Second hottest October on record, just above October 2016

Summary: October 2017 was the second hottest October on record. The 12 month period to October 2017 was the second hottest November to October period on record.

According to GISS NASA, the average global surface temperature anomaly for October was 0.90 °C, which is 0.18 °C less than the October 2015. October 2017 was just 0.01 °C hotter than the next hottest October in 2016.

Below is a chart of the average of 12 months to October each year. The 12 months to October 2017 averaged 0.90 °C above the 1951-1980 mean, which was 0.13 °C cooler than the 12 months to October 2016.

This makes it the second hottest September to October 12 month period on record.

Figure 1 | Global mean surface temperature anomaly for the 12 months to October each year. The base period is 1951-1980. Data source: GISS NASA

Next is a chart of the month of October only. This October was 0.90 °C above the 1951-1980 average and was the second hottest October on record. It was 0.18 °C cooler than October 2016, which was 1.08 °C above the 51-80 mean. Hover over the chart to see the anomaly in any October:

Figure 2 | Global mean surface temperature anomaly for the the month of October only. The base period is 1951-1980. Data source: GISS NASA

ENSO year comparisons

In the chart below you can see the global mean temperature trend by month. It shows the strongest El Niño years since 1950, which were followed by a La Nina. I've included the 2015-17 period for comparison. The BoM ENSO update is now showing La Nina watch. It's not made the global temperature drop yet, however.

Of the seven very strong, strong and strong to moderate El Ninos since 1950, there were only three that were followed by a La Nina (not including 2015-17, when there was no La Nina).

The chart below spans a three year period. That is, for the 2015-16 El Niño and subsequent, it goes from January 2015 to December 2017, or would if the data allowed. (For a more detailed explanation see the HW articles: El Niño to La Niña years with more detail here.)

Figure 4 | Global mean surface temperature for strong or moderate/strong El Nino years that were followed by a La Nina. Also includes the 2015/16 El Nino for comparison. Data source: GISS NASA

Year to date chart

For the record, here is the year to date progressive chart. You need to understand what it is to make sense of it. The chart below shows the average temperature for the year at each point on each separate line on the chart. The topmost line is last year (2016). At January, the point is just the anomaly for January. At February, the point is the average anomaly for January and February. At October, it's the average of January to October inclusive - all the way to December, which is the average for the whole year.

So the 2017 year shows that the average for the period January to October is 0.91 °C. This is 0.11 °C lower than the average for the year was this time last year at the end of the massive El Nino. The average over the entire 2016 year is 0.99 °C (the point marked for December on the 2016 line) so by now it's virtually impossible for this year to be another hottest year.

Figure 6 | Progressive year to date global mean surface temperature anomaly. The base period is 1951-1980. Data source: GISS NASA

The anomalies for the rest of the year would have to average more than 1.4 °C for 2017 to be hotter overall than last year. That won't happen. If the anomalies have an average of 0.67 °C or more then 2017 will end up hotter than 2015. In that case 2017 will be the second hottest year on record. That's quite likely even with a La Nina.