Indicator Assessment

Ocean heat content

Indicator Assessment

Prod-ID: IND-350-en

Also known as: CLIM 044

Published 20 Dec 2016 Last modified 04 Oct 2021

10 min read

Topics: Climate change adaptation Water and marine environment

This page was archived on 04 Oct 2021 with reason: No more updates will be done

The warming of the oceans has accounted for approximately 93 % of the warming of the Earth since the 1950s. Warming of the upper (0–700 m) ocean accounted for about 64 % of the total heat uptake.
A trend for increasing heat content in the upper ocean has become evident since the 1950s. Recent observations also show substantial warming of the deeper ocean (between depths of 700 and 2 000 m and below 3 000 m).
Further warming of the oceans is expected with the projected climate change. The amount of warming is strongly dependent on the emissions scenario.

This indicator has been archived and will no longer be updated.

Information on the development of ocean heat content globally and in the Mediterranean Sea is available from the Ocean Monitoring Indicators "Global Ocean Heat Content (0-700m)" and "Mediterranean Sea Heat Content (0-700m)" maintained by the Copernicus Marine Service:
https://marine.copernicus.eu/access-data/ocean-monitoring-indicators?category=105

Time series of global ocean heat content at different depths

Chart

Data sources:

Global Ocean Heat and Salt Content provided by National Oceanic and Atmospheric Administration (NOAA)

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Past trends

The warming of the oceans has accounted for approximately 93 % of the warming of the Earth since the 1950s [i]. Ocean heat content (OHC) has increased since around 1970 (Figure 1). Differences in the values for yearly and five-yearly averages are the result of the particular method used for spatial gap filling. The linear warming trends of the uppermost 700 m of the ocean and the 700–2 000 m layer over the time period 1955–2013 were 0.27 W/m² and 0.39 W/m² (per unit area of the ocean), respectively. It is likely that the ocean warmed between 700 and 2 000 m from 1957 to 2014 and between 3 000 m and the bottom of the ocean from 1992 to 2005, while trends in ocean temperature between depths of 2 000 and 3 000 m were not statistically significant [ii].

Two-thirds of the observed increase in OHC has occurred in the upper 700 m of the ocean, with increases in the layers below a depth of 700 m accounting for the remaining third. The strongest warming is found near the sea surface, with the upper 75 m having warmed by more than 0.1 °C per decade since 1971. It has been estimated that heat uptake has doubled in recent decades [iii]. At a depth of 700 m, the warming decreases to about 0.015 °C per decade [iv]. Recently, it has been determined that past increases in OHC have been substantially underestimated because of poor sampling of the Southern Hemisphere and limitations of the analysis methods [v]. These concerns have not yet been considered in the datasets presented here.

Projections

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

The rate of increase of OHC is approximately proportional to the global mean change in surface air temperature. Under the low-to-medium (RCP4.5) emissions scenario, half of the energy taken up by the ocean by the end of the 21st century will be by the uppermost 700 m, and 85 % will be by the uppermost 2 000 m. Projected ocean warming varies considerably across forcing scenarios. Globally averaged projected surface warming ranges from about 1 °C for RCP2.6 to more than 3 °C for RCP8.5 during the 21st century, and at a depth of 1 000 m ranges from 0.5 °C for RCP2.6 to 1.5 °C for RCP8.5 [vi].

[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; New York: Cambridge University Press, 2013), 255–316, http://www.climatechange2013.org/images/report/WG1AR5_Chapter03_FINAL.pdf; E. L. Howes et al.,The Oceans 2015 Initiative, Part I. An Updated Synthesis of the Observed Impacts of Climate Change on the Physical and Biological Processes in the Oceans, No 02/15 March (Paris: IDDRI, 2015).

[ii] 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; S. Levitus et al., ‘World Ocean Heat Content and Thermosteric Sea Level Change (0–2000 M), 1955–2010’,Geophysical Research Letters 39 (17 May 2012): L10603, doi:10.1029/2012GL051106; J. P. Abraham et al., ‘A Review of Global Ocean Temperature Observations: Implications for Ocean Heat Content Estimates and Climate Change: Review of Ocean Observations’,Reviews of Geophysics 51, no. 3 (September 2013): 450–83, doi:10.1002/rog.20022; Rhein et al., ‘Observations: Ocean’.

[iii] Peter J. Gleckler et al., ‘Industrial-Era Global Ocean Heat Uptake Doubles in Recent Decades’,Nature Climate Change 6, no. 4 (18 January 2016): 394–98, doi:10.1038/nclimate2915.

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

[v] Paul J. Durack et al., ‘Quantifying Underestimates of Long-Term Upper-Ocean Warming’,Nature Climate Change 4, no. 11 (5 October 2014): 999–1005, doi:10.1038/nclimate2389.

[vi] 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; 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; New York: Cambridge University Press, 2013), 1137–1216, http://www.climatechange2013.org/images/report/WG1AR5_Chapter13_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; New York: Cambridge University Press, 2013), 1029–1136, http://www.climatechange2013.org/images/report/WG1AR5_Chapter12_FINAL.pdf; 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; New York: Cambridge University Press, 2013), 953–1028, http://www.climatechange2013.org/images/report/WG1AR5_Chapter11_FINAL.pdf.

Supporting information

Indicator definition

Observed change in global ocean heat content at different depths

Units

Ocean heat content (Joule)

Rationale

Justification for indicator selection

The ocean is the most dominant component of the Earth’s heat balance, and most of the total warming caused by climate change is manifested in increased ocean heat content (OHC). Isotherms (i.e. contour lines of a given temperature) in the ocean have moved at comparable or faster rates than on land, causing species distribution shifts. Good estimates of past changes in OHC are essential for understanding the role of the oceans in past climate change, and for assessing future climate change. OHC integrates temperature change, the density of seawater and specific heat capacity from the surface down to the deep ocean. OHC is an anomaly calculated in comparison with 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 global sea level. This thermosteric effect has contributed about one-quarter to global sea level rise since 1993.

Scientific references

No rationale references available

Policy context and targets

Context description

In April 2013, the European Commission (EC) presented the EU Adaptation Strategy Package. This package consists of the EU Strategy on adaptation to climate change (COM/2013/216 final) and a number of supporting documents. The overall aim of the EU Adaptation Strategy is to contribute to a more climate-resilient Europe.

One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which will be achieved by bridging the knowledge gap and further developing the European climate adaptation platform (Climate-ADAPT) as the ‘one-stop shop’ for adaptation information in Europe. Climate-ADAPT has been developed jointly by the EC and the EEA to share knowledge on (1) observed and projected climate change and its impacts on environmental and social systems and on human health, (2) relevant research, (3) EU, transnational, national and subnational adaptation strategies and plans, and (4) adaptation case studies.

Further objectives include Promoting adaptation in key vulnerablesectors through climate-proofing EU sector policies and Promoting action by Member States. Most EU Member States have already adopted national adaptation strategies and many have also prepared action plans on climate change adaptation. The EC also supports adaptation in cities through the Covenant of Mayors for Climate and Energy initiative.

In September 2016, the EC presented an indicative roadmap for the evaluation of the EU Adaptation Strategy by 2018.

In November 2013, the European Parliament and the European Council adopted the 7^th EU Environment Action Programme (7^th EAP) to 2020, ‘Living well, within the limits of our planet’. The 7^th EAP is intended to help guide EU action on environment and climate change up to and beyond 2020. It highlights that ‘Action to mitigate and adapt to climate change will increase the resilience of the Union’s economy and society, while stimulating innovation and protecting the Union’s natural resources.’ Consequently, several priority objectives of the 7^th EAP refer to climate change adaptation.

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.

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

No methodology references available.

Uncertainties

Methodology uncertainty

See under "Methodology".

Data sets uncertainty

In general, changes related to the physical and chemical marine environment are better documented than biological changes. For example, systematic observations of sea surface temperature began around 1880. More recently, these manual measurements have been complemented by satellite-based observations that have a high resolution in time and a wide geographical coverage, as well as by Argo floats that automatically measure temperature and salinity below the ocean surface.

Recently, it has been determined that past increases in OHC have been substantially underestimated because of poor sampling of the Southern Hemisphere and limitations of the analysis methods. These concerns have not yet been considered in the datasets presented here.