Resolves YES if in NASA's monthly GISTEMP data (Land+Ocean column) any entry for 2023 has the (strictly) highest monthly temperature anomaly, and NO otherwise. The current record is 1.37° (for "2016.13", which I think means Feb 2016). If the format of the GISTEMP data changes, I'll resolve according to whatever part of the GISTEMP data best corresponds to the meaning of the question title. If monthly GISTEMP data is unavailable at resolution, I'll look for another dataset of monthly global temperature anomalies.

For the first week that this market is open, I reserve the right to make whatever changes to the resolution criteria I feel best capture the spirit of the question title. I won't bet on this market during that week, but may bet after that week.

Edit 04-25: To clarify, one of the 2023 months has to be the highest number in the entire Land+Ocean column, not just the highest out of all the months with that name.

Edit 08-15: If multiple 2023 months have the same rounded anomaly that is higher than any pre-2023 anomaly, this still resolves YES.