115: Chad - temperature trends WARMING 2°C after 1980

Any analysis of the climate of Chad is complicated by two factors: the first is the lack of data; the second is the gap in the temperature data from 1979 to 1986 that coincides with a sudden jump in temperatures. The longest temperature record for Chad has less than 800 months of data before 2014 and only extends back to 1941. In total there are only seven medium stations with over 480 months of data and a further five stations with over 300 months of data. The locations of these stations are shown on the map in Fig. 115.1 below. All but one of these twelve stations are in the southern half of the country.

Fig. 115.1: The (approximate) locations of the twelve longest weather station records in Chad. 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.

In order to quantify the changes to the climate of Chad since 1941 the temperature anomalies for each of the twelve stations shown in Fig. 115.1 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. 115.2 below and clearly shows that temperatures declined slowly before 1980 by about 0.4°C in total, and rose more rapidly by up to 2°C thereafter. However, the large gap in data from 1979 to 1987, together with the abrupt jump in temperatures after 1987, together raise questions over the reliability of the post-1987 data.

Fig. 115.2: The mean temperature change for Chad relative to the 1951-1980 monthly averages. The best fit is applied to the monthly mean data from 1941 to 1975 and has a negative gradient of -1.04 ± 0.38 °C per century.

The process of determining the MTA in Fig. 115.2 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. 115.2 each month is indicated in Fig. 115.3 below. The peak in the frequency between 1950 and 1980 suggests that the 1951-1980 interval was indeed the most appropriate to use for the MRTs.

Fig. 115.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Chad in Fig. 115.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 115.4 below. This again was determined by averaging each monthly anomaly from all the available stations and suggests that the climate was stable before 1980 and only warmed by about 1°C thereafter. So there is less cooling before 1980 and less warming after. It also appears that the data after 1987 has been offset downwards by about 0.5°C in order to eliminate some of the temperature rise seen in Fig. 115.2.

Fig. 115.4: Temperature trends for Chad based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1941-1975 and has a very slight negative gradient of -0.04 ± 0.16°C/century.

If we compare the curves in Fig. 115.4 with the published Berkeley Earth (BE) version in Fig. 115.5 below we see that there is good agreement between the two sets of data at least as far back as 1950. This indicates that the simple averaging of anomalies used to generate the BE MTA in Fig. 115.4 gives similar results to the more complex gridding method used by Berkeley Earth in Fig. 115.5. The official BE temperature trend also claims to know the temperature trend before 1940 and as far back as 1870, and also for all of the 1980s, even though there is no actual data in Chad for either time period.

Fig. 115.5: The temperature trend for Chad since 1850 according to Berkeley Earth.

The differences between the MTA in Fig. 115.2 and the BE version using adjusted data in Fig. 115.4  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. 115.2 and Fig. 115.4. The magnitudes of these adjustments are shown graphically in Fig. 115.6 below. The blue curve is the difference in MTA values between adjusted (Fig. 115.4) and unadjusted data (Fig. 115.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. It can be seen that the main adjustment is a vertical offset of data after 1987.

Fig. 115.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 115.4 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 has a negative gradient of -1.17 ± 0.04 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Summary

According to the raw unadjusted temperature data, the climate of Chad cooled by 0.4°C until 1975 and then warmed by about 2°C (see Fig. 115.2). However, the gap in data from 1979-1987 and the subsequent jump in temperatures thereafter may mean the rise post-1987 is only about 1.4°C and the net rise only 1°C.

Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of Chad has warmed by about 1.0°C (see Fig. 115.4). This is possibly the same as the raw data overall, but the pattern over time is different.


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Link to list of all stations in Chad and their raw data files.