Guest essay By Frank Bosse
In a recent blog post at Dr. Judith Curry’s website the author Nicholas Lewis analyzes the climate sensivity from observations and concludes a TCR of about 1.33 which is very stable versus different periods (see Table 1 of the linked post).
Here I want to use a slightly different method and another temperature record, the Cowtan/Way (C/W) http://www-users.york.ac.uk/~kdc3/papers/coverage2013/series.html ) series. In the discussions of the post at Judy Currys website there were big “?” if the result of TCR would also stand if one uses the “land- infilled” Data mostly for the polar regions of the earth. Therefore I’ll use this record to show the difference to HadCRUT4 (that was used by N. Lewis in his calculations) in the output.
I investigate the span 1940…2015. This includes the latest increase of the Global Mean Surface Temperature (GMST) , see Fig. 1, and avoids the periods of temperature data with great uncertainty in the early years of the observations.
Fig.1: The GMST anomalies (GMSTA) following the record of C/W for 1940…2015.
The Forcing-data I take from the IPCC AR5 appendix (Tab. A II 1.2 https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_AnnexII_FINAL.pdf ) This record ends in 2011. For the time span 2012…2015 I calculated the CO2-forcing with the observed concentration data and the other forcings I extrapolated for a difference (the sum of forcings) of 0.25 W/m² between 2011 and 2015.
I excluded every volcano forcing because the few events during 1940…2015 were all before 1991 and would insert a bias.
One of the biggest pitfalls in the forcing data is the magnitude of the aerosol-forcing. In an actual paper http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-14-00656.1 the author Björn Stevens made some thoughts about a required reducing of it. N. Lewis could show https://climateaudit.org/2015/03/19/the-implications-for-climate-sensitivity-of-bjorn-stevens-new-aerosol-forcing-paper that the downscaling, which is implicid in the Stevens-paper, is about 50% (see the appendix in the “Climate Audit”-Post). This makes some difference in the total forcing:
Fig. 2: The total forcing (but for volcano) with Aerosol-forcing as calculated in AR5 (black) and reduced by 50% (magenta) as implicid suggested by Stevens (2015).
To avoid a “single study syndrome” I’ll calculate the TCR for both cases: With and without the Aerosol-Forcing reduction.
For the detection of the TCR I calculate a linear regression (least square) for the forcings of every year 1940 to 2015 versus the observed temperatures (annual means):
Fig. 3: Regression of Forcing vs. temperature anomalies. The forcings account for 78% of the variance of the GMSTA.
The slope of the trend line in Fig.3 stands for the observed relation of a forcing-change of 1W/m² to the GMST-change from this.
It’s 0.37K/(W/m²) for the unchanged aerosol-forcing as shown in Fig.3, the reduction included gives a slope of 0.32 K/(W/m²), not shown.
Let’s take a look at the “rest” which is not explained by the forcings, excluded volcano. The residuals between the linear regression slope and the observed GMSTA over the time:
Fig. 4: The residuals between forcings and observations with a 15year-smoothing (Loess).
Fig. 4 shows the natural variability like the ENSO-events 1998/2000 and the volcanic eruptions, such as in 1992/1993. There is also a low-frequency pattern as the low-pass in fig.4 shows. I want to compare it with the AMO-pattern as it’s described in a modern record suggested by v. Oldenborgh et al. (2009) http://dspace.library.uu.nl/bitstream/handle/1874/43930/os-5-293-2009.pdf?sequence=2
Fig. 5: The AMO-Index, see the rapid shift in the 90s.
The AMO seems to be a part of the internal variability as it’s pattern is well replicated in the Fig. 4. The amplitude of the impact of this index on the GMST is about 0.2K and a shift from negative to positive arose also during the years between 1976 and 2005 from which we know that many models were “tuned” ( see Mauritsen et al. 2012 http://onlinelibrary.wiley.com/doi/10.1029/2012MS000154/full ).
The models don’t replicate the AMO, they don’t “know” of it.
Finally let’s have a look at the TCR-values. It’s well known that a doubling of the GHG-concentrations will lead to a forcing of 3.71W/m². This gives for the discussed trend slopes:
1. TCR 1940…2015 for full aerosol-forcing: 1.39 K/(2*CO2)
2. TCR 1940…2015 for reduced aerosol-forcing: 1.19K/(2*CO2)
3. TCR 1976…2005 (Model “tuning” span): 2.3 K/(2*CO2)
The results of N. Lewis for TCR (1.33 with full aerosol-forcing and 1.22 for reduced aerosol forcing) are confirmed with only a deviation of 4% for another temperature record.
The residuals show a clear AMO-like pattern which is an essential part of the internal variability.
If one ignores this pattern one gets unrealistic high TCR-values greater than 1.8.
Many GCM do so, see Forster et al. (2013) http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50174/full .