76. Indian Ocean - temperature trend WARMING 1°C

There are sixteen weather stations in the Indian Ocean region with over 480 months of data, most of which are situated on the islands surrounding Madagascar (see Fig. 76.1 below). The exceptions are the stations on Diego Garcia (Berkeley Earth ID: 173559) which is in the eastern half of the ocean, and Minicoy (Berkeley Earth ID: 155480) which is off the coast of India north of the Maldives (Berkeley Earth ID: 156700). The rest are located on, or near, the islands of Comoros, Mayotte, Seychelles, Agalega, Mauritius, Réunion, Ile Tromelin, Ile Juan de Nova and Ile Europa.

Of all these stations, only two have temperature records with more than 1000 months of data (Seychelles Airport and Minicoy) while thirteen have over 600 months of data. Overall, twenty-two stations have at least 300 months of data.

 

Fig. 76.1: The (approximate) locations of the long and medium temperature records in the Indian Ocean. Those stations with a high warming trend between 1911 and 2010 are marked in red while those with a cooling or stable trend (Comoros) are marked in blue. Those denoted with squares are long stations with over 1200 months of data, while diamonds denote medium stations with over 480 months of data.

 

Summing the anomalies from these stations gives the mean temperature trend shown in Fig. 76.2 below. The anomalies for each station where determined relative to the monthly reference temperature (MRT) averages for the period 1941-1970 using the method outlined in Post 47. Only stations with twelve years of data in this MRT interval are included in the final calculation of the regional mean temperature trend. This means that seventeen stations were included in the calculation, including fifteen long and medium stations. The one medium station to be excluded was that at Vacoas in Mauritius (Berkeley Earth ID: 156749) which had insufficient data in the MRT interval.

 

Fig. 76.2: The temperature trend for the Indian Ocean based on an average of anomalies from stations with over 300 months of data. The best fit is applied to the monthly mean data from 1916 to 1975 and has a positive gradient of +0.33 ± 0.07 °C per century. The monthly temperature changes are defined relative to the 1941-1970 monthly averages.

 

It can be seen that the temperature trend in Fig. 76.2 has two distinct intervals. Before 1975 there is a gentle warming of about 0.33°C per century. This gives a total warming of about 0.2°C since 1920. After 1975 the warming increases significantly and this adds a further 0.8°C to the total. This suggests that the total warming since 1900 has been about 1°C.

 

 

Fig. 76.3: The number of station records included each month in the mean temperature trend for Bangladesh in Fig. 76.2.

The reliability of this trend can be estimated by considering the number of stations included in the mean. Previous analysis for central Europe (see Post 57) suggests that between fifteen and thirty stations are needed for the mean trend to be truly representative of the actual temperature trend. This is because the individual temperature records are prone to measurement errors. However, many of these errors from different stations cancel when averaged in sufficiently large numbers (Regression Towards The Mean).

The temperature trend in Fig. 76.2 is the result of averaging up to seventeen different records, but this is only true for data from the 1950s (see Fig. 76.3 above). For data after 1960 the typical number of records in the average is only about twelve, while before 1950 its is typically less than five. This suggests that the trend after 1960 is much more reliable than that before 1950. So we can be reasonably confident in the magnitude of the warming post-1960, but much less so for the data before 1950.

 

Fig. 76.4: Temperature trends for the Indian Ocean based on the average of anomalies for all long and medium stations using Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1911-2010 and has a gradient of +1.03 ± 0.02°C/century.

 

Finally, if we compare the result in Fig. 76.2 with the equivalent Berkeley Earth result using their adjusted data we find only a small difference as Fig. 76.4 illustrates. In fact the only significant difference is the warming seen before 1950 which is twice as large in Fig. 76.4 as it is in Fig. 76.2. But given the scarcity and hence unreliability of this data, it is impossible to ascertain which, if any, of the two results is the more accurate.

 

Summary

The islands of the Indian Ocean have warmed by about 0.8°C since 1960.

There is some evidence of a slight warming before 1950, but with insufficient data available for this period, it is difficult to quantify this with any certainty.