst the rise in average temperatures dramatically slowed. There are two things that this suggests. First, the impact could be explained by rising GHG emissions being a minor element in temperature rise, with natural factors both causing some of the warming in the 1976-1998 period, then reversing, causing cooling, in the last few years. Second is that there is a darn funny lagged response of rising GHGs (especially CO2) to rises in temperature. That is, the amount of warming in the pipeline has increased dramatically. If either idea has any traction, then the implied warming to come of just 0.06°C is a false estimate. This needs to be elaborated.
Climate Sensitivity
If a doubling of CO2 leads to 3.00°C of warming (the assumption of the IPCC in their emissions calculations), then a rise in CO2 levels from 290ppm to 398 ppm (1870 to 2014) eventually gives 1.37°C of warming. With other GHGs, this figure should be around 1.80°C. Half that warming has actually occurred, and some of that is natural. So there is well over 1.0°C still to emerge. It is too late to talk about constraining warming to 1.5°C as the cause of that warming has already occurred.
The implication from the paper in claiming that 0.94°C will result from human emissions in the period 1870-2014 is to reduce the climate sensitivity estimate to around 2.0°C for a doubling of CO2, if only CO2 is considered, or around 1.5°C for a doubling of CO2, if all GHGs are taken into account. (See below) Compare this to AR5 WG1 section D.2 Quantification of Climate System Responses
The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multicentury time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence).
The equilibrium climate sensitivity ECS is at the very bottom of the IPCC’s range, and equilibrium climate response is reached in 5-6 years instead of mutlicentury time scales. This is on top of the implied assumption that there is no net natural warming between 1870 and 2015.
How much are GHG emissions?
With respect to policy, as global warming is caused by human greenhouse gas emissions, to prevent further human-caused warming requires reducing, and possibly eliminating global greenhouse emissions. In conjunction with the publication of the AR5 Synthesis report, the IPCC produced a slide show of the policy case laid out in the three vast reports. It was effectively a short summary of a summary of the synthesis report. Approaching the policy climax at slide 30 of 35:-

Apart from the policy objective in AR5 was to limit warming from 2°C, not 1.5°C, it also mentions the need to constrain GHG emissions, not CO2 emissions. Then slide 33 gives the simple policy simplified position to achieve 2°C of warming.

To the end of 2011 1900 GTCO2 of CO2 was estimated to have been emitted, whilst the estimate is around 1000 GTCO2 could be emitted until the 2°C warming was reached.
This is the highly simplified version. At the other end of the scale, AR5 WG3 Ch6 p431 has a very large table in a very small font to consider a lot of the policy options. It is reproduced below, though the resolution is much poorer than the original.

Note 3 states
For comparison of the cumulative CO2 emissions estimates assessed here with those presented in WGI AR5, an amount of 515 [445 to 585] GtC (1890 [1630 to 2150] GtCO2), was already emitted by 2011 since 1870
The top line is for the 1.5°C of warming – the most ambitious policy aim. Of note:-
The CO2 equivalent concentration in 2100 (ppm CO2eq ) is 430-480ppm.
Cumulative CO2 emissions (GtCO2) from 2011 to 2100 is 630 to 1180.
CO2 concentration in 2100 is 390-435ppm.
Peak CO2 equivalent concentration is 465-530ppm. This is higher than the 2100 concentration, and if for CO2 alone with ECS = 3 would eventually produce 2.0°C to 2.6°C of warming.
The Probability of Exceeding 1.5 °C in 2100 is 49-86%. They had to squeeze really hard to say that 1.5°C was more than 50% likely.
Compare the above to this from the abstract of Millar et. al 2017.
If CO2 emissions are continuously adjusted over time to limit 2100 warming to 1.5◦C, with ambitious non-CO2 mitigation, net future cumulative CO2 emissions are unlikely to prove less than 250 GtC and unlikely greater than 540 GtC. Hence, limiting warming to 1.5◦C is not yet a geophysical impossibility, but is likely to require delivery on strengthened pledges for 2030 followed by challengingly deep and rapid mitigation.
They use tonnes of carbon as the unit of measure as against CO2 equivalent. The conversion factor is 3.664, so cumulative CO2 emissions need to be 870-1010 GtCO2 range. As this is to the end of 2015, not 2011, as in the IPCC report, it will be different. Subtracting 150 from the IPCC reports figures would give a range of 480 to 1030. That is, Millar et. al 2017 have reduced the emissions range by 75% to the top end of the IPCC’s range. Given the IPCC considered a range of 1.5-1.7°C of warming, this seems somewhat odd to then say it is related to the lower end of the warming band, until you take into account that ECS has been reduced. But then why curtail the range of emissions instead calculating your own? It appears that again the authors are trying to squeeze a result within existing constraints.
However, this does not take into account the much higher levels of peak CO2 equivalent concentrations in table 6.3. Peak CO2 concentrations are around 75-95ppm higher than in 2100. Compare this to the green line in the central graph in Millar et. al 2017.

This is less than 50ppm higher than in 2100. Further in 2100 Millar et. al 2017 has CO2 levels of around 500ppm as against a mid-point of 410 in AR5. CO2 rising from 290 to 410ppm with ECS = 3.0 produced 1.50°C of warming. CO2 rising from 290 to 410ppm with ECS = 2.0 produced 1.51°C of warming. Further, this does not include the warming impact of other GHGs. To squeeze into the 1.5°C band, the mid-century overshoot in Millar et. al 2017 is much less than in AR5. This might be required in the modeling assumptions due to the very short time assumed in reaching full equilibrium climate response.
Are the authors playing games?
The figures do not appear to stack up. But then they appear to be playing around with figures, indicated by a statement in the explanation of Figure 2
Like other simple climate models, this lacks an explicit physical link between oceanic heat and carbon uptake. It allows a global feedback between temperature and carbon uptake from the atmosphere, but no direct link with net deforestation. It also treats all forcing agents equally, in the sense that a single set of climate response parameters is used in for all forcing components, despite some evidence of component-specific responses. We do not, however, attempt to calibrate the model directly against observations, using it instead to explore the implications of ranges of uncertainty in emissions, and forcing and response derived directly from the IPCC-AR5, which are derived from multiple lines of evidence and, importantly, do not depend directly on the anomalously cool temperatures observed around 2010.
That is:-
The model does not consider an “explicit physical link between oceanic heat and carbon uptake.” The IPCC estimated that over 90% of heat accumulation since 1970 was in the oceans. If the oceans were to belch out some of this heat at a random point in the future, the 1.5°C limit will be exceeded.
No attempt has been made to “calibrate the model directly against observations”. Therefore there is no attempt to properly reconcile beliefs to the real world.
The “multiple lines of evidence” in IPCC-AR5 does not include a glaring anomaly that potentially falsifies the theory and therefore any “need” for policy at all. That is the divergence in actual temperatures trends from theory in this century.
Conclusions
The authors of Millar et. al 2017 have pushed out the boundaries to continue to support climate mitigation policies. To justify constraining emissions sufficient stop 1.5°C of warming the authors would appear to have
Assumed that all the warming since 1870 is caused by anthropogenic GHG emissions when there is not even a valid statistical test that confirms even half the warming was from this source.
Largely ignored any hidden heat or other long-term response to rises in GHGs.
Ignored the divergence between model predictions and actual temperature anomalies since around the turn of the century. This has two consequences. First, the evidence appears to strongly contradict the belief that humans are a major source of global warming and by implication dangerous climate change. Second, if it does not contradict the theory, suggests the amount of warming in the pipeline consequential on human GHG emissions has massively increased. Thus the 1.5°C warming could be breached anyway.
Made ECS as low as possible in the long-standing 1.5°C to 4.5°C range. Even assuming ECS is at the mid-point of the range for policy (as the IPCC has done in all its reports) means that warming will breach the 1.5°C level without any further emissions.
The authors live in their closed academic world of models and shared beliefs. Yet the paper is being used for the continued support of mitigation policy that is both failing to get anywhere close to achieving the objectives and is massively net harmful in any countries that apply it, whether financially or politically.
Kevin Marshall
Commentary at Cliscep, Jo Nova, Daily Caller, Independent, The GWPF
Update 25/09/17 to improve formatting.