Spherical triangulation of Hadcrut4.5 = Cowtan & Way

The spatial integration of irregular temperature data (CRUTEM4) using spherical triangulation removes any coverage bias. 

I have made a spherical triangulation of the CRUTEM4 station data combined with HSST3 ocean temperature data. This is essentially Hadcrut4.5, but instead of using a (lat,lon) grid, it is averaged over the surface of a sphere including the poles.

Hadcrut4.5 displayed on a 3D spherical grid – January 2017

So how does this compare to existing temperature data? The next plot shows the annual global temperature anomaly as compared to the standard Hadcrut4 values and those of Cowtan & Way.

Comparison of recent yearly anomalies based on Hadcrut4.5

The spherical results are essentially identical to those of Cowtan and Way! They use a complex 2D kriging algorithm to interpolate the 5 degree T(lat,lon) values into higher latitudes, partly guided by UAH satellite data.

This result demonstrates that spatial integration of irregular temperature data (CRUTEM4) using 3D spherical triangulation alone removes any coverage bias.

  • Thanks to Tim Osborne for providing IDL code that processes CRUTEM4 station data.
  • Thanks to Nick Stokes for the basic idea. He uses something similar in TempLS

Download data for Hadcrut4.5 Spherical Triangulation (ST):

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The Ed Hawkins zonal animation

There is a new animation by Ed Hawkins which shows a comparison of zonal mean temperatures with latitude, between observations and models.

Observations compared to models

It is a very nice animation but it all looked a little bit too good to me, so I decided to check the result. So firstly here is  Cowtan and Way  plotted on the same scale.

Cowtan and Way kigging extrapolation of Hadcrut4. The curves are coloured according to their annual global temperature anomaly.

The result is very similar but not quite the same. Several years fall below the scale, but none do on Ed’s plot. The 10th warmest Arctic year actually had a zero global temperature anomaly. The main problem I have with using Cowtan and Way is that their results are essentially smoothed. They  use satellite measurements to extrapolate Hadcrut4 into areas not covered by stations, but the satellite data is only available after 1980, so why are early years also so smooth? My guess is that they use a generic dependence based on their fit to satellite data also for earlier years. If true,  then I think it is unfair to label these observations as such. So lets instead look at the actual observations, Hadcrut4.5 itself.

Hadcrut4 latitude dependence. CLICK HERE for animation

The overall trend is fairly similar but the data are now far more variable, especially at high latitudes. We also see the largest increases in the Arctic for years with average global temperature anomalies, and vice versa. If we expand the scale further we can see just how much variability there really is in the underlying data.

Hadcrut4.5 results on an expanded scale shows large variability at high latitudes.

The conclusion is that Ed’s plot is basically correct, but he seems to have used a smoothed version of Cowtan and Way, which itself is a ‘kriged’ version of Hadcrut4.5 rather than using direct ‘observations’. If he used direct observations from Hadcrut4.5 then it would not look nearly so good.

The ‘simulated’ data is not the output of any actual climate model. Instead it is the average of all of them. Individually they disagree enormously about the amount of future warming. So the error on the 2100 simulated curves is really of the order of ±1C

Comparison of Hadcrut4.4 monthly data with an ensemble of 40 CMIP5 models

 

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Decadal Temperature Spatial Averages

The ten year averages of temperature anomalies within cells of an icosahedral grid  covering the earth’s surface are shown below.  The images are taken from four different directions. To view each decade yourself using WebGL follow the first set of header links. To enlarge each image just click on it.

Western Hemisphere
1951-1960 (50s) 1961-1970 (6os) 1971-1980 (70s)
1981-1990 (80s) 1991-2000 (9os) 2001-2010 (2000s)
Eastern Hemisphere
50s 6os 70s
80s 9os 2000s
North Pole
50s 6os 70s
80s 9os 2000s
South Pole
50s 6os 70s
80s 9os 2000s

The climate has clearly warmed over the last 60 years, with the exception of Antarctica. The mean regional warming in 2010 was 0.48C since 1950. The maximum warming was 2.14C and the maximum ‘cooling’ was -1.1C. The distribution of the 2001-2010 anomalies compared to the 1950s across all 2652 cells of the grid is shown below.

Figure 1. Distribution of the 10 year average of temperature anomalies within each grid cell.

High cell numbers are in general at higher latitudes. These figures also agree with a 10 year average calculated from the monthly data triangulated on a sphere ( see figure 2.).

Figure 2. Simple 10 year average of annual global anomalies.

One needs to be a bit cautious of colour coded temperature maps because they can give an illusion of extreme warming simply by a judicial choice of colour boundaries. However there is no doubt that the earth has warmed by ~0.5C since 1950 mainly concentrated at high northern latitudes.

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