Mississippi has even better temperature data than Louisiana which I examined in Post 97. It has 46 long stations with over 1200 months of data and another 60 medium stations with over 480 months of data. Of these, 98 stations in total have over 600 months of data. The graph in Fig. 99.1 below shows the result of averaging the monthly temperature anomalies for the 100 longest station records in Mississippi. It shows that there was no warming in the 120 years before 2010. In fact the state's climate cooled by 0.5°C.
Fig. 99.1: The mean temperature change for Mississippi relative to the 1951-1980 monthly averages. The best fit is applied to the monthly mean data from 1891 to 2010 and has a negative gradient of -0.42 ± 0.13 °C per century.
The scatter plot graph in Fig. 99.1 of mean temperature anomalies (MTA) is the result of averaging the temperature anomalies from 100 separate temperature records. These anomalies were calculated by subtracting the raw monthly mean temperature of each station from its monthly reference temperature (MRT) using the method outlined in Post 47. In this case the MRTs for each station were determined for the period 1951-1980 as this corresponded to the thirty year period with the most station data as shown in Fig. 99.2 below.
Fig. 99.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for Mississippi in Fig. 99.1.
The data in Fig. 99.2 indicates that after 1900 the MTA in Fig. 99.1 is the result of averaging over more than forty separate station records, and peaks at over ninety. However, before 1890 this number drops to less than fifteen, which as I have noted before in Post 57 seems to be the minimum needed in order to obtain reliable values for the MTA.
The locations of the stations with the 100 longest temperature records are shown on the map in Fig. 99.3 above. This shows them to be evenly distributed across the state. This means that advanced numerical techniques of homogenization, gridding and Kriging used by Berkeley Earth (BE) and others are probably unnecessary for determining an accurate MTA.
Fig. 99.4: Temperature trends for Mississippi based on Berkeley Earth adjusted data from the 100 longest station data records. The best fit linear trend line (in red) is for the period 1891-2010 and has a gradient of +0.54 ± 0.04°C/century.
This is confirmed by a comparison of the data in Fig. 99.4 above and Fig. 99.5 below. The MTA in Fig. 99.4 above was determined by averaging the Berkeley Earth (BE) adjusted anomalies. These values are found in the data files for each station on the Berkeley Earth website along with the raw data that I use. This allows me to do two things: (i) compare the MTA based on raw data (Fig. 99.1) with the same based on BE adjusted data (Fig. 99.4); (ii) assess the accuracy of the averaging process by comparing the average without homogenization, gridding and Kriging (Fig. 99.4) with the BE average that uses all three (Fig. 99.5).
A comparison of Fig. 99.1 and Fig. 99.4 indicates that the BE adjustments have turned a strong negative (cooling) temperature trend into a significant warming trend. Yet this cannot be due to any fault in the averaging process that I use because that yields virtually identical results for the BE adjusted MTA in Fig. 99.4 as the official published version in Fig. 99.5. So it is the BE temperature adjustments, and only those adjustments that are making the difference.
Fig. 99.5: The temperature trend for Mississippi since 1750 according to Berkeley Earth.
Finally it is possible to determine the magnitude of the BE adjustments by subtracting the unadjusted data in Fig. 99.1 from the adjusted data in Fig. 99.4. The result is shown in Fig. 99.6 below and indicates that the BE adjustments add over 1.1°C of warming from 1890 onwards, and possibly as much as 1.4°C. This is a huge amount of warming to add to a trend that is otherwise generally cooling.
Fig. 99.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 99.4 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 for the period 1891-2010 has a positive gradient of +0.949 ± 0.008 °C per century. The orange curve shows the contribution just from breakpoint adjustments.
Finally, we need to consider the discontinuity in the MTA data in 1957. This was discussed for Louisiana and Texas in Post 98 and it occurs again for Mississippi as shown by the arrow and red line in Fig. 99.7 below.
The size of this discontinuity can be estimated by averaging the anomalies for a thirty year period either side of it. If the periods chosen are 1921-1950 and 1961-1990, then the difference in the mean anomalies for those periods is 0.81°C. This is the magnitude of the discontinuity (red curve) in Fig. 99.7.
Fig. 99.7: The mean temperature change for Mississippi relative to the 1951-1980 monthly averages. The arrow and red line indicate the position and size of the data discontinuity.
If we then correct for this discontinuity by adjusting upwards all the anomalies after 1957 by 0.81°C we get the MTA time series shown in Fig. 99.8 below. The best fit to the data after 1891 now has an upward slope indicating a warming of about 0.7°C in total. This is now almost identical to the trend in the BE adjusted data in Fig. 99.4, but is this correct? After all, the BE adjusted data in Fig. 99.4 shows the same discontinuity in 1957 as the unadjusted data in Fig. 99.1. In other words, the BE adjustments have failed to erase the most striking irregularity in the data.
Fig. 99.8: The mean temperature change for Mississippi after breakpoint adjustment in 1957. The best fit is applied to the monthly mean data from 1891 to 2010 and has a positive gradient of +0.58 ± 0.13 °C per century.
A further complication can be found in the data in 1919. Here there is also what appears to be an abrupt discontinuity, but this one adds warming of approximately 0.43°C. It therefore partially cancels the effect of the discontinuity in 1957. The result is that the warming trend is dramatically reduced to a mere 0.19°C per century, or about 0.23 °C in total from 1891 to 2010.
Fig. 99.9: The mean temperature change for Mississippi after breakpoint adjustments to data before 1919 of 0.43°C and after 1957 of 0.81°C. The best fit is applied to the monthly mean data from 1891 to 2010 and has a positive gradient of +0.19 ± 0.12 °C per century.
What this highlights is the difficulty in using breakpoint adjustments. In the two examples shown above the data discontinuities in 1919 and 1957 can be seen in many of the original data sets of the 100 individual station records used to determine the MTA. So these different stations act as corroboration for each other, and this also explains why the discontinuities are so pronounced. But that does not explain the cause of the discontinuities, which could be natural and so should just be accepted, or could be due to a systematic and simultaneous change in the data gathering methods which then needs to be adjusted for.
The Berkeley Earth breakpoint algorithm, on the other hand, instead identifies numerous potential breakpoints in most station records in addition to the ones I have outlined. These additional breakpoints are often unique to each station record and so have no way of being corroborated. The other issue here is whether the Berkeley Earth identification algorithm actually works. But even if it does, regression towards the mean would render many of these adjustments redundant as I demonstrated in Post 57 and Post 67. Only the coincident ones would then remain after averaging such as those at 1919 and 1957.
Conclusions
Analysis of the raw temperature data suggests that the climate of Mississippi has cooled by almost 0.5°C since 1890 (see Fig. 99.1).
This is totally at odds with the result presented by Berkeley Earth (Fig. 99.4 and Fig. 99.5) which claims that the climate warmed by over 0.6°C over the same period. This is still less than the mean temperature rise for land-based stations (see Fig. 98.1 in Post 98).
Two major discontinuities in the mean temperature anomaly (MTA) data remain unexplained. Compensating for either or both would change the temperature trend of the MTA from strong cooling in Fig. 99.1 to either moderate (Fig. 99.8) or slight (Fig. 99.9) warming.
Acronyms
BE = Berkeley Earth.
MRT = monthly reference temperature (see Post 47).
MTA = mean temperature anomaly.
Link to list of all stations and their raw data files.
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