What are the ocean heat content and the Earth's energy imbalance?
An unbalanced climate and land system
Planet Earth's climate system relies on a perfect balance between the heat received from the sun and the heat re-emitted by the Earth. However, since the industrial era, anthropogenic emissions of greenhouse gases (GHGs) into the atmosphere have been steadily increasing, trapping part of the energy emitted by the Earth. This is known as Earth Energy Imbalance ( EEI). This EEI takes the form of an accumulation of energy in the form of heat (trapped since the 1970s, according to an ESSD study), making it the main cause of climate change.
This excess heat in the climate system accumulates mainly in the ocean (≈ 91% versus ≈ 4% for land and ≈ 5% for ice) and drastically disrupts the global water and energy cycle, leading to climate disruption and numerous phenomena threatening biodiversity and life on Earth. In fact, the ocean plays a fundamental role in climate regulation, as it is a major CO2 sink and stores a large proportion of excess heat.
The heat of our oceans as an essential measure
Thus, Ocean Heat Content (OHC) is an essential variable to monitor, measured using both in-situ data and a combination of Earth observation space data. It is the essential variable for calculating the EEI. OHC and EEI are therefore key indicators for analyzing climate change, and it is essential to measure them accurately.
Led by Magellium Artal Group in collaboration with LEGOS, and funded by ESA, CNES and CNRS, the main aims of the MOHeaCAN and 4DAtlantic-OHC projects are to provide accurate measurements of OHC and EEI, in order to improve understanding of the Earth's climate system and develop tools to better predict and manage the impacts of climate change. The data provided also helps to inform public policy on environmental and social issues.
MOHeaCAN project, global monitoring of oceans and climate change using space data
The MOHeaCAN project and its objectives
The MOHeaCAN(Monitoring Ocean HEAt Contentand earth energy imbalANce) project is supported by ESA and CNES, and led by Magellium Artal Group in collaboration with scientific experts from the LEGOS laboratory.
MOHeaCAN has successfully explored the feasibility of monitoring the global heat content of the oceans using a product based on geodetic space observations (altimetry and gravimetry).
The MOHeaCAN project has multiple objectives:
- It aims to establish a global estimate by developing a system for the continuous monitoring of OHC and IEE;
- The aim is to use the data collected to refine existing climate models, providing a better understanding of ocean thermodynamic processes and their impact on global climate.
Magellium Artal Group's contribution to the project
Magellium Artal Group's solid expertise in spatial data processing and the development of processing chains makes it a key player in geodetic data processing.
The MOHeaCAN product contains monthly time series of several variables, the main ones being the global OHC and the EEI indicator. Uncertainties are provided for variables at the global scale, by propagating errors in measurements of sea level (altimetry) and ocean mass content (gravimetry).
Time series of OHC changes can be deduced by various methods. One indirect approach is the spatial geodetic approach, based on the sea-level budget equation. The MOHeaCAN project served as a proof of concept, describing the application of the spatial geodetic approach on a global scale. The space geodetic approach aims to measure the thermosteric sea level variation due to temperature-induced seawater density change, based on the differences between the total sea level variation derived from satellite altimetry measurements and the barystatic sea level variation derived from satellite gravity measurements. Halosteric sea level variations due to saline contraction are estimated from in situ data and removed from total sea level variations.
The latest version of the global OHC/EEI product (v5.0) based on a spatial geodesic approach is available on the ODATIS/AVISO portal and accessible to all.
This project lays the foundations for more regional projects such as 4DAtlantic-OHC, which focuses primarily on measuring the OHC of the Atlantic Ocean.
4DAtlantic-OHC, the spatial geodetic approach for the Atlantic Ocean
The 4DAtlantic-OHC project and its objectives
The 4DAtlantic-OHC project focuses specifically on estimating the OHC of the AtlanticOcean. It is a regional extension of the MOHeaCAN project, aimed at developing and testing space geodetic techniques (satellite altimetry and gravity) for global monitoring on a regional scale, both for data generation and uncertainty estimation. The project addresses specific regional challenges associated with more complex variations that could be overlooked on a global scale.
More specifically, halosteric variations due to sea-level salt variations, which are not negligible on a regional scale, are estimated from in situ data and removed from steric sea-level variations derived from altimetry minus gravimetry.
4DAtlantic enables realistic monitoring of the Atlantic Ocean's heat absorption, and a better understanding of the Atlantic Ocean's role in the Earth and climate system.
Magellium Artal Group's work in heat content calculation
Magellium Artal Group's contribution to 4DAtlantic is similar to its contribution to MOHeaCAN, as our teams are working in parallel on both projects.
For the time being, uncertainties in OHC trends are mainly due to uncertainties in gravimetry (GRACE(-FO) data).
Involving experts in geophysics, oceanography and climatology, 4DAtlantic-OHC is an example of interdisciplinary collaboration aimed at merging various fields of expertise for a more comprehensive understanding of ocean dynamics.
Our commitment to monitoring climate change
In the current context of climate disruption, the need to understand the Earth's energy imbalance and its medium- and long-term implications has become absolutely urgent. Climate disruption, resulting from this excess of energy, is having serious consequences for biodiversity and human populations. Among other things, global warming is causing sea levels to rise and modifying the hydrological cycle, which can lead to extreme events such as drought and flooding, affecting all populations. These events will affect France in particular, with a 56% increase in the risk of flooding and an increase in summer droughts by 2050.
The MOHeaCAN and 4DAtlantic-OHC projects provide answers to these issues. By providing analyses of these data, these projects play a decisive role in helping public policies to quantify impacts and adapt strategies to anticipate and mitigate the effects of climate change and adapt our society to the new climatic realities.
The diagrams below show the evolution of the Earth's surface air temperature (top diagram) and the Earth's energy imbalance (bottom diagram) over the course of the century, according to two scenarios described in the latest IPCC report (AR6):
- the red curves correspond to the "business as usual" scenario
- the blue curves correspond to the Paris Agreement scenario, i.e. keeping the global temperature rise below 2°C.
In these figures, the trajectories between the two scenarios differ significantly, around 15 years earlier for the Earth's energy imbalance than for temperature. Measuring the Earth's energy imbalance will therefore enable us to observe more quickly the impact of public policies to reduce greenhouse gas emissions on climate change.
On a European and continental scale, other projects are responding to the climate emergency with space hydrology. In this context, HydroVenture is an initiative set up by Magellium Artal Group and 14 partners to create the first operational European space hydrology center for the study and analysis of continental waters. Similarly, the SEED-FD project, for which Magellium Artal Group is consortium leader, aims to improve the hydrological models of the Copernicus Emergency Management Service (CEMS ) to better predict floods and droughts on a global scale.
