Ocean heat content and earth energy imbalance: climate indicators from space

Over the past decades, anthropogenic emissions of greenhouse gases (GHG) in the atmosphere have reduced the energy emitted by Earth toward space. Now the Earth emits less energy towards space than it receives energy from the sun leading to a radiative imbalance at the Top Of the Atmosphere (TOA). This energy imbalance called EEI (for Earth Energy Imbalance) is responsible for the accumulation of heat in the climate system making it the primary cause for climate change. It is absolutely essential to monitor EEI to evaluate the amount of energy that is accumulating in the system and to understand how this energy changes the climate.

Measuring the EEI is challenging because it is a globally integrated variable whose variations are small (0.5-1 W.m⁻²) compared to the typical annual and year-to-year variations of the energy fluxes in and out of the climate system (incoming solar radiation is about 340 W.m⁻²). Ideally, we would need EEI estimates with an accuracy around ±0.1 W.m⁻² at decadal time scales to be able to monitor not only the EEI variations caused by GHG emissions but also the variations in EEI caused by volcanic eruptions or internal variability (such as the Hiatus). This objective pushes the challenge further.

EEI can be estimated by an inventory of heat changes in the different reservoirs of the climate system - namely the atmosphere, the land, the cryosphere and the ocean. As the ocean concentrates the vast majority of the excess of energy (~93%) in the form of heat, global variations in Ocean Heat Content (OHC) place a strong constraint on the EEI estimate.

From the Ocean Heat Uptake...

The OHC variations can be estimated directly from net ocean surface heat fluxes measured based on CERES space measurements, from in situ data observed by the ARGO floats or from ocean model reanalyses. We estimate here the OHC from an alternative method based on spatial altimetry and gravimetry observations which complements these other approaches and which is very promising to reduce uncertainty estimates.  

The OHC is estimated from the measurement of the thermal expansion of the ocean based on differences between the total sea-level content derived from altimetry measurements and the mass content derived from gravimetry data, noted “altimetry-gravimetry”. 
This “altimetry-gravimetry” approach provides consistent spatial and temporal sampling of the ocean. It samples nearly the entire global oceans, except in polar regions where the sea is completely covered by sea ice (i.e. essentially north of 80°N), and it provides estimates of the OHC variations over the ocean’s entire depth. 

… to an estimate of the Earth energy Imbalance

The EEI indicator is derived from the temporal variations of the ocean heat content, i.e. by calculating its derivative (called the ocean heat uptake). Energy uptakes from the land, cryosphere and atmosphere reservoirs represent about 7% of the EEI and are not accounted for. The average value of the EEI is +0.84 W.m^-2 with an error of +/- 0.2 W.m^-2 (within a 90 % confidence level) and shows that on average the Earth stores energy. This EEI value represents an enormous amount of energy when it is integrated into the entire Earth's surface at the top of the atmosphere (20 km) since the EEI represents a total energy uptake of the Earth of about 430 TW (i.e. about 1000 times the power of the world's nuclear power plant).

Results show that on average the Earth stores energy. Clarifying the uncertainties on the EEI indicator will help to address the question: are EEI’s variations significant?

The OHC-EEI product

This product is referenced with a DOI: 10.24400/527896/a01-2020.003

Users will be mainly interested in:

• time series of gridded ocean heat content (3° x 3°)
• global ocean heat content time series (representative of the globe within the extent of data availability) 
• earth energy imbalance time series (from global OHC filtered-out from signals lower than 3 years)

All data used to calculate the indicators are given in the product in full transparency. Thus users can rebuild their own OHC or EEI indicators. 

This product is accessible (NetCdf file) directly via the FTP download (not supported by some browsers)

Documentation

The Algorithm Theoretical Basis Document (ATBD) can be accessed here.
The Product User Manual (PUM) can be accessed here.

Contacts

For any technical issues or additional information related to the OHC-EEI product, users are advised to contact Florence Marti (technical coordinator) : florence.marti(at)magellium.fr

Acknowledgment

This work has been supported by ESA in the framework of the MOHeaCAN project (Monitoring Ocean Heat Content and Earth Energy ImbalANce from Space):   eo4society.esa.int/projects/moheacan/. This work is also supported by the CNES for the dissemination of the products through ODATIS, and for the future evolutions of this product.
 

References

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