https://judithcurry.com/2018/10/18/rema ... g-of-1-5c/
A close reading of Chapters 1 and 2 of the IPCC Special Report on Global Warming of 1.5°C (SR15) reveals some interesting changes from the IPCC 5th Assessment Report (AR5), and other science-relevant statements. This article highlights statements in SR15 relating to carbon emission budgets for meeting the 1.5°C and 2°C targets.
It seems fairly extraordinary to me that the AR5 post-2010 carbon budget for 1.5°C, which was only published four years ago, has in effect been now been increased by ~700 GtCO2 – equal to 21st century emissions to date – despite SR15’s projections of future warming being based very largely on the transient climate response to cumulative emissions (TCRE) range exhibited by the models used in AR5.
Key points
- The SR15 estimates of the carbon budgets that will allow us to remain within the 1.5°C and 2°C targets are far larger than those given in AR5 – over five times as high from end 2017 for a 66% probability of not exceeding 1.5°C warming.
- SR15 switches the measure of past (up to 2010) warming for the 1.5°C and 2°C targets from near-surface air temperatures (SAT) everywhere (as in AR5) to a blend of near-surface air temperatures over land and sea-surface water temperatures (SST).
- SR15 bases its estimates of the relationship of future warming to future CO2 emissions very largely on the behaviour of the current generation of Earth system models (ESMs), as used for AR5. However, unlike AR5 it does not do so directly. Instead, it assumes a fixed probabilistic relationship between post-2010 cumulative CO2 emissions and the warming they cause, and derives (using simplified climate models) an allowance for warming from other causes.
- SR15 ignores ESM simulation estimates of warming to date, instead estimating it using observational data.
- The resulting SR15 estimate of the post-1875 cumulative CO2 emissions that would give a 50% probability of meeting the 1.5°C target is approximately 720 GtCO2 larger than per AR5, partially offset by a 210 GtCO2 increase in estimated 1876–2010 emissions, giving a net increase of 510 GtCO2 for the post-2010 carbon budget.
- Approximately 180 GtCO2 of the ~720 GtCO2 increase in the post-1875 budget is due to lower projected post-2010 warming relative to post-2010 cumulative CO2 The lower projected warming appears to be because of two factors:
-- The TCRE value used in SR15 matches the average of the full set of ESMs in AR5; however the budgets calculated for AR5 were based on a subset of ESMs that had a higher average TCRE value.
-- Lower non-CO2 warming is projected in SR15 than in AR5
So, the IPCC just expanded the CO2 budget by 5 times? FIVE TIMES? For a 66% probability of doom. ROFL
SR15 explains that ‘The IPCC has traditionally defined changes in observed global mean surface temperature (GMST) as a weighted average of near-surface air temperature (SAT) changes over land and sea surface temperature (SST) changes over the oceans’. Consistent with that, the SR15 1.5°C remaining carbon budgets are based on anthropogenic warming up to 2006–2015 of 0.87°C, which is based on surface temperature datasets that mostly combine near-surface air temperature over land and sea surface temperature over the (open) ocean.
Average global warming simulated over the historical period (1850 to date) by the ESMs used in AR5 exceeds that shown by the observational temperature records used in SR15.[iii] It is likely that part of that difference in warming is due to the ESMs using SAT as the measure of temperature over the ocean as well as land, and to incomplete global coverage of observations. The importance of this issue is reflected by SR15’s statement that ‘the use of blended SAT/SST data and incomplete coverage together can give approximately 0.2°C less warming from the 19th century to the present relative to the use of complete global-average SAT.’
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Although the reasons for deciding to measure warming for the purposes of the 1.5°C and 2°C targets by combining SAT over land with SST over ocean (rather than SAT everywhere, as in AR5) are not entirely clear, it is in my view a sensible decision scientifically. Surface air temperature over the ocean has been (and still is) less well measured than SST, and also has much less direct relevance to humans and the biosphere than does SST. The change has the effect of making the remaining carbon budgets larger. However, SR15 is inconsistent in applying its decision to use a weighted average of SAT and SST: it only does so in respect of past warming; future warming is in effect still projected using a fully SAT-based measure.
Lord.
My interest was piqued by the statement that, although:
[i]considerably uncertainties remain, there is high agreement across various lines of evidence assessed in this report that the remaining carbon budget for 1.5°C or 2°C would be larger than the estimates at the time of the AR5.[x]
How much larger? Well, later in the section, SR15 says this:
This assessment finds a larger remaining budget from the 2006-2015 base period than the 1.5°C and 2°C remaining budgets inferred from AR5 from the start of 2011, [which were] approximately 1000 GtCO2 for the 2°C (66% of model simulations) and approximately 400 GtCO2 for the 1.5°C budget (66% of model simulations).[xi] In contrast, this assessment finds approximately 1600 GtCO2 for the 2°C (66th TCRE percentile) and approximately 860 GtCO2 for the 1.5°C budget (66th TCRE percentile) from 2011.
So, the remaining carbon budget from 1 January 2011 for a 66% probability of keeping below 1.5°C has been increased by 460 GtCO2, from 400 to 860 GtCO2 – more than doubled. Deducting the estimated 290 GtCO2 emissions during the 2011 to 2017 period,[xii] the change from 1 January 2018 is from 110 GtCO2 to 570 GtCO2 – over five times as high.
This is damning. I applaud them improving the stability of their data sources, that's laudable and exactly what should happen in the course of refining methods and working through the science towards greater understanding. But this behind-the-scenes reality does not jive with their rhetoric and puts a huge spotlight on how immature this science is.
As an aside, the most sophisticated TCRE study cited in AR5 was Gillett et al. (2013)[xv] – one of the two key observational-constrained, scaling-based ‘detection and attribution’ studies underlying the main AR5 human-caused warming finding.[xvi] Gillett et al. scaled CMIP5 ESM patterns of temperature responses to greenhouse gas warming so that they were consistent with the observed warming and found a TCRE range of 0.7–2.0°C, with a mean of 1.35°C. [b]The range adopted in SR15 has a 22% higher central value and a 25% higher upper bound than this observationally-constrained range.[/b]
Politics. Not a science report, a political report.
SR15 claims that Figure 2.3 (a version of which is reproduced below as Figure 1) illustrates that ‘the change since AR5 is, in very large part, due to the application of a more recent observed baseline to the historic temperature change and cumulative emissions’.
In my view this statement in SR15 lends itself to misinterpretation. A naïve interpretation of this statement is that both observed warming and observed emissions were lower than projected (by ESMs, under the RCP8.5 scenario) in AR5 over the period since then, with both of these factors contributing to an increase in the remaining carbon budgets consistent with 1.5°C or 2°C warming.
In fact, observed emissions between 2005 (the observational baseline date for emissions per the RCP scenarios) and the end of 2017 were almost identical to those per RCP8.5. And, if the ‘blended-masked’ CMIP5 models’ temperature (thin black line in Figure 1) is a fair comparison with the global temperature observations used in SR15 (thin blue line in Figure 1), then there is little difference between models and observations over that period.
I wonder how many actually understand what he is explaining? IPCC wants you to think the models are just fine, that warming decreased because emissions decreased, but in truth emissions increased at the same time warming was decreasing.
