The Earth energy imbalance (EEI) is a fundamental climate variable that characterizes the energy state of the climate system. When integrated over multiple years, EEI estimates provide the net energy gain (or loss) by the climate system. In addition, measuring accurately the EEI along with surface temperature and atmospheric composition is essential to separate the role of different radiative forcing from the role of feedbacks on the global energy budget enabling further to constraint effective and equilibrium climate sensitivities. In this presentation I review the current EEI observing system performance and uncertainty. I intercompare the different EEI datasets, originating from in-situ and space-based observing systems to evaluate their differences and to assess their uncertainty.

Since 2000 the Clouds and the Earth’s Radiant Energy System (CERES) project provides satellite-based observations of the Earth radiation budget and the EEI with the highest precision (±0.3W.m^-2 -1s- on a monthly basis). Nevertheless, because of limitation in the absolute calibration of CERES radiometers the CERES final product needs a bias correction (of about ±2.5W.m^-2 -1s-) on the EEI mean. The current best approach to estimating the mean EEI is to estimate the ocean heat uptake (OHU)  which represent 89% of the energy storage  due to the EEI.  Today, the OHU can be derived with the highest accuracy (±0.18W.m^-2 -1s- on the mean OHU), from in situ ocean temperature measured by Argo or from the thermal expansion estimated by the difference between satellite altimetry sea level and ocean mass from GRACE. On 2-yr and longer time scales, OHU and CERES EEI estimates show good agreement in EEI variability. But OHU approaches cannot resolve the EEI variability below 1 yr because the energy gain (or loss) induced by EEI over such small time-scales is of the same order of magnitude as the global exchanges of energy between the atmosphere and the ocean.

The different EEI measurements have enabled since 2005 a robust estimate of the mean EEI of +0.75±0.18W.m^-2 that is essentially due to anthropogenic emissions of greenhouse gases (GHG). They have also allowed to detect a significantly positive trend in EEI of 0.4±0.3W.m^-2 per decade, leading to a doubling of the EEI during the past 20 years in response to continued increases in GHG emissions combined with decreases in aerosol emissions. In addition, on interannual time scales, they showed that the variability in EEI is mostly sensitive to low cloud variability, with ENSO controlling the ±0.5W.m^-2 variability on the 4-7yr time scale.  Today, new scientific challenges related to EEI are emerging like the closure of the energy budget from top of the atmosphere to the bottom of the ocean at monthly to decadal time scales, the estimate of the current effective climate sensitivity, the monitoring of the physical climate system response to GHG mitigation policies and others. These new challenges lead to new requirements on the EEI observing system ranging from sustained continuity to higher precision and accuracy. I discuss briefly the need to refine these requirements and some opportunities to meet them in the future.

How to cite: Meyssignac, B.: Mean, Trend, variability and uncertainty in Earth's Energy Imbalance over the last two decades, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16610, https://doi.org/10.5194/egusphere-egu24-16610, 2024.

The Earth energy imbalance (EEI) at the top of the atmosphere (TOA) is the cause of the energy accumulation in the climate system. 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 amount of energy entering and leaving the climate system (~ 340 W.m-2). 91% of the excess of energy stored by the planet in response to the EEI is accumulated in the ocean in the form of heat making the ocean heat content (OHC) change an accurate proxy of EEI.

In this work, we adopt the space geodetic approach which relies on the sea level budget equation to estimate the OHC changes. The thermosteric sea level change is derived at regional scale from a combination of space altimetry and space gravimetry observations, and divided by the integrated expansion efficiency of heat  to estimate the OHC changes. The global OHC (GOHC) change is then estimated by a spatial integration of the regional OHC changes. The uncertainty in GOHC is estimated by propagation of the uncertainty of input data using the input data error variance-covariance matrix to account for the instrumental and post-processing errors and for the time correlation in errors.

Regional estimates of the OHC changes are validated over the Atlantic Ocean directly against data from in-situ Argo profiles and indirectly by an energy budget approach. In the energy budget approach, surface heat flux derived from ERA5 and CERES TOA radiation budget are combined with regional OHC changes to estimate the north Atlantic meridional heat transport which is then validated against in-situ RAPID and OSNAP estimates. Both validations show good agreement in terms of signal amplitudes and variability with time correlations above 0.6. 

Over the period 1993-2022, the GOHC shows a significant positive trend of 0.75 W m-2 [0.61, 1.04] at the 90% confidence level, indicating a positive mean ocean heat uptake or EEI. Comparisons with GOHC estimates based on in-situ ocean temperature measurements over the full ocean depth show good agreement over 2005-2019 (Marti et al. 2023, in review). Over 2000-2020, the ocean heat uptake presents a positive trend of 0.33 W/m²/decade, significant at the 90% confidence level and in agreement with CERES estimate. This EEI trend  reflects an acceleration in ocean warming.

The two space geodetic products based on space altimetry and space gravimetry are freely available on the AVISO website. One estimating the GOHC and EEI (https://doi.org/10.24400/527896/a01-2020.003), the other estimating regional OHC over the Atlantic Ocean (https://doi.org/10.24400/527896/a01-2022.012).

How to cite: Fraudeau, R., Marti, F., Meyssignac, B., Blazquez, A., Fourest, S., Ablain, M., Rousseau, V., Larnicol, G., Restano, M., Benveniste, J., Sabia, R., and Dibarboure, G.: Estimate of the global and regional Ocean Heat Content changes from space gravimetry and altimetry observations to assess the Earth Energy Imbalance variations and trend, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16132, https://doi.org/10.5194/egusphere-egu24-16132, 2024.

TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) is an operational climate mission, aiming to enhance, up to an order-of-magnitude, our ability to estimate the Earth radiation budget, spectrally resolved to support attribution. Through direct measurements of incoming total and spectrally resolved solar irradiances and Earth reflected radiances, spatially resolved, it establishes ‘benchmarks’ against which change/trends can be detected in as short a time as possible. These fiducial reference data sets can be combined with data from other sensors and also serve as ‘gold standard’ references to anchor and upgrade the performance of other space sensors through in-orbit calibration.

TRUTHS will become a founding member of a new class of satellites called SITSats, SI-Traceable Satellites, with payloads explicitly designed to achieve and evidence an uncertainty, in-orbit, at a level commensurate with the exacting goals of long-time-base climate studies. SITSats also facilitate interoperability and enhanced trust in the data from the Earth observation system as a whole, helping to provide observational evidence-based confidence in actions addressing the climate emergency. 

The unprecedented uncertainty of TRUTHS’ globally sampled hyperspectral data underpins many additional applications:

• Establishing an interoperable, harmonised Earth Observing system incorporating agency and commercial satellites: large and small
• Top and Bottom of atmosphere reflectances impacting carbon cycle (e.g. land cover, ocean colour, vegetation, methane etc together with similar applications of other hyper/multi-spectral missions). Low uncertainty also facilitates improvements in retrieval algorithms.
• Transferring radiometric reference values to existing Cal/Val infrastructure (e.g. RadCalNet, Pseudo-Invariant Calibration sites, In-situ ocean colour reference observations; selected surface reflectance test-sites (fluxnet, …), both nadir and multi-angular) and Moon observations.

The mission comprises an “agile” satellite capable to point and image the Earth, Moon and Sun from a 90°polar orbit by the Hyperspectral Imaging Spectrometer (HIS). The HIS provides spectrally continuous observations from 320 to 2400 nm, with a spectral sampling between 2 and 6 nm and a spatial sampling of 50 m. The payload utilises a novel SI-traceable on-board calibration system (OBCS), comprising of the Cryogenic Solar Absolute Radiometer (CSAR), able to realise SI-traceability in space and also measure incoming solar radiation. Together with other optical elements the OBCS links the HIS observations to the CSAR with a target expanded uncertainty 0.3% (k=2).

TRUTHS is implemented by the European Space Agency (ESA) as a UK-led Earth Watch mission in collaboration with Switzerland, Czech Republic, Greece, Romania and Spain. The mission was conceived by the UK national metrology institute, NPL, in response to challenges highlighted by the worlds space agencies, through bodies such as CEOS addressing observational needs of GCOS. The mission is under development by an industrial consortium led by Airbus Defence and Space UK, with a target launch date of 2030 and minimal operations life-time of 5 years with a goal of 8 yrs.

Together with FORUM (ESA) and IASI-NG (CNES/EUMETSAT) it will provide spectrally resolved Earth radiance information from the UV to the Far-Infrared in the coming decade, and in partnership with CLARREO-Pathfinder (NASA) and CSRB (CMA) inaugurate a future constellation of SITSats.

How to cite: Fox, N., Fehr, T., Marini, A., August, T., and Remedios, J.: Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) – A ‘gold standard’ imaging spectrometer in space for radiation imbalance and in support of the climate emergency , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18864, https://doi.org/10.5194/egusphere-egu24-18864, 2024.

The Earth’s energy imbalance (EEI), i.e. the difference between incoming solar radiation and outgoing reflected and emitted radiation, is the one quantity that ultimately controls the evolution of our climate system. Despite its importance, the exact magnitude of the energy imbalance is not well known, and because it is a small net difference of about 1 Wm⁻² between two large fluxes (approximately 340 Wm⁻²), it is difficult to measure directly. There has recently been a renewed interest in applying wide-field-of-view radiometers onboard satellites to measure the outgoing radiation, and hence deduce the global annual mean energy imbalance.

Here we investigate how to sample with a limited number of satellite orbits, in order to correctly determine the global annual mean imbalance. Using observational and model data, we have investigated the importance of the local and global diurnal cycles, as they are observed by a satellite, in the determination of the EEI. We simulate satellites in polar (90° inclination), sun-synchronous (98°) and precessing orbits (73°, 82°), as well as constellations of these types of satellite orbits. We present the results of ongoing work concerning different orbits, and how they affect the estimated global annual mean EEI.

How to cite: Hocking, T., Mauritsen, T., and Megner, L.: Sampling the diurnal and annual cycles of Earth’s energy imbalance with constellations of satellite-borne radiometers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15999, https://doi.org/10.5194/egusphere-egu24-15999, 2024.

Global climate change, which causes, among other things, an accumulation of energy in the oceans, may cause irreversible changes to their ecosystems. This can be particularly quickly apparent in bodies of water that are shallow and small in relation to the Oceans, such as the Baltic Sea. In the SatBałtyk System (http://www.satbaltyk.pl), which aims to observe the state of the Baltic Sea environment based on satellite data, maps of the distributions of values of a number of physical biological and chemical parameters of the sea are collected and made available. Within the framework of this System, the SBRB (SatBałtyk Radiation Budget) model was launched, determining data on radiation budget (NET) at the sea surface. Daily maps of the spatial distribution of the radiation budget  and its components at the Baltic Sea surface are created based on data from SEVIRI, AVHRR, MODIS, SBUV/2, TOVS radiometers, and forecast auxiliary models. The component algorithms of this model were developed and validated against empirical data measured directly in the Baltic Sea (Zapadka et al. 2020). The uncertainties in the estimation of the radiation budget for the monthly averages are: RMSD 4 Wm^-2 and BIAS -0.5 Wm^-2. The individual downward and upward shortwave radiation fluxes are determined with an accuracy of RMSD 3 Wm^-2, 1 Wm^-2, BIAS 3 Wm^-2, 0.1 Wm^-2 respectively, and downward and upward longwave radiation fluxes are RMSD 4.5 Wm^-2, 3.7 Wm^-2, BIAS -0.8 Wm^-2, 2.6 Wm^-2 respectively. The uniform methodology used since 2010 has enabled an analysis of the variability of the radiation budget and its components at the surface of the Baltic Sea covering 14 years. Despite the natural variation in NET values and its components year-on-year, the analyses showed an annual growth trend of c. 0.7 Wm^-2. Interestingly, the increasing trend applies to all NET components. An analysis of the possible causes of the trend observed in recent years may confirm the role of the anthropological factor in these changes.

How to cite: Zapadka, T., Ostrowska, M., Stoltmann, D., and Paszkuta, M.: Radiation budget at the Baltic Sea surface in 2010 – 2023 from SatBałtyk System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11721, https://doi.org/10.5194/egusphere-egu24-11721, 2024.

The average global temperature of Earth is governed by the energy balance equation, comparing energy entering and leaving the Earth system. A key parameter in this balance is the Earth’s albedo, determining the ratio of the Sun’s energy being reflected from or absorbed by Earth. The global albedo varies on several different timescales – daily due to changes in cloud cover, seasonally due to changes in foliage and snowfall, and on greater timescales a change in albedo is a reflection of our changing climate. To measure these changes, multi-decadal data is needed.

Data of top-of-the-atmosphere shortwave radiation used in albedo estimation, are primarily gathered by LEO satellites using absolute measurement techniques. These are however affected by the harsh space environment, especially radiation, which causes drift errors in the data, requiring in-flight calibration. The purpose of NASA’s and ESA’s upcoming missions CLARREO and TRUHTS respectively, is to provide state of the art calibration data to account for these errors. However, they do not remove the issue all together.

As an alternative to these absolute measurements, the space based earthshine telescope juLIET (ju Lunar Imaging Earthshine Telescope) aims to estimate the albedo through relative measurements. The Earthshine albedo technique is based on comparing the intensity of Moonlight coming from the visible dayside of the Moon and the Earthshine reflected off the visible nightside of the Moon. As a relative measurement, it is more resilient to calibration drift.

Albedo measurements using the Earthshine technique have been successfully carried out from Earth, but due to Moonlight being several magnitudes brighter than Earthshine, atmospheric scattering of Moonlight reduces the possible precision on the Earthshine intensity. While the issue of atmospheric scattering is removed by going into orbit, measuring the dim Earthshine with a sufficiently high precision to be used for albedo estimation, using the same sensor that measures the Moonlight, still poses a significant challenge, due to scattering and diffraction of Moonlight within the telescope.

To determine the feasibility of the juLIET instrument, an analysis of the optical noise of the telescope is conducted. This analysis is carried out using Zemax OpticStudio and MATLAB, where main contributors to the uncertainty of the measurement are isolated and quantified.

The results of this noise analysis will be extended to determine which lunar phases juLIET can provide measurements of the Earth albedo, during its mission time as primary payload on the small-sat ROMEO developed by IRS, University of Stuttgart. 

How to cite: Winther, K., Thejll, P., and Fléron, R.: Lunar Imaging Earthshine Telescope, juLIET, for Earth Albedo Measurements , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10172, https://doi.org/10.5194/egusphere-egu24-10172, 2024.

The NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) launched on December 15^th, 2017 and was integrated on the International Space Station (ISS) to measure long-term total solar irradiance (TSI) and solar spectral irradiance (SSI). The direct measurement of the SSI is made by the LASP Spectral Irradiance Monitor (SIM) and provides data essential to interpreting how the Earth system responds to solar spectral variability. Extensive advances in TSIS-1 SIM instrument design and new SI-traceable spectral irradiance calibration techniques have resulted in improved absolute accuracy with uncertainties of less than 0.5% over the continuous 200 to 2400 nm spectral range. Furthermore, improvements in the long-term spectral stability corrections provide lower trend uncertainties in SSI variability from those of the previous SORCE SSI instruments. We present the early mission results of the TSIS-1 SIM SSI observations for the first 5 years of operations – a time-period that includes the descending phase of solar cycle 24, the last solar minimum, and the ascending phase of solar cycle 25. Comparisons are made to previous spectral measurements both in the absolute scale of the solar spectrum and the time dependence of the SSI variability. The TSIS-1 SIM SSI spectrum shows lower IR irradiance (by as much as 6% near 2400 nm) and small visible irradiance increases (~0.5%) from the previous ATLAS3 and WHI reference solar spectra, but more consistent agreement with recent SCIAMACHY and SOLAR2 reanalysis results. We also show initial comparisons to current NRLSSI2 and SATIRE-S SSI model results both for short-term (solar rotation) spectral variability and, for the first time, the longer-term (near half solar cycle) spectral variability across the solar spectrum from the UV to the IR.

How to cite: Richard, E., Coddinton, O., Harber, D., Pilewskie, P., and Woods, T.: Long-term Solar Spectral Irradiance Observations by the TSIS-1 Spectral Irradiance Monitor, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2928, https://doi.org/10.5194/egusphere-egu24-2928, 2024.

Multiple satellites capture images of the Sun in Extreme Ultraviolet (EUV) light. However, only the Solar Dynamics Observatory (SDO) was equipped with instruments that measure the Sun's EUV spectral irradiance (i.e., MEGS-A and MEGS-B onboard the Extreme Ultraviolet Variability Experiment (EVE) suite). The MEGS-A instrument malfunctioned in 2014, making it impossible to measure the full irradiance spectrum ever since. 

Using AI, we explore the translation of a set of EUV images of the Sun into spectral irradiance, effectively replacing the malfunctioning MEGS-A instrument onboard SDO. In other words, we generate a virtual irradiance instrument, MEGS-AI, for SDO. Using an Image-to-Image translation tool (ITI), this virtual instrument can also be trained and added on other EUV-observing satellites like STEREO, GOES, SolO, and the upcoming VIGIL satellite, enabling unprecedented irradiance estimates from additional satellite missions. In the case of the STEREO twin-satellites and VIGIL, this enables estimates of spectral irradiance prior to the Sun rotating into Earth’s view, which directly enables the forecast of enhanced irradiance. Additionally, we explore different combinations of images in different EUV channels and evaluate their contributions in estimating different irradiance channels. Finally, when combined with a neural radiance field model of the Sun (SuNeRFs), MEGS-AI can estimate spectral irradiance from any viewpoint in the solar system, enabling for the first time a complete 4pi spectral irradiance map of the Sun. This can be directly used to estimate the Sun’s impact on other planets in the solar system and to determine the total solar irradiance output in multiple EUV spectral bands.

How to cite: Tremblay, B., Jarolim, R., Jungbluth, A., Munoz-Jaramillo, A., Bintsi, K.-M., Santos, M., Mason, J. P., Vourlidas, A., and Sundaresan, S.: AI to Enhance the Capabilities of EUV-observing Satellites and Estimate Spectral Irradiance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13691, https://doi.org/10.5194/egusphere-egu24-13691, 2024.