Global warming, a phenomenon that has significantly affected our planet in recent decades, is a direct consequence of the disruption of the delicate balance between incoming solar radiation and that reflected and emitted. radiation from the Earth. This imbalance, known as Earth's energy imbalance (EEI), leading to the accumulation of energy in the atmosphere, oceans and land and the melting of the cryosphere. The consequences are increasingly evident: rising temperatures, rising sea levels and increasingly extreme weather around the world. Surprisingly, recent observations revealed that Earth's energy imbalance has more than doubled in recent decades, which represents an unexpected and worrying trend.

The nature and surprising rise of energy imbalance

The energy imbalance is a critical indicator of the state of the climate, the fundamental role of which has been known for more than two centuries. Essentially, it is the difference between the energy that the Earth receives from the Sun and the energy that it radiates back into space. The causes of this imbalance are mainly anthropogenic greenhouse gas emissions, which reduce the amount of infrared radiation emitted into space, thereby driving a gradual increase in imbalance.

Observations from space show that energy imbalance is rising much faster than expectedIn 2023, it reached the values 1.8 Wm−2, which is twice the best estimate of the Intergovernmental Panel on Climate Change (IPCC). This strong upward trend is difficult to reconcile with climate models; even when taking into account the increase in anthropogenic radiative forcing and the associated climate response, state-of-the-art global climate models can only barely reproduce the rate of change to 2020 within the observational uncertainty. The continued increase in EEI since 2020 suggests that the actual signal has moved away from the model predictions.

Complex factors influencing imbalance

The energy imbalance is the result of several factors, including forcing, feedback, and internal variability. The main forcing is, as mentioned, anthropogenic emissions. Part of the positive greenhouse forcing is compensated by the presence of anthropogenic aerosols, which cool the climate by reflecting sunlight back into space. However, there is evidence that the cooling effect of aerosols is weakening, as governments address air quality issues.

In addition, higher surface temperatures lead to greater emission of infrared radiation into space, which reduces the energy imbalance, creating negative feedback. Warming also activates other climate feedbacks from clouds, water vapor, cryosphere, etc., which together act to amplify global warming. Internal variability, such as weather events or El Niño, can also cause interannual fluctuations in the energy imbalance. The observed decrease in EEI in the second half of 2023 and in 2024 after the El Niño event suggests the activity of stabilizing feedbacks, but this decrease copies the overall upward trend.

The crisis of observation capacity

Despite the fundamental role of energy imbalance in regulating the climate system, our the ability to observe it is rapidly threatened as satellites are decommissionedThe current components of Earth's energy imbalance are monitored using a combination of tools. NASA CERES on board polar-orbiting satellites and an instrument TSIS-1 on the International Space Station. However, it is likely that within ten years there will be Libero (CERES successor mission, planned for 2027) the only tool in the universe, which represents single point of failure with no formal plans to continue this vital record after its mission. For solar radiation monitoring, a successor instrument, TSIS-2, is scheduled for 2025, but has only a three-year planned lifespan.

Complementary initiatives are being proposed that aim to directly measure the energy imbalance itself, such as spheroidal satellites with accelerometers or a constellation of satellites with radiometers. However, these missions are not yet funded and are unlikely to be operational until the second half of the 2030s. However, their successful implementation would provide crucial redundancy and independent measurements.

The key to the future of climate policy

Monitoring and quantitatively understanding changes in Earth's energy storage will be crucial in the coming decades, particularly in the context of international efforts to keep global warming well below 2°C. If mitigation measures are successful, they will first be reflected in a peak, followed by a slowly decreasing trend in the Earth's energy imbalance, basically decades before the temperature signalIt is in the energy imbalance that we can monitor and evaluate the effectiveness of mitigation measures in real time. If there were surprises, such as unexpectedly large aerosol impacts, the imbalance would be the first place to detect them.

It is therefore essential that the scientific community systematically work to understand and quantify the causes of changes in energy imbalances, not only for the future but also on an annual basis. Together with funding agencies and policymakers, we must strive to ensuring robust and reliable capacity to observe energy imbalances – perhaps the most fundamental quantity in the climate system – at this pivotal moment in history. Spring

Report published in a journal agu.org

Glossary of key terms

• Earth's Energy Imbalance (EEI): The net difference between the amount of solar energy that the Earth absorbs and the energy it radiates back into space as infrared radiation. A positive imbalance means that the Earth is receiving more energy than it is radiating, leading to warming.
• Anthropogenic emissions: Greenhouse gas and aerosol emissions caused by human activities, such as the burning of fossil fuels.
• Greenhouse gases (GHG): Gases in the atmosphere that absorb and emit infrared radiation, thereby trapping heat and contributing to warming (e.g. carbon dioxide, methane).
• Radiative forcing: The rate of change in the Earth's energy balance caused by an external change in the climate system, such as an increase in greenhouse gas concentrations.
• Aerosols: Small solid particles or droplets suspended in the air. Anthropogenic aerosols can have a cooling effect by reflecting sunlight back into space and affecting cloud formation.
• Climate models: Mathematical representations of the Earth's climate system that are used to simulate and predict climate change.
• IPCC (Intergovernmental Panel on Climate Change): The leading international body for assessing climate change, providing regular scientific assessments of climate change, its impacts and future risks.
• Cryosphere: All parts of the Earth's surface where water is in solid form, including glaciers, snow cover, sea ice, and permafrost.
• Albedo of the Earth: The measure of reflectivity of a surface or body. A higher albedo means more reflection of incoming sunlight. A decrease in the Earth's albedo means that the Earth absorbs more sunlight.
• Feedback (climatic): A process in which an initial change in one component of the climate system triggers changes in other components that either amplify (positive feedback) or reduce (negative feedback) the initial change.
• Internal variability: A natural, unpredictable fluctuation in the climate system that is not caused by external forcing (e.g. El Niño, La Niña).
• El Nino: A natural climatic phenomenon characterized by unusually warm surface waters in the central and eastern Pacific Ocean that affects weather around the world.
• CERES (Clouds and the Earth's Radiant Energy System): A set of NASA satellite instruments that measure the Earth's incoming and outgoing radiation.
• TSIS-1 (Total Solar Irradiance Sensor-1): An instrument on the International Space Station that measures total solar irradiance (the amount of solar energy reaching Earth).
• Argo floats: A network of autonomous robotic floats that monitor temperature, salinity and currents in the upper 2000 meters of the ocean, contributing to estimates of ocean heat accumulation.
• Libero: A planned follow-up mission to CERES with similar or improved capabilities is scheduled to launch in 2027.
• Mitigation efforts: Steps taken to reduce greenhouse gas emissions and mitigate climate change, such as phasing out the burning of fossil fuels.