Like Chad (see Post 15) and the Central African Republic (see Post 116), Cameroon has no significant temperature data before 1950. However, the change in its climate is more reminiscent of that of West Africa (see Post 114). Before 1990 the climate is stable; thereafter the mean temperature appears to increase by about 0.5°C (see Fig. 117.1 below). This is a modest temperature rise and much less than the often quoted IPCC global value.
Fig. 117.1: The mean temperature change for Cameroon since 1940 relative to the 1951-1980 monthly averages. The best fit is applied to the monthly mean data from 1956 to 1990 and has a slight negative gradient of -0.02 ± 0.20 °C per century.
In order to quantify the changes to the climate of Cameroon since 1940 the temperature anomalies for the fifteen stations with the most data (i.e. over 300 months of data) were determined and averaged. This was done using the usual method as outlined in Post 47 and involved first calculating the temperature anomaly each month for each station, and then averaging those anomalies to determine the mean temperature anomaly (MTA) for the country. This MTA is shown as a time series in Fig. 117.1 above and clearly shows that temperatures declined continuously from 1940.
The process of determining the MTA in Fig. 117.1 involved first determining the monthly reference temperatures (MRTs) for each station using a set reference period, in this case from 1951 to 1980, and then subtracting the MRTs from the raw temperature data to deliver the anomalies. If a station had at least twelve valid temperatures per month within the MRT interval then its anomalies were included in the calculation of the mean temperature anomaly (MTA). The total number of stations included in the MTA in Fig. 117.1 each month is indicated in Fig. 117.2 below. The peak in the frequency between 1950 and 1990 suggests that the 1951-1980 interval was probably the most appropriate to use for the MRTs.
Fig. 117.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for Cameroon in Fig. 117.1.
The locations of the sixteen stations whose data was used to determine the MTA in Fig. 117.1 are shown in the map in Fig. 117.3 below. Eight are medium stations with over 480 months of data, but only one
station has over 800 months of data before 2014, and only two have any data
before 1950 (see here for a full list). In addition, there are another
eight stations with over 300 months of data. According to the map below the geographical spread of stations is fairly uniform, in which case the simple average of the anomalies from all stations used to construct the MTA in Fig. 117.1 should yield a fairly accurate temperature trend for the country as a whole.
Fig. 117.3: The (approximate) locations of the sixteen longest weather station records in Cameroon. Those stations with a high warming trend between 1911 and 2010 are marked in red while those with a cooling or stable trend are marked in blue. Those denoted with squares are medium stations with over 480 months of data, while diamonds denote stations with more than 300 months of data.
The MTA in Fig. 117.1 shows the temperature change over the time period where the data is most numerous and therefore reliable. There are, however, two stations with data before 1940. These are in the two main cities of Douala and Yaoundé. The fact that in both cases the data is discontinuous with large gaps in the data between 1900 and 1940, and both stations are located in large urban areas, would suggest the data for both is not representative of the country as a whole. If we do include this earlier data we get the extended MTA shown in Fig. 117.4 below. This appears to imply an additional warming of 1.3°C occurred before 1940 when increases in carbon dioxide levels were small which also raises questions about the quality of the data. For these reasons I would tend to discount all the MTA data before 1950.
Fig. 117.4: The mean temperature change for Cameroon since 1880
relative to the 1951-1980 monthly averages. The best fit is applied to
the monthly mean data from 1956 to 1990 and has a slight negative
gradient of -0.02 ± 0.20 °C per century.
If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 117.5 below. This again was determined by averaging each monthly anomaly from the sixteen longest stations and suggests that the climate was fairly stable from 1940 until 1980 but then warmed by about 0.7°C thereafter.
Fig. 117.5: Temperature trends for Cameroon based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1952-2011 and has a positive gradient of +1.46 ± 0.05°C/century.
Comparing the curves in Fig. 117.5 with the published Berkeley Earth (BE) version for Cameroon in Fig. 117.6 below shows that there is good agreement between the two sets of data. This indicates that the simple averaging of anomalies used to generate the BE MTA in Fig. 117.5 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 117.6. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 117.1. How Berkeley Earth managed to determine the temperature change in Cameroon between 1900 and 1950 in Fig. 17.6 when there is virtually no data for this period is a point of debate.
Fig. 117.6: The temperature trend for Cameroon since 1840 according to Berkeley Earth.
The differences between the MTA in Fig. 117.4 and the BE versions using adjusted data in Fig. 117.5 are instead mainly due to the data processing procedures used by Berkeley Earth. These include homogenization, gridding, Kriging and most significantly breakpoint adjustments. These lead to changes to the original temperature data, the magnitude of these adjustments being the difference in the MTA values seen in Fig. 117.4 and Fig. 117.5. The magnitudes of these adjustments are shown graphically in Fig. 117.7 below. The blue curve is the difference in MTA values between adjusted (Fig. 117.5) and unadjusted data (Fig. 117.4), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Neither are larger than about 0.2°C.
Fig. 117.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 117.5 after smoothing with a 12-month moving average. The blue curve represents the total BE adjustments including those from homogenization. The linear best fit (red line) to these adjustments for the period 1952-2011 has a positive gradient of +0.41 ± 0.03 °C per century. The orange curve shows the contribution just from breakpoint adjustments.
Summary
According to the raw unadjusted temperature data, the climate of Cameroon was stable until 1990 and then warmed by between 0.3°C and 0.6°C (see Fig. 117.1).
Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of Cameroon has warmed by about 0.8°C (see Fig. 117.5).
Acronyms
BE = Berkeley Earth.
MRT = monthly reference temperature (see Post 47).
MTA = mean temperature anomaly.
Link to list of all stations in Cameroon and their raw data files.
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