NZ vs S. Hemisphere temperaturesGuest author | January 9, 2011
The “Seven-Station Series” (7SS) constituting the official New Zealand Temperature Record (NZTR) is analysed and compared with the Southern Hemisphere (SH) temperature record using an interesting new data analysis technique called Empirical Mode Decomposition (EMD).
Analysis of temperature trends usually employs extrinsic data smoothing techniques such as regression, moving average and Fourier filtering, but there is a more appropriate technique available.
Empirical mode decomposition (EMD) is an intrinsic data analysis technique now being used across a number of disciplines, including climatology. You can find out more from these two background papers: On the trend, detrending, and variability of nonlinear and nonstationary time series (Wu et al., 2007)(pdf) and Analysis of Temperature Change under Global Warming Impact using Empirical Mode Decomposition (Molla et al., 2007)(pdf). If you want to study EMD in detail, there’s a lot of help available — even a free command line utility.
EMD uses a sifting algorithm that filters the data until an overall adaptive trend (monotonic residual) is revealed. The first paper linked above shows how a decadal trend was also extracted from the global record but the 100-year 7SS time-frame used here is too short to do the same. A longer 7SS record would probably reveal an intermediate decadal trend similar to that presented plus an overall trend that cannot be extracted (by this author) from the 7SS at its current length.
The SH normal climate of 0.14 °C/century in this analysis is the Southern Hemisphere (SH) component of the 0.5 °C/century global recovery from the Little Ice Age (LIA) (Akasofu, 2010).
In lieu of any component already established that is to date unknown to this author, the SH component was arrived at by two different means: north/south landmass distribution (39:19) and north/south decadal temperature trend (0.21:0.08 °C December 9, 2010). The trend starts at the global average temperature of 13.6 °C in 1850.
Here is Figure 9 from On the recovery from the Little Ice Age (Akasofu, 2010):
Here is the comparison plot of NZ and SH temperatures:
The first and lowest 7SS EMD data point at 1910 is 11.93 °C, the last and highest is 12.54 °C at 2009, so the temperature rise from 1910-2009 is 0.61 °C in total. Of that, 0.14 °C is attributable to normal climate, leaving a rise of 0.47 °C unaccounted for. It should be noted that the 0.61 °C rise is considerably less than the 0.91 °C/century warming promoted by NIWA but arrived at by linear regression.
Possible causes for the increase of 0.47 °C include:
A) Unnecessary adjustment of raw data.
B) Urban heat island effect (UHI).
C) Part of a multi-decadal cycle.
D) Combinations of A, B and C.
UPDATE 22:28 p.m. NZDT
As a trial, the values of two cool year extremes, 1912 and 1930, were removed from the input dataset and replaced by a duplicate of the previous year’s values (1911 and 1929 respectively). In other words, smoothed — the emd.exe utility does this automatically when data is removed.
Two other trials were run where, first, 1912 alone was removed, then 1930 alone was removed, but neither changed the general shape of the EMD trend in the way that removing both did, which appears somewhat counter-intuitive.
No conclusions should be drawn from this experiment unless the reader has complete mathematical understanding of the EMD algorithm (something this author certainly does not have yet).