Indicator Assessment

Ocean heat content

Indicator Assessment

Prod-ID: IND-350-en

Also known as: CLIM 044

Published 01 Aug 2014 Last modified 11 May 2021

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Topics: Climate change adaptation Water and marine environment

This page was archived on 20 Dec 2016 with reason: Other (New version data-and-maps/indicators/ocean-heat-content-1/assessment was published)

The warming of the World Ocean accounts for approximately 93 % of the warming of the Earth system during the last six decades. Warming of the upper (0–700 m) ocean accounted for about 64% of the total heat uptake.
An increasing trend in the heat content in the uppermost 700 m depth of the World Ocean is evident over the last six decades. Recent observations show substantial warming also of the deeper ocean (between 700 m and 2 000 m depth and below 3000 m depth).

Further warming of the oceans is expected with projected climate change. The amount of warming is strongly dependent on the emissions scenario.

Change in global ocean heat content at different depths

Chart

Data sources:

Global Ocean Heat Content - World - Pentadal - 0-2000 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Pentadal - 0-700 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Yearly - 0-2000 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Yearly - 0-700 m provided by National Oceanic and Atmospheric Administration (NOAA)

Explore chart interactively

Table

Data sources:

Global Ocean Heat Content - World - Pentadal - 0-2000 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Pentadal - 0-700 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Yearly - 0-2000 m provided by National Oceanic and Atmospheric Administration (NOAA)
Global Ocean Heat Content - World - Yearly - 0-700 m provided by National Oceanic and Atmospheric Administration (NOAA)

Explore chart interactively

Past trends

The warming of the World Ocean accounts for approximately 93 % of the warming of the Earth during the last six decades [i]. Two thirds of the observed increase of global heat content has occurred in the upper 700 m of the ocean, with increases in the layers below 700 m depth accounting for the remaining one third. The strongest warming is found near the sea surface, which warmed more than 0.1°C per decade in the upper 75 m since 1971. The warming decreased to about 0.015°C per decade at 700 m depth [ii].

The heat content of the World Ocean has increased since around 1970 (Figure 1; small differences in the values for yearly and 5-year averages are due to the particular method used for spatial gap filling). The linear trend of the uppermost 700 m and 2 000 m layer over the time period 1955–2013 was 0.27 Wm-2 and 0.39 Wm-2 (per unit area of the World Ocean), respectively. It is likely that the ocean warmed between 700–2000 m from 1957 to 2013 and from 3000 m to the bottom from 1992 to 2005, while no significant trends in global average temperature were observed between 2000 and 3000 m depth from circa 1992 to 2005 [iii].

Several global ocean data assimilation products are available to compare observation-based estimates with independent reanalysis data. Global and basin-scale heat content warming trends in the upper 700 m of the ocean computed from a set of global ocean reanalyses fall within the range of the most recent observation-based estimates derived using different methods [iv].

Projections

All available ocean temperature projections suggest that the global ocean will continue to warm. The largest warming is projected for the top few hundred meters of the subtropical gyres, similar to the observed pattern of ocean temperature changes. Mixing and advection processes gradually transfer the additional heat to deeper levels.

The rate of increase of ocean heat content is approximately proportional to the global mean change in surface air temperature. Under the RCP4.5 scenario, half of the energy taken up by the ocean by the end of the 21^st century is in the uppermost 700 m, and 85% is in the uppermost 2000 m. Projected ocean warming varies considerably across forcing scenarios. Globally averaged surface warming ranges from about 1°C for RCP2.6 to more than 3°C for RCP8.5 during the 21^st century; warming at 1 km depth ranges from 0.5 °C for RCP2.6 to 1.5 °C for RCP 8.5 [v].

[i] J. Hansen et al., ‘Earth’s Energy Imbalance and Implications’,Atmospheric Chemistry and Physics 11, no. 24 (22 December 2011): 13421–49, doi:10.5194/acp-11-13421-2011; M. Rhein et al., ‘Observations: Ocean’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 3, http://www.climatechange2013.org/images/report/WG1AR5_Chapter03_FINAL.pdf.

[ii] Rhein et al., ‘Observations: Ocean’.

[iii] Levitus et al., ‘World Ocean Heat Content and Thermosteric Sea Level Change (0–2000 M), 1955–2010’; Sarah G. Purkey and Gregory C. Johnson, ‘Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets*’,Journal of Climate 23, no. 23 (December 2010): 6336–51, doi:10.1175/2010JCLI3682.1; Rhein et al., ‘Observations: Ocean’.

[iv] John M. Lyman et al., ‘Robust Warming of the Global Upper Ocean’,Nature 465, no. 7296 (May 2010): 334–37, doi:10.1038/nature09043; Simona Masina et al., ‘Global Ocean Re-Analyses for Climate Applications’,Dynamics of Atmospheres and Oceans 52, no. 1–2 (September 2011): 341–66, doi:10.1016/j.dynatmoce.2011.03.006; Rhein et al., ‘Observations: Ocean’.

[v] Karl E. Taylor, Ronald J. Stouffer, and Gerald A. Meehl, ‘An Overview of CMIP5 and the Experiment Design’,Bulletin of the American Meteorological Society 93, no. 4 (April 2012): 485–98, doi:10.1175/BAMS-D-11-00094.1; B. Kirtman et al., ‘Near-Term Climate Change: Projections and Predictability’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 11, http://www.climatechange2013.org/images/report/WG1AR5_Chapter11_FINAL.pdf; M. Collins et al., ‘Long-Term Climate Change: Projections, Commitments and Irreversibility’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 12, http://www.climatechange2013.org/images/report/WG1AR5_Chapter12_FINAL.pdf; J. A. Church et al., ‘Sea-Level Change’, inClimate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. T. F. Stocker et al. (Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press, 2013), Chapter 13, http://www.climatechange2013.org/images/report/WG1AR5_Chapter13_FINAL.pdf.

Supporting information

Indicator definition

Observed change in global ocean heat content

Units

Ocean heat content (Joule)

Rationale

Justification for indicator selection

The World Ocean is the dominant component of the Earth’s heat balance. Oceans cover roughly 72 % of the planet’s surface, and water has a heat uptake capacity that is around 20 times greater than that of the atmosphere. Most of the total warming caused by climate change is manifested in increased ocean heat content (OHC). Hence, a precise estimate of OHC is essential for understanding the role of oceans in past climate change, and for assessing future climate change. OHC is defined as the integrated temperature change times the density of sea water, times specific heat capacity from the surface down to the deep ocean. In other words, OHC is an anomaly calculated in comparison to a reference period. OHC is estimated based on temperature measurements or on reanalyses using a combination of models and observations.

Changes in heat content cause the ocean to expand or contract, thereby changing sea level regionally and globally. This thermosteric effect has contributed about one quarter to global sea-level rise since 1993.

Scientific references

IPCC, 2013. Climate Change: 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, 1535 pp.

Policy context and targets

Context description

In April 2013 the European Commission presented the EU Adaptation Strategy Package (http://ec.europa.eu/clima/policies/adaptation/what/documentation_en.htm). This package consists of the EU Strategy on adaptation to climate change /* COM/2013/0216 final */ and a number of supporting documents. One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which should occur through Bridging the knowledge gap and Further developing Climate-ADAPT as the ‘one-stop shop’ for adaptation information in Europe. Further objectives include Promoting action by Member States and Climate-proofing EU action: promoting adaptation in key vulnerable sectors. Many EU Member States have already taken action, such as by adopting national adaptation strategies, and several have also prepared action plans on climate change adaptation.

The European Commission and the European Environment Agency have developed the European Climate Adaptation Platform (Climate-ADAPT, http://climate-adapt.eea.europa.eu/) to share knowledge on observed and projected climate change and its impacts on environmental and social systems and on human health; on relevant research; on EU, national and subnational adaptation strategies and plans; and on adaptation case studies.

Targets

No targets have been specified.

Methodology

Methodology for indicator calculation

Ocean heat content is defined as the integrated temperature change times the density of seawater, times specific heat capacity from the surface down to the deep ocean. It is calculated here base on observations from the upper 700 metres of ocean water.

The warming of the world ocean since 1955 is estimated using different kinds of observational data: historical data not previously available, additional modern data, correcting for instrumental biases of bathythermograph data, and correcting or excluding some Argo float data.

Methodology for gap filling

Not applicable

Methodology references

Levitus et al. 2012: World ocean heat content and thermosteric sea level change (0–2000 m), 1955–2010 S. Levitus, J. I. Antonov, T. P. Boyer, O. K. Baranova, H. E. Garcia, R. A. Locarnini, A. V. Mishonov, J. R. Reagan, D. Seidov, E. S. Yarosh and M. M. Zweng (2012): World ocean heat content and thermosteric sea level change (0–2000 m), 1955–2010. Geophysical Research Letters 39(10). doi:10.1029/2012GL051106

Uncertainties

Methodology uncertainty

Not applicable

Data sets uncertainty

Ocean temperature data are sparse in the polar and subpolar regions of the world. In general, however, changes related to the physical and chemical marine environment are better documented than biological changes because links between cause and effect are better understood and often time series of observations are longer. For example, systematic observations of both sea-level and sea surface temperature were started around 1880 and are today complemented by observations from space that have high resolution in time and geographical coverage and by Argo floats that also automatically measure temperature and salinity below the ocean surface.

Further information on uncertainties is provided in Section 1.7 of the EEA report on Climate change, impacts, and vulnerability in Europe 2012 (http://www.eea.europa.eu/publications/climate-impacts-and-vulnerability-2012/)