102. Georgia (US) - temperature trends COOLING

In a number of previous posts I have analysed the temperature data for all the US states along the Gulf of Mexico. None has experienced any global warming and the same is true for Georgia. In fact over the last 100 years the climate of Georgia has cooled by about 0.7°C as shown by the mean temperature anomaly (MTA) data for the state illustrated in Fig. 102.1 below.

Fig. 102.1: The mean temperature change for Georgia relative to the 1951-1980 monthly averages. The best fit is applied to the monthly mean data from 1911 to 2010 and has a negative gradient of -0.76 ± 0.16 °C per century.

The MTA in Fig. 102.1 was calculated by averaging the temperature anomalies from the 100 longest temperature records for the state. All these records had over 480 months of temperature data before the end of 2013 and 37 were long stations that each had more than 1200 months of data in total. For a full list of stations see here.

The anomalies for each station 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 within the MRT interval then its anomalies were included in the MTA calculation. In total 98 stations were included with only two being excluded for lack of data between 1951 and 1980. These were Greensboro (Berkeley Earth ID: 28632) and Columbus (Berkeley Earth ID: 28632).

The total number of stations included in the MTA in Fig. 102.1 each month is indicated in Fig. 102.2 below. The broad peak from 1955 to 1990 suggests that the 1951-1980 interval was probably the most appropriate although a 1961-1990 interval could have been equally optimal.

Fig. 102.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for Georgia in Fig. 102.1.

The locations of the one hundred stations is shown in the map in Fig. 102.3 below. This appears to show that the geographical spread is fairly uniform and in turn suggests that a simple average of all the anomalies should yield an accurate temperature trend for the state as a whole.

Fig. 102.3: The (approximate) locations of the 100 longest weather station records in Georgia. 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.

Next I compare the MTA based on raw unadjusted data with the MTA result based on data that has been adjusted by Berkeley Earth (BE). The result is shown in Fig. 102.4 below.

Fig. 102.4: Temperature trends for Georgia 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 1911-2010 and has a gradient of +0.35 ± 0.05°C/century.

Comparing the curves in Fig. 102.4 with the published Berkeley Earth (BE) version in Fig. 102.5 below indicates remarkably good agreement. This indicates that the 100 longest records are sufficient to determine the MTA and that simple averaging of anomalies is also highly effective and accurate.

Fig. 102.5: The temperature trend for Georgia since 1750 according to Berkeley Earth.

The differences between the MTA in Fig. 102.1 and the BE versions in Fig. 102.4 and Fig. 102.5 are 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 in Fig. 102.1 and Fig. 102.4. The magnitudes of these adjustments are shown graphically in Fig. 102.6 below. The blue curve is the difference in MTA between adjusted (Fig. 102.4) and unadjusted data (Fig. 102.1), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Both are considerable with the former leading to an additional warming since 1935 of over 1°C.

Fig. 102.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 102.4 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 1911-2010 has a positive gradient of +1.018 ± 0.011 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Finally there is the question of the negative discontinuity in the temperature data in 1957 that was observed in Texas (Post 98), Louisiana (Post 98), Mississippi (Post 99) and Alabama (Post 101). It is again present in the data for Georgia and amounts to a temperature jump of 0.84°C. It can also be seen even more starkly in the BE adjusted data in Fig. 102.4. Correcting for this jump yields the MTA time series shown in Fig. 102.7 below.

Fig. 102.7: The mean temperature change for Georgia after breakpoint adjustment in 1957. The best fit is applied to the monthly mean data from 1911 to 2010 and has a positive gradient of +0.48 ± 0.16 °C per century.

The net result of making this correction is that the temperature trend since 1910 changes from a negative value of -0.76°C per century in Fig. 102.1 to a positive one of 0.48°C per century. Yet the origin of this discontinuity is still unclear. So the validity of this correction is therefore not known either.

Summary 

According to the raw unadjusted temperature data, over the past century the climate of Georgia has cooled by around 0.76°C (see Fig. 102.1).

Over the same period adjusted temperature data from Berkeley Earth claims to show that the climate of Georgia has warmed by over 1°C (see Fig. 102.4 and Fig. 102.5).

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.