In a recent Yale CC article "Schmidt says that scientists still can’t explain the unexpected spike in temperatures. When I talked with him recently, he called the continuing confusion “a little embarrassing” for researchers."

A spike that lasts a year or more is not consistent with the secular, monotonic rise in temperature predicted by a GHG model. It could be a tipping point perhaps, but if the spike disappears, that does not make sense. So that leaves an El Nino or ocean-cycle cause, Hunga-Tonga response, or an aerosol change as has been suggested. No way is it a sunspot-related event IMO. As Gavin said, it's somewhat embarrassing not understanding the cause, but that's really in the context of not having a consensus to the cause of El Nino events in the first place. Or having an agreement of whether the AMO is an oscillation or not.

It's good that Gavin Schmidt is honest in his appraisal of the difficulty in pinning this all down.