  The Earth system is accumulating energy due to human-induced activities. More
than 90 percent of this energy has been stored in the ocean as heat since 1970,
with about 64 percent of that in the upper 700 m. Differences in upper ocean
heat content anomaly (OHCA) estimates, however, exist. Here, we evaluate spread
in upper OHCA estimates arising from choices in instrumental bias corrections
and mapping methods, in addition to the effect of using a common ocean mask.
The same dataset was mapped by six research groups for 1970 to 2008, with six
instrumental bias corrections applied to expendable bathythermograph (XBT)
data. We find that use of a common ocean mask may impact estimation of global
OHCA by +- 13 percent. Uncertainty due to mapping method dominates over XBT
bias correction at a global scale and is largest in the Indian Ocean and in the
eddy-rich and frontal regions of all basins. Uncertainty due to XBT bias
correction is largest in the Pacific Ocean within 30N to 30S. In both mapping
and XBT cases, spread is higher since the 1990s. Important differences in
spatial trends among mapping methods are found in the well-observed Northwest
Atlantic and the poorly-observed Southern Ocean. Although our results cannot
identify the best mapping or bias correction schemes, they identify where and
when greater uncertainties exist, and so where further refinements may yield
the largest improvements. Our results highlight the need for a future
international coordination to evaluate performance of existing mapping methods.
