Thermal Expansion

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Thermal Expansion

As the ocean warms, the water expands, contributing to sea level rise. This thermal expansion is one part of "steric" sea level change, which is the part associated with changes in density. At the global scale, steric sea level change is driven by thermosteric (temperature related) changes, although halosteric (salinity related) changes can be important at regional scales.

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Observed and projected global sea-level 1900-2100

Observed and projected global sea-level 1900-2100

Observed global mean sea level change (difference from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Observed and projected thermal expansion component of sea level 1900-2100

Observed and projected thermal expansion component of sea level 1900-2100

Observed global mean contribution of thermal expansion to sea level change (from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Global ocean heat content change

Global ocean heat content change

Annual global ocean heat content to a depth of 700m difference from 1981-2010 average. Four data sets - EN4, Levitus, JMA and Cheng et al. - are shown. They agree on the overall warming of the upper ocean.

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Why is thermal expansion important for understanding sea level?

The oceans are currently warming, and substantial further warming is "locked in" due to the slow ocean response to climate forcings. When water warms it expands, causing sea levels to rise. Thermal expansion is responsible for 50% of the observed sea level rise in the period 1971 - 2018.

Sea water has a large heat capacity, which means it takes a lot of energy to warm it. However, the oceans are taking up over 90% of the excess energy trapped in the climate system by increased concentrations of greenhouse gases and, as a result, their heat content is increasing. Because oceans are slower to respond to forcing, their heat uptake and resulting thermal expansion are robust measures of climate change as they are less susceptible to the short term climate variability events, such as El Niño, that can cause fluctuations in surface temperature (Allison et al., 2020).

How has thermal expansion affected sea level?

Steric sea level has been increasing on average since at least the 1950s. The increase in heat content, leading to thermal expansion, is not restricted to the relatively well observed upper 700m of the ocean, but is now also evident in the 700-2000m layer and below 2000m. According to IPCC AR6, the contribution to sea level rise from full depth ocean heat uptake has increased from an estimated 0.36mm/yr between 1901 and 1990 to 1.39mm/yr in the period 2006 to 2018.

Why has the thermal expansion component of sea level changed?

The thermosteric component of sea level has increased because of the heat uptake of the ocean in response to greenhouse gas emissions from human activities. IPCC AR6 states: "It is extremely likely that human influence was the main driver of the ocean heat content increase observed since the 1970s, which extends into the deeper ocean (very high confidence)".

How is the thermal expansion component of sea level projected to change in the future?

The oceans are "locked in" to continued warming for a century or more, even under the lowest greenhouse gas emissions scenario. IPCC AR6 states that the amount of ocean heat uptake observed between 1971 and 2018 (396 ZJ) will likely at least double by 2100 under a low warming scenario and will increase by 4 to 8 times under a high warming scenario, with associated sea level rise.

How is the thermal expansion contribution to sea level measured?

Ocean temperatures and salinities have been measured by in situ observing systems, ranging from ship-based bottle and Conductivity Temperature Depth (CTD) observations, to autonomous profiling floats. For more details see the dedicated page on ocean heat content.

Frederikse et al., 2020 calculated the thermosteric contribution to sea level rise from 1957 to 2018 using gridded datasets based on these in situ measurements for the upper 2000m of the ocean. Prior to 1957, the in situ observations were too sparse to use. For the thermosteric contribution before 1957, the approach from Zanna et al. (2019) was used, which uses the principle that all ocean heat anomalies must have been transported from the ocean surface. Ocean heat pathways from the ECCO (Estimating the Circulation of the Climate and the Ocean) model were combined with observed sea-surface temperature anomalies to reconstruct ocean heat content changes and the associated thermosteric expansion.

Below 2000m, measurement coverage has always been sparse, and thus, the approach from Zanna et al. (2019) has been used to estimate this contribution.

How are projections of the thermal expansion component of sea level made?

Projections of thermosteric sea level rise come from a simple climate model "emulator". This is a simplified representation of the climate system that represents the key processes at global scales and provides a consistent set of projections across scenarios. The emulator was constrained by historical observations and estimates of climate sensitivity to provide projections that are consistent with the underlying science assessment of AR6.

Find out more?

Zanna, L., Khatiwala, S., Gregory, J. M., Ison, J. & Heimbach, P. Global reconstruction of historical ocean heat storage and transport. Proc. Natl Acad. Sci. USA 116, 1126–1131 (2019).

References

AR6 Projections

Steric Contribution

AR6 SSP1-2.6

AR6 SSP2-4.5

AR6 SSP5-8.5

Frederikse et al.

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Sea Level

Steric Contribution