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Call it hair splitting, if you want, but I see at least three numbers in the latest IPCC report (e.g. on pages 7-51–7-52). The first one, "the total human forced GSAT change from 1750–2019", is 1.29°C. The second one, "the total (anthropogenic plus natural) emulated GSAT between 1850–1900 and 2010–2019", is 1.14°C. The third one, "the assessed GSAT", is 1.06°C. Suppose, the second one is a simulation and the third one is an observation (or so I understood). But why is the first estimate considerably higher than the second one even though the former doesn't include small natural warming, I take it? Keep in mind, 1750 and 1850 are almost the same baselines and differ by a measly 0.1°C so it doesn't explain the discrepancy. I'm not a science skeptic, by the way

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  • $\begingroup$ Have you thoroughly studied the report? What does it say on the discrepancies? $\endgroup$
    – Erik
    Dec 14, 2021 at 11:24
  • $\begingroup$ @eric what do you mean? $\endgroup$ Dec 14, 2021 at 16:03
  • $\begingroup$ I imagine it depends on which stretch of years you're referring to--starting and ending dates--and also which set of observations you use. Hadley CRUTEM uses 1850-present, for example, while NASA GISS and NOAA start from 1880. Before that you have to use proxies like ice cores. You might also get a different answer by choosing two years as endpoints, as opposed to doing a linear regression on all the points and then using the estimated figures AT the endpoints. The latter is the way I'd do it. $\endgroup$
    – bpl1960
    Dec 15, 2021 at 20:18
  • $\begingroup$ You left out ranges. The 1.29 °C value, for example has a "high confidence" range of 1.00 to 1.65 °C. $\endgroup$ Dec 16, 2021 at 10:39
  • $\begingroup$ @davidhammen but it's the best estimate that counts, isn't it? Besides, those are three separate numbers still. How do they differ? Why do they differ? $\endgroup$ Dec 16, 2021 at 16:02

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Some of the relevant AR6 headline values are:

  • +1.09 °C = Observed GMST and GSAT change between the periods 1850-1900 and 2011-2020

  • +1.06 °C = Observed GMST and GSAT change between the Chapter 7 attribution assessment periods 1850-1900 and 2010-2019

  • +1.14 °C = Emulated GSAT change between the Chapter 7 attribution assessment periods 1850-1900 and 2010-2019

  • +1.29 °C = Emulated GSAT change between end points 1750 and 2019, attributed to anthropogenic forcings

The +1.06 °C and +1.09 °C values are the same observed estimates of temperature change, it’s just that the latter covers the most recent “current” period that was available at the time the report was being written. Yes, the 0.03 °C difference comes from only one year of difference in the meaning periods, but that just reflects the effect of interannual variability on 10-year means. The +1.06 °C value is only really included in the report for comparison with the work in Chapter 7.

The +1.06 °C and +1.14 °C values are two different estimates of the same GSAT metric. They're not independent estimates because the latter is constrained by the GSAT observations, as well as other observations and models. When you consider their uncertainty bounds, these estimates are consistent with each other, but, as noted in Chapter 7 p7-52:

As the emulated response attempts to constrain to multiple lines of evidence (Supplementary Material 7.SM.2), only one of which is GSAT, they should not necessarily be expected to exactly agree.

The GSAT observations tell us what happened, but they don’t tell us why it happened: how much does each forcing agent contribute to the record? In theory we could estimate these contributions by running full ESMs many many times, covering uncertainty in the forcing agents, perhaps also constrained by observations and their uncertainties, in an attempt to see the signals through the noise of internal variability. But in practice, it’s just not computationally possible to do that, which is where the emulators come in.

Cross-Chapter Box 7.1

Climate model emulators are simple physically-based models that are used to approximate large-scale climate responses of complex Earth System Models (ESMs). Due to their low computational cost they can populate or span wide uncertainty ranges that ESMs cannot. They need to be calibrated to do this and, once calibrated, they can aid inter-ESM comparisons and act as ESM extrapolation tools to reflect and combine knowledge from ESMs and many other lines of evidence.

The emulators don’t (and don’t attempt to) capture the full, noisy temperature record including interannual variability. Think of them as emulating the underlying forced response of temperature without the obscuring effects of noise.

The difference between emulated values +1.14 °C and +1.29 °C is a mixture of several things: the 1750 versus 1850-1900 baselines (about 0.1 °C), the non-anthropogenic forcings (about -0.02 °C), and using end points rather than end periods (about 0.2 °C). Note that just looking at the end points is normally not recommended in observations and ESMs because a lot of the difference will be internal variability, but that’s not really a problem in these smooth emulations. You just have to be careful what you do with them, i.e., what other values you’re comparing them to.

So, the bottom line is that the best estimate of the warming that has been expressed by the system to date is +1.09 [0.95 to 1.20] °C. From the AR6 SPM headline statements (my emphasis):

A.1.2 Each of the last four decades has been successively warmer than any decade that preceded it since 1850. Global surface temperature8 in the first two decades of the 21st century (2001-2020) was 0.99 [0.84-1.10] °C higher than 1850-1900. Global surface temperature was 1.09 [0.95 to 1.20] °C higher in 2011–2020 than 1850–1900, with larger increases over land (1.59 [1.34 to 1.83] °C) than over the ocean (0.88 [0.68 to 1.01] °C). The estimated increase in global surface temperature since AR5 is principally due to further warming since 2003–2012 (+0.19 [0.16 to 0.22] °C). Additionally, methodological advances and new datasets contributed approximately 0.1ºC to the updated estimate of warming in AR6.

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There has been a component of natural cooling over that period - not warming. 1.14C is the modeled amount of warming overall, which comes from 1.29C of anthropogenic, less 0.15 of natural cooling. There appears about 0.8C of difference between modeled and assessed from actual temperature data - which is almost certainly within the expected range of uncertainty (which were not provided here).

It would be more remarkable for modeling and temperature data to work out exactly the same than being a bit different, but within the ranges of uncertainty.

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About 1 degree Celsius since the 1850s. That's up from a another one since the 1600's. Earth's temperature rose about 1.5- degrees since the 1700's. enter image description here If you want a good guess, use the CET since 1659. NOTE the temperature curve, failed to obey the CO2 curve. Average temperatures are hard to come by largely because corrupted data from temperature data from weather stations placed in poor locations..... Subtract Urban heat island effect it spits in eye of climate alarmists. And many temp sensors are in cities, thou cities only make up 2.7% landscape they have vast majority of data sets. Outliers often Always offset averages, Urban heat islands affect, even at nighttime temps; An asphalt parking lot can store a days worth of solar energy and heat air an added 10 degrees in daytime, 2-5 degrees at night. Contaminated temperature data come from temperature sensors and weather stations in parking lots and urban building zones. enter image description here enter image description here

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  • $\begingroup$ Downvoted. Graph is not global average and location appears chosen because it has less temperature rise than the global average. The blanket disparagement of temperature records as poor data dismisses the professional efforts made to "homogenize" - ie account for known distorting factors including heat island effect and, without any evidence suggests global average warming is a consequence of poor data. The pics, like the graph are cherry picked. $\endgroup$
    – Ken Fabian
    Dec 19, 2021 at 6:29

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