The mean monthly Sea Surface Temperature (SST) in each box was computed (1) using the SST values measured concurrently with seawater pCO2, and (2) the same interpolation algorithm. Our SST values are listed as MOD_SST in the data table, and are compared to the climatological SST compiled by Shea, et al. (A global monthly sea surface temperature climatology, Jour. Climate, 5, 987-1001(1992)). These values were used to estimate possible errors in our global pCO2 values attributable to undersampling and our interpolation method. There are a total of 12 months, 1,759 individual 4° X 5° boxes for each month for a total of 21,108 boxes. Graphs of these results follow, including monthly maps of the difference.

(1) Overall, the mean difference (Interpolated - Climatological) was 0.091 ± 1.379, (N = 21,108).

(2) In the boxes where there was at least one observation, the mean difference was 0.036 ± 1.019 (N = 10,648).

(3) In the boxes where there were no observations, the mean difference was 0.146 ± 1.666 ( N = 10,460).