“Increasing greenhouse gases in the atmosphere cause heat uptake in the Earth system (Section 1.2) and as reported since 1970, there is high confidence that the majority (more than 90%) of the extra thermal energy in the Earth’s system is stored in the global ocean (IPCC, 2013). Mean ocean surface temperature has increased since the 1970s at a rate of 0.11 (0.09–0.13)ºC per decade (high confidence), and forms part of a long-term warming of the surface ocean since the mid-19th century. The upper ocean (0–700 m, virtually certain) and intermediate ocean (700 to 2,000 m, likely) have warmed since the 1970s. Ocean heat uptake has continued unabated since AR5 (Sections 22.214.171.124.1, 5.2), increasing the risk of marine heat waves and other extreme events (Section 6.4). During the 21st century, ocean warming is projected to continue even if anthropogenic greenhouse gas emissions cease (Sections 1.3, 5.2).” 
“The ocean heat uptake efficiency is a measure (W m –2 °C –1) of the rate at which heat storage by the global ocean increases as global mean surface temperature rises. It is a useful parameter for climate change experiments in which the radiative forcing is changing monotonically, when it can be compared with the Climate Feedback Parameter to gauge the relative importance of climate response and ocean heat uptake in determining the rate of climate change. It can be estimated from such an experiment as the ratio of the rate of increase of ocean heat content to the global mean surface air temperature change.” 
 Source: Abram, N., J.-P. Gattuso, A. Prakash, L. Cheng, M.P. Chidichimo, S. Crate, H. Enomoto, M. Garschagen, N. Gruber, S. Harper, E. Holland, R.M. Kudela, J. Rice, K. Steffen, and K. von Schuckmann, 2019: Framing and Context of the Report. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. In press.
 Source: IPCC, 2013: Annex III: Glossary [Planton, S. (ed.)]. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
The above definition was taken from the IPCC report available at: