The background for my forecast:
In recent years, CO2 has been increasing by around 2.2 ppm, per year. Barring anything unusual, we would therefore expect next year’s September value to be around 399.3 ppm, just barely below 400 ppm, and we’d expect the lowest daily minima to be around 398 ppm or so. But we seem now to be on the verge of the largest El Niño event since 1997. This is significant because CO2 tends to rise much faster during and just following El Niño events. From September 1997 to September 1998, for example, CO2 rose by a whopping 3.7 ppm. If this El Niño is comparable, the rise from September 2015 to September 2016 could easily be 4.4 ppm, allowing for an El Niño boost and allowing that fossil-fuel emissions rates globally are larger now than in 1998. Taking these factors into account, a reasonable forecast for next year’s September minimum is around 402 ppm, with the lowest daily minima also over 400 ppm.