How 2005 & 201o got “ranked” the warmest years.
The new temperature data from CRUTEM4 has added 628 new weather stations, including strangely enough over 50 from Kyrgyzstan. Most of these stations are in far northern latitudes. There are none in the Southern Hemisphere. The general perception is that the greenhouse effect is a global phenomenon and all parts of the world will experience “climate change”. However GISS, UAH and Hadcrut trends show recent larger increases in the Arctic. Antarctica on the other hand shows little sign of any warming. The Arctic is surrounded by land masses and this is precisely where the vast majority of the new station data have been added by CRU. CRUTEM4 has significantly increased sampling across the (still very cold) Arctic borders. The best way to see this is through the effect it has on global average temperatures. I will ignore all the counter-arguments as to why “global” temperatures cannot be shown. These are the temperatures to which the station anomaly measurements refer and reflect their geographic distribution. Figure 1 shows the area averaged temperatures for CRUTEM3 compared to CRUTEM4.
The effect of adding so many Arctic stations is to essentially drop the average temperatures in the northern hemisphere by up to 1-2 degrees C. This is accentuated during recent years where more of the stations have data. What effect does all this have on the temperature anomalies ? The anomalies for CRUTEM4 will naturally tend to increase slightly over CRUTEM3 for two reasons.
1. The addition of stations in an area which is already known to show strong warming will lead to a higher global average anomaly as available grid points get filled in preferentially there.
2. The density of grid points in a 5×5 degree grid increase rapidly as we get closer to the poles. In fact if you stand near the north pole and simply walk around it you will then pass through 75 grid points. This means that there are far more grid points available in which to place new stations than for example at the equator. Furthermore because there is so much land area close to the north pole there are stations nearby unlike at the south pole. It is true that the area averaging does weight according to latitude. However CRUTEM4 has filled as many grid points as possible just near the north pole. It is clear that this is the explanation as to why the post 1998 anomalies have increased sufficiently for 2010 to become the “warmest year”, although within errors this is anyway meaningless. A detailed comparison of the anomalies after 1994 is shown in Figure 2.
To look further into this I compared missing grid points between CRUTEM3 and CRUTEM4. A missing grid point is simply one single 5×5 cell which does not contain any stations. There are 2592 cells in a 5×5 degree world grid of which perhaps 65% contain just ocean. If all available land points were covered with stations then there would be an ideal minimum of ~1700 missing cells. Figure 3 shows the actual number of missing cells per year (and month) for CRUTEM3 and CRUTEM4.
The geospatial changes can be seen in more detail by comparing the ratio of sampled cells from the tropics (LAT < 25) with those at large latitudes (LAT >25) – see Figure 3.
CRUTEM4 accentuates further the sampling bias away from the Tropics. It is pretty clear that oversampling of the arctic region leads to an increase in the “global” temperature anomaly, as is now “measured” with CRUTEM4. However, it should be remembered when reading various press releases and news headlines that the error on a single annual anomaly value is ~ 0.1 deg.C , so statistically it is meaningless to state that 2010 is warmer than 1998 or vice versa.
P.S. I wonder what is the real origin of the spike in 2009? Why did so many of the new arctic stations suddenly disappear that year ?