Robust warming of the global upper ocean

A large (~1023 J) multi-decadal globally averaged warming signal in the upper 300 m of the world\textquoterights oceans was reported roughly a decade ago1 and is attributed to warming associated with anthropogenic greenhouse gases2, 3. The majority of the Earth\textquoterights total energy uptake during recent decades has occurred in the upper ocean3, but the underlying uncertainties in ocean warming are unclear, limiting our ability to assess closure of sea-level budgets4, 5, 6, 7, the global radiation imbalance8 and climate models5. For example, several teams have recently produced different multi-year estimates of the annually averaged global integral of upper-ocean heat content anomalies (hereafter OHCA curves) or, equivalently, the thermosteric sea-level rise5, 9, 10, 11, 12, 13, 14, 15, 16. Patterns of interannual variability, in particular, differ among methods. Here we examine several sources of uncertainty that contribute to differences among OHCA curves from 1993 to 2008, focusing on the difficulties of correcting biases in expendable bathythermograph (XBT) data. XBT data constitute the majority of the in situ measurements of upper-ocean heat content from 1967 to 2002, and we find that the uncertainty due to choice of XBT bias correction dominates among-method variability in OHCA curves during our 1993\textendash2008 study period. Accounting for multiple sources of uncertainty, a composite of several OHCA curves using different XBT bias corrections still yields a statistically significant linear warming trend for 1993\textendash2008 of 0.64 W m-2 (calculated for the Earth\textquoterights entire surface area), with a 90-per-cent confidence interval of 0.53\textendash0.75 W m-2.