Globally September 2017 was the fourth warmest September since reliable measurements began in 1880, 0.80C warmer than the 1951-1980 mean. Warmest Septembers were 2014 and 2016 at +0.87C, with 2015 third at 0.82C.
The 2017 January to September 9-month mean is second highest, behind 2016, compared to the first nine months of other years, as shown above.
If the temperature anomaly for the next three months (relative to the 1951-1980 mean) is less than 0.75C (i.e., if October-December 2017 is more than 0.33C cooler than October-December 2015) the annual 2017 temperature would fall below that of 2015, making 2017 the third warmest year. Even though a La Nina may be in the offing for this fall-winter, a cooling that large seems unlikely, so 2017 will probably be the second warmest year.