The influence of subjectivity in tree ring-based retrospective climate history reconstructions

This is very embarrassing for the science of climatology! Scientists are all too human. Sometimes the the grey area between science and pseudoscience is very large like in this case!

This undoubtedly puts more doubt on so called retrospective climate history reconstruction for the Common Era!

The infamous 1.5°C temperature increase above preindustrial levels in coming decades that we have to avoid under all circumstances is more and more exposed as a measurement error if not propaganda and demagoguery!

In short, the current approach to tree ring-based climate reconstruction is not much better than tea leaf reading (a.k.a. Tasseography)!

See e.g. figure 5a below: The mean temperature values reconstructed by each of the 15 groups are all over in a range between about 0°C and -0.8°C. 

Caveat: Presuming this study is not seriously flawed!

From the abstract:

"Tree-ring chronologies underpin the majority of annually-resolved reconstructions of Common Era climate. However, they are derived using different datasets and techniques, the ramifications of which have hitherto been little explored. Here, we report the results of a double-blind experiment that yielded 15 Northern Hemisphere summer temperature reconstructions from a common network of regional tree-ring width datasets. Taken together as an ensemble, the Common Era reconstruction mean correlates with instrumental temperatures from 1794–2016 CE at 0.79 (p < 0.001), reveals summer cooling in the years following large volcanic eruptions, and exhibits strong warming since the 1980s. Differing in their mean, variance, amplitude, sensitivity, and persistence, the ensemble members demonstrate the influence of subjectivity in the reconstruction process. We therefore recommend the routine use of ensemble reconstruction approaches to provide a more consensual picture of past climate variability."

From the paper:

"... each of the 15 groups who contributed independently to this experiment (referred to here as R1–R15) have experience in developing tree ring-based climate reconstructions. However, each group employed a distinct reconstruction approach (Fig. 2; Supplementary Table 1), manifested in different series and site selections, detrending methods, temperature targets, and calibration techniques (see the “Methods” section). ...
No two temperature target datasets are the same. Each group identified different regions within the NH [Northern Hemisphere], and extracted different seasons between May and October within the years 1750 and 2016 from different gridded temperature products. ...
However, each group employed a distinct reconstruction approach ..., manifested in different series and site selections, detrending methods, temperature targets, and calibration techniques ..."

The influence of decision-making in tree ring-based climate reconstructions | Nature Communications (open access)

Credits: Klimaforscher kritisiert Fridays for Future - Klimaschau 108

Fig. 5: Reconstruction characteristics

a–d Mean, standard deviation, first-order autocorrelation (and highest Hurst exponent H), and the difference between industrial and pre-industrial temperature means (after and before 1850 CE) of the 15 ensemble reconstructions (R1–R15), as well as their mean and median (orange and red).