Earth's energy budget

• 1. Introduction

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  Earth's energy budget describes the balance between the sunlight our planet absorbs and the heat it radiates back into space. Get that balance wrong by even a fraction, and the climate shifts. Right now, it is wrong. Since at least 1970, Earth has been absorbing more energy than it releases, and that imbalance is measurable from orbit.

  The numbers involved are staggering. Every second, the Sun delivers roughly 340 watts of energy for each square meter of Earth's surface area. Meanwhile, the planet is steadily warming at about 0.18 degrees Celsius per decade since around 1970. Those two facts are connected by a cascade of physical processes spanning the upper atmosphere down to ocean water a kilometer deep.

  What keeps the books from balancing? Why do greenhouse gases matter as much as scientists say? What happens to all that extra heat once it enters the system? And what would it actually take to stop the warming? The answers lie inside the accounting.

• 2. Sun, Reflection, and Absorption

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  About 340 watts per square meter of solar energy arrive at the top of Earth's atmosphere every second, averaged across the globe and across the year. Not all of it stays. Around 77 W/m2 bounces back to space from clouds and the atmosphere, and another 23 W/m2 reflects off bright surfaces like snow and ice. What remains, roughly 240 W/m2, is the absorbed solar radiation that actually drives the climate.

  That reflective fraction has a name: the Bond albedo, which averages close to 0.3 for Earth as a whole. Clouds are responsible for about half of that reflectivity. They are also the single most complex variable in the energy budget, acting simultaneously as mirrors, insulators, and feedback mechanisms depending on their altitude, thickness, and location.

  The Sun does not heat the planet evenly. Tropical regions receive far more direct radiation than the poles, and that gradient is what sets weather systems in motion. As the energy seeks equilibrium across latitudes, it drives the atmosphere, the oceans, ice sheets, and living ecosystems, producing what we experience as climate.

• 3. How Heat Leaves the Planet

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  Energy that the Earth absorbs cannot stay forever; physics demands it return to space. It does so as outgoing longwave radiation, thermal energy emitted at wavelengths far longer than sunlight, in the infrared band.

  The path from absorbed sunlight to escaping heat is not direct. Some of the surface's thermal emission passes straight through the atmosphere via what scientists call the atmospheric window, escaping unimpeded. The rest must travel upward through the atmosphere by other routes: evaporation carries heat away from wet surfaces as latent heat, convection and turbulence lift warm air masses, and infrared radiation passes layer by layer through the air until it can finally escape at the top.

  Greenhouse gases complicate that exit route. Molecules of carbon dioxide, water vapor, and methane absorb outgoing infrared radiation and re-emit it in all directions, including back toward the surface. An increase in those gases forces a decrease in outgoing longwave radiation, producing a warming energy imbalance. Surface temperatures must then rise until enough additional heat is radiated to restore balance. The atmosphere's emissivity, a measure of how effectively it radiates, captures this effect mathematically, and the fourth-power sensitivity of thermal radiation to temperature keeps the system from running away, at least in the long run.

• 4. The Ocean as a Heat Vault

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  The top few meters of Earth's oceans hold more energy than the entire atmosphere. That single fact explains why climate change unfolds over decades rather than years.

  Ocean water is an extraordinarily effective absorber of solar energy, and its sheer mass gives it a thermal inertia that dwarfs the land surface or the cryosphere. When the atmosphere receives extra heat from the greenhouse effect, most of it flows into the sea. Over 90 percent of the extra energy that has accumulated on Earth from ongoing global warming since 1970 has been stored in the ocean. About one-third of that ocean heating has reached depths below 700 meters.

  The pace of ocean heat uptake has accelerated. As of 2020, the rate of growth was approaching 500 TW, equivalent to roughly 1 W/m2. That translated to about 14 zettajoules of heat gain in a single year, a figure that exceeds the total primary energy consumed by all of humanity by a factor of at least 20. Since at least 1990, ocean heat content has increased at a steady or accelerating rate, and since the year 2000 a global network of nearly 4,000 Argo robotic floats has tracked that warming in real time.

• 5. Measuring the Imbalance

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  During 2005 to 2019, the Earth's energy imbalance averaged about 460 terawatts, or 0.90 watts per square meter globally. That figure is not a model projection; it is an observed measurement assembled from satellites, ocean floats, and geodetic surveys.

  The NASA Earth Radiation Budget Experiment put three satellites into orbit in the mid-1980s: the Earth Radiation Budget Satellite, launched in October 1984; NOAA-9, launched in December 1984; and NOAA-10, launched in September 1986. Since March 2000, NASA's Clouds and the Earth's Radiant Energy System, or CERES, instruments have extended and refined those measurements. CERES data showed that the energy imbalance rose from roughly +0.42 W/m2 in 2005 to +1.12 W/m2 in 2019. Contributing factors included more water vapor, fewer clouds, rising greenhouse gas concentrations, and declining ice cover, partially offset by rising temperatures.

  Other researchers found a radiative forcing rise of +0.53 from 2003 to 2018 using CERES, AIRS, and CloudSat data together. About 80 percent of that increase traced to reduced outgoing longwave radiation caused by growing concentrations of greenhouse gases. Estimates of the imbalance across different time periods show a clear upward trend: the 1971-2006 average was 0.50 W/m2, rising to 0.57 for 1971-2018, and then to 0.99 for the 2012-2024 window.

• 6. Minor Sources, Major Context

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  Not all of Earth's energy comes from the Sun. Geothermal heat flowing from the interior contributes an estimated 47 terawatts, split roughly equally between heat left over from the planet's formation and heat generated by radioactive decay in the crust and mantle. That translates to an average flux of 0.087 W/m2, just 0.027 percent of the solar contribution.

  Human energy production in 2019 averaged about 18 TW, equivalent to roughly 160,000 TW-hours for the year. That is also small compared to solar input, but the combustion that generates most of that energy loads additional greenhouse gases into the atmosphere, producing a warming effect more than 20 times larger than the direct heat of burning.

  Photosynthesis captures an estimated 140 TW of incident energy, around 0.08 percent, allowing plants to build biomass. That same energy flows back out again as heat when plants are eaten or burned. Joseph Fourier, in a paper often cited as the first scientific work on the greenhouse effect, once argued that radiation from deep space played a meaningful role in Earth's temperature; later analysis showed it was negligible compared to the processes he was among the first to recognize.

• 7. Why the Budget Matters for Policy

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  Climate scientists Kevin Trenberth, James Hansen, and colleagues have described the monitoring of Earth's energy imbalance as the most important metric for guiding climate policy. Because the ocean absorbs heat slowly and releases it just as slowly, the climate system carries changes in the pipeline that surface temperatures have not yet fully expressed. Tracking EEI lets scientists forecast how much additional warming is already locked in, regardless of current emissions.

  In 2012, NASA scientists reported that halting global warming would require reducing atmospheric CO2 to 350 parts per million or below, assuming all other forcings remained fixed. By 2020, atmospheric CO2 had reached 415 ppm, and all long-lived greenhouse gases together exceeded a 500 ppm equivalent concentration.

  The rate of heating from the current human-caused imbalance is, in the source's words, without precedent. Natural climate forcings, such as the changes in solar output over an 11-year solar cycle, produce swings in radiative forcing that are smaller in magnitude than the shifts caused by recent trends in greenhouse gas concentration alone. Understanding how much energy is flowing where, and how fast, is not just an academic exercise; it is the foundation for every estimate of how much warming to expect and how quickly action must be taken.