Ocean observations from space confirm disruption of Earth's energy balance

By Florence Marti (Magellium), Rob Roebeling, Remko Scharroo and Hayley Evers-King (EUMETSAT)

Key to a stable climate on Earth is the delicate balance between energy coming in from the Sun and emitted back to space by the Earth. In our present climate, this balance has become disturbed and may even continue to grow. Prolonging this imbalance would have major implications for the future climate of our Earth. 

Over the past century, ‘climate drivers’, such as the emissions of anthropogenic greenhouse gases (GHG) in the atmosphere and volcanic eruptions, disturbed this delicate balance (IPCC, 2021). Today, the Earth emits less energy towards space than it receives from the Sun. This energy imbalance, also called Earth Energy Imbalance (EEI), is responsible for the accumulation of heat in the climate system, making it the primary cause for climate change. Understanding how this imbalance affects our climate requires accurate and global measurements of incoming and reflected solar radiation, as well as of emitted and backward radiated thermal radiation (see Figure 1). The latest estimates of EEI, combined with observations of other ‘climate drivers’, have provided the United Nations' International Panel for Climate Change (IPCC) with sufficient evidence to conclude that is very likely that our climate will warm between 2°C to 5°C until the end of this century, relative to pre-industrial times, if greenhouse gas emissions are not reduced rigorously and quickly (IPCC, 2021).

Observing Earth Energy Imbalance (EEI)

The Earth Energy Imbalance 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. Because oceans absorb the vast majority (~90%) of excess energy in the form of heat, changes in ocean heat content (OHC) are an accurate proxy of the EEI.

The absorption of heat by the oceans makes its temperatures rise and its water expand. Over the two past decades, this process of thermal expansion caused about 35% of global sea level rise (Horwarth et al., 2022). Whereas, the prime driver of sea level rise (about 65%) is the increase of the ocean mass due to melting of glaciers and ice sheets — of which the glaciers on Greenland and Antarctica are the most important ones. Hence, changes in global sea level, when corrected for the contribution of land-based glacier melt, provide a good indicator for changes in ocean heat content (see Figures 2 and 3). Satellite observations allow us to disentangle sea level rise induced by mass variation from sea level rise induced by thermal expansion. Hereto, one needs to combine two types of satellites observations. Firstly, space-based altimetry measurements to observe changes in sea level. Secondly, space-based gravimetry measurements to observe changes in ocean mass.

20 years of increasing EEI

The paper of Marti et al. (2022) analyses 20 years of ocean heat content and the associated EEI observations, derived from estimates of ocean's thermal expansion. To do so, they combine three observational datasets:

1. A dataset of sea level anomalies above the reference mean sea surface derived from space-based radar altimetry observations (Taburet et al., 2019; Legeais et al., 2021), also referred as the total sea level change.
2. A dataset of ocean mass variations obtained from space-based gravimetry observations from the Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow-on missions (Tapley et al., 2004).
3. An estimate of Expansion Efficiency of Heat (EEH) derived from in-situ observations of ocean temperature and salinity.

With these datasets it is possible to estimate the Ocean Heat Content and the Earth Energy Imbalance. Where the latter is be derived from the temporal variations of Ocean Heat Content.

Marti et al. (2022) found positive trends in ocean heat content over almost all oceans between 2002 and 2020. Still, the trends in ocean heat content are not the same around the globe. The strongest positive trends were observed over the Atlantic Ocean (see Figure 4) and Indian Ocean. The positive anomaly in the Indian Ocean is likely related to the warm pool, recording higher temperature increase during the last decades than the global ocean (e.g. Weller et al., 2016). Scientists associate the warming over the southern part of the Atlantic Ocean with slowing down of the Atlantic Gulf Stream — or Atlantic Meridional Overturning Circulation (AMOC) — which reduces the transport of heat from the tropics to the North Atlantic (Rahmstorf et al., 2015).  In contrast, the North Atlantic is one of the few areas where negative trends in ocean heat content were observed. As this area is closer to the Greenland coast, its cooling could be mainly attributed to strong outflows of cool melt water (see the case study Melting Greenland ice sheet cools North Atlantic Ocean).

Taking into account energy uptakes from the land, cryosphere and atmosphere reservoirs, Marti et al. (2022) estimated global variations of the energy imbalance during the last 20 years (see Figure 5, update of Marti et al. (2022)). They found that our oceans have been accumulating heat and storing large amounts of energy. Apart from some years with a lower Earth Energy Imbalance (2007 and 2010), the overall the imbalance was positive, at about 0.8 W.m^-2.

At first glimpse, an Earth Energy Imbalance value of about 0.8 W.m^-2 may seem small. However, this value is representative for the imbalance of every square meter on Earth every second. When integrated over the entire Earth's surface the time series of Global Ocean Heat Content Change (see Figure 6, update of Marti et al. (2022)) reveals an enormous energy uptake of the Earth system of about 400 Terawatt since 2002. This is equivalent to about 1000 times the power produced by all nuclear power plants in the world. Further studies are needed to better estimate variations of the energy imbalance at inter-annual time scales and to be able to assess whether there is a significant positive trend reflecting an accelerated warming of the Earth system.

Continued and better measuring the oceans in the future

The extended and improved observations of the topography and the mass of Earth's oceans rely on high quality, satellite instruments. New altimetry missions complement the legacy of missions that have contributed to nearly 30 years of altimetry measurements. Under the Copernicus programme, EUMETSAT operates two Sentinel-3 satellites (with two further missions planned) providing altimetry measurements using modern Synthetic Aperture Radar technology.

Similarly, under Copernicus, and as continuity of the Jason series of altimeters, EUMETSAT also operates the Sentinel-6 mission. Sentinel-6 Michael Freilich was launched in 2020, and has since become the altimetry reference mission. It will be joined by Sentinel-6B in 2025. Bridging the gap between low- and high-resolution modes of altimetry, the Sentinel-6 mission is essential for extending climate quality data records.

The two Copernicus expansion missions CIMR (Copernicus Imaging Microwave Radiometer) and CRISTAL (Copernicus Polar Ice and Snow Topography Altimeter) will also offer enhanced observations of ocean and ice, particularly in the polar regions.

The continued observations of ocean mass rely on past and present gravity missions, such as NASA's Gravity Recovery And Climate Experiment (GRACE) missions, ESA's Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), and ESA's planned Next Generation Gravity Mission (NGGM). The latter mission is currently under definition, and aims to improve our knowledge and monitoring of geophysical phenomena revealed by Earth’s gravity field.

Benefits

The IPCC concludes in its Sixth Assessment Report (AR6) that there is high confidence that Earth energy imbalance was positive by almost 0.8 W.m^-2 during the past 20 years (IPCC, 2021). This Climate Use Case demonstrates the synergetic use of space-based estimates of sea level from altimetry observations and sea mass from gravimetry observations for estimating the rate at which oceans accumulate heat and, in turn, giving an estimate of the Earth's energy imbalance. The results of this Climate Use Case confirm IPCC's conclusions. The new Sentinel-3 data may help further narrowing the uncertainties of future Earth Energy Imbalance estimates.

Acknowledgments

The study on the assessment of the Earth's energy imbalance from space geodetic observations was supported by ESA in the framework of the MOHeaCAN project (Monitoring Ocean Heat Content and Earth Energy ImbalANce from Space, ESA Contract number 4000129563-19-I-DT) and is still ongoing with the support of CNES. The results of this study rely on data of (jointly) operated satellites of CNES, NASA, NOAA, ESA, and EUMETSAT.

Data used

A daily satellite-based data record of sea level observations, derived from different altimetry missions, such as EUMETSAT's operated Jason and Sentinel-3 missions, covering the period 1993-date. The data are provided by the Copernicus Climate Change Service (C3S) and is made available via the Copernicus Data Store at https://cds.climate.copernicus.eu/cdsapp#!/dataset/satellite-sea-level-global?tab=overview (accessed on 2022-03-04).

A satellite-based data record of ocean mass variations, derived from observations of the Gravity Recovery And Climate Experiment (GRACE) and GRACE Follow –on missions, covering the period 2002-date. The data were derived by the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS) based on Blazquez et al. (2018) and is available at ftp://ftp.legos.obs-mip.fr/pub/soa/gravimetrie/grace_legos/V1.5.1/ (accessed on 2022-05-24).

A satellite-based data record of geodetic Ocean Heat Content (OHC) and Earth Energy Imbalance (EEI) covering the period 2002-2020, and referred to as "Version 4.0 of the OHC/EEI product from space altimetry and space gravimetry produced by Magellium/LEGOS and distributed by AVISO+ with support from CNES and ESA". The data record is freely available at https://doi.org/10.24400/527896/a01-2020.003 (accessed on 2022-05-24).

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