There are fifteen countries that together make up West Africa: Mauritania, Mali, Burkina Faso, Niger, Nigeria, Benin, Togo, Ghana, Ivory Coast (Côte d'Ivoire), Liberia, Sierra Leone, Guinea, Guinea-Bissau, Senegal, and The Gambia. Unfortunately there are only seven long stations in the entire region with over 1200 months of data, most of which have cooling trends (see Fig. 114.1 below). There are, however, another 111 medium stations with over 480 months of data. Of these nearly 100 have over 600 months of data, nearly 50 have over 800 months of data and 11 have over 1000 months of data. For a full list see here.
While the region as a whole has reasonably good data, most individual countries do not. Most countries have fewer than ten long and medium stations in total. For that reason it makes sense to determine the temperature change across the region as a whole rather than concentrating on the data from individual countries. On the positive side, the distribution of stations is fairly uniform so averaging of the mean temperature anomaly (MTA) data should yield accurate results for the temperature change at least for the most recent 800 months (i.e. as far back as 1950), and possibly as far back as 1920.
Fig. 114.1: The (approximate) locations of the 118 longest weather station records in West Africa. 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 long stations with over 1200 months of data, while diamonds denote medium stations with more than 480 months of data.
The temperature anomalies for each station in Fig. 114.1 were determined using the usual method as outlined in Post 47. This involved first calculating 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 regional mean temperature anomaly (MTA). The resulting MTA after 1900 is shown in Fig. 114.2 below. It can be seen that before 1975 the climate was slowly cooling with the mean temperature falling by about 0.2°C over the preceding 75 years. Then after 1975 there was a rapid warming of about 0.75°C. The net warming since 1900 is therefore just over 0.5°C.
Fig. 114.2: The mean temperature change for West Africa since 1880 relative to the 1951-1980 monthly averages. The best fit is applied to the monthly mean data from 1901 to 1970 and has a negative gradient of -0.25 ± 0.08 °C per century.
The total number of stations included in the MTA in Fig. 114.2 each month is indicated in Fig. 114.3 below. The peak in the frequency around 1970 suggests that the 1951-1980 interval for the MRTs was indeed the most appropriate to use. It also indicates that there are a couple of stations with data before 1880 and about six with data before 1900.
Fig. 114.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for West Africa in Fig. 114.2.
Unfortunately the data before 1880 is fragmented with large fluctuations in its values, as the MTA in Fig. 114.4 below shows. It is therefore likely to be very unreliable and thus adds nothing to our understanding of climate change in the region.
Fig. 114.4: The mean temperature change for West Africa since 1840. The best fit has a slight negative gradient of -0.25 ± 0.08 °C per century.
In contrast to Fig. 114.2, the corresponding MTA dataset based on data that has been adjusted by Berkeley Earth (BE) exhibits a strong warming trend before 1975 with temperatures rising by over 1.0°C since 1900 (see Fig. 114.5 below).
Fig. 114.5: Temperature trends for West Africa based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1901-2010 and has a gradient of +0.85 ± 0.03°C/century.
If we next compare the curves in Fig. 114.5 with the published Berkeley Earth (BE) version for West Africa in Fig. 114.6 below we see that there is remarkably good agreement between the two sets of data at least as far back as 1900. This indicates that the simple averaging of anomalies used to generate the BE MTA in Fig. 114.5 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 114.6. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 114.2. This also suggests that any difference between the two averages cannot be due primarily to the averaging process, but must instead be due at least in part to the temperature adjustments made by Berkeley Earth.
Fig. 114.6: The temperature trend for West Africa since 1840 according to Berkeley Earth.
The differences between the MTA in Fig. 114.2 and the BE versions using adjusted data in Fig. 114.5 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. 114.2 and Fig. 114.5. The magnitudes of these adjustments are shown graphically in Fig. 114.7 below.
Fig. 114.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 114.5 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 1921-2010 has a slight negative gradient of -0.057 ± 0.009 °C per century. The orange curve shows the contribution just from breakpoint adjustments.
The blue curve in Fig. 114.7 above is the difference in MTA values between adjusted (Fig. 114.5) and unadjusted data (Fig. 114.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Both adjustments are negligible after 1920, but before 1920 they add an additional warming of up to 0.8°C. But as Fig. 114.3 shows, the MTA data before 1920 is based on temperature data from at most 21 stations. So how reliable is the raw data before 1920? And how reliable are the BE adjustments before 1920?
The case in favour of believing the raw unadjusted data before 1920 is two-fold: that it is the real data, and that it follows the same trend as the raw data after 1920. In addition, the case against the adjusted data is that the adjustments are so large relative to any made after 1920. In reality, it is difficult to say categorically which is the more reliable, but the unadjusted data does also correlate with raw unadjusted data from elsewhere, such as southern Africa (see Posts 37, 77, 78, 79) and the Southern Hemisphere (see Post 64).
Summary
According to the raw unadjusted temperature data, the climate of West Africa cooled until 1975 and then warmed by about 0.75°C (see Fig. 114.2). The net warming since 1900 is only about 0.5°C.
Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of West Africa has warmed by over 1.0°C (see Fig. 114.5).
The data before 1920 is based on a small sample (only 21 stations) and so could be considered highly uncertain.
Acronyms
BE = Berkeley Earth.
MRT = monthly reference temperature (see Post 47).
MTA = mean temperature anomaly.
Link to list of all stations in West Africa and their raw data files.
