Pattern effect warming has been largely absent in the historic surface temperature record. This is due to the impacts of anthropogenic aerosols on Pacific Ocean surface temperatures and their associated surface wind patterns. This 'third aerosol forcing parameter' has not yet been strongly identified by climate sciences even though it has suppressed surface warming through the modern period and is only now being evidenced as SE Asia continues to reduce high temperature stack emissions of SOx. By (around) 2029 the Pacific Decadal Oscillation will exist in a nearly permanent positive state, causing much higher regional forcing and a doubling of the historic GMST decadal warming trend.
The latest generation of climate models (CMIP6) suggests that anthropogenic warming will pass 2C warming relative to preindustrial levels between 2038 and 2072 in a current-policy-type scenario (SSP2-4.5). However, ENSO-driven natural variability results in +/- 0.2C annual temperatures compared to the long-term average, so it would be possible to see a single year above 2C even if the human contribution to warming will not exceed 2C for another decade or so. That said, I am still willing to take up this bet, in part because I'd win the wager under most individual CMIP6 model projections, and because I'm hopeful that the world will reduce emissions to a greater extent over the next few decades than under a SSP2-4.5 scenario.
John Mitchell will win the bet if the Berkeley Earth Global Average Temperature Anomaly with Sea Ice Temperature Inferred from Air Temperatures dataset reports an annual (January through December) temperature anomaly of over 2C relative to the 1850-1899 baseline period on or before the published value corresponding to the calendar year 2037. Zeke Hausfather will win the bet if an annual temperature anomaly of 2C relative to the 1850-1899 baseline period does not occur before the published calendar year 2037 value. The latest version of the Berkeley Earth global temperature dataset will be used to adjudicate this bet. The temperature anomaly with respect to 1850-1899 will be calculated by subtracting the mean of monthly temperature anomalies over that period (January 1850-December 1899) from the record reported by Berkeley Earth. In the case in which the Berkeley Earth product is discontinued, the Hadley Centre/UEA HadCRUT dataset will be used.