New: Simulation of a 20% uncertainty in proxy resolution time and peaks practically disappear.
Have there been previous warming periods over the last 10,000 years comparable to current warming ? If so would Marcott proxy data have been able to detect them? Tamino has a post where he attempts to prove that indeed such peaks would have been detected and therefore there is nothing comparable to the recent warming spike. In order to show this he artificially generated three 200-year long triangular peaks of amplitude 0.9C at dates 7000BC, 3000BC and 1000BC. These were added to the underlying data and processed as before. The resultant signals would have easily been detected (he claims).
Lets do exactly the same thing on the individual raw proxy data and then process them in the same way as I described previously. I simply increased all proxy temperatures within 100 years of a peak by DT = 0.009*DY, where DY=(100-ABS(peak year-proxy year)). Each spike is then a rise of 0.9 deg.C over a span of 100 years, followed by a return to zero over the next 100 years. The same three dates were used for the peaks as those used by Tamino. The results are shown below for anomaly data binned in 50 year time intervals.
The peaks are indeed visible although smaller than those claimed by Tamino. In addition, I believe we have been too generous by displacing the proxy data linearly upwards since the measurement standard deviation should be properly folded in. I estimate this would reduce the peaks by ~30%.
What I find very interesting is that there actually do appear to be smaller but similar peaks in the real data (blue arrows), one of which corresponds to the Medieval Warm Period !
New: Simulation of a 20% time resolution error in proxy measurement time.
Several people have commented that proxies measure an average temperature over an extended period rather than instantaneously, plus there is also uncertainty in the exact timing of the measurements. I have looked into this effect by simply randomizing the proxy measurements within a window of 20% of the proxy resolution. So a proxy with a resolution of 100 years is moved randomly to a new time within +- 20 years of the recorded time. The data is otherwise treated exactly the same way as before but now the new time is used to calculate the peak increment. This simulates proxy time synchronization. The result is shown below.
Uncertainties in the time synchronisation across proxies effectively smears out the peaks completely. With a 20% of resolution random time shift the pulses are essentially smeared out. Only the pulse at 3000BC remains a candidate. Note this does not simulate long Proxy measurement times.
The code used in this post can be downloaded here. There are 3 PERL scripts.
- convert.pl reads the proxy data from the excel spreadsheet and creates “station files” like Hadcrut4.
- marcott_gridder.pl bins the data both in time(50y) and space (5×5 deg)
- marcott_global_average_ts_ascii.pl reads the grid and calculates the global averages – as plotted above.
Place the Marcott excel spreadsheet (provided with the Science Paper’s supplementary material) in the working directory. Convert it to .xls, remove the two spurious zero temperature entries in Proxy 62, and ensure the metadata sheet cells do not span 2 rows.
>mkdir station-peaks >cd station-peaks >perl ../convert.pl >cd ..
>perl marcott_gridder.pl | perl marcott_global_average_ts_ascii.pl > peaks-anomalies
The last 2 scripts are modified versions of the Met Office analysis software for CRUTEM4 – British Crown Copyright (c) 2009, the Met Office.