Earth's energy balance is the way Earth manages the energy it receives from the Sun and the energy it sends back into space. Energy from Earth's inside, like heat from deep within the planet, is also considered, but it has very little effect compared to energy from the Sun. The energy balance also includes how energy moves through Earth's climate system, which includes water, ice, air, land, and living things. The Sun warms the tropical areas near the equator more than the colder polar regions. This causes the amount of sunlight reaching different parts of Earth to be uneven. As energy moves to balance across the planet, it affects Earth's climate system. This process creates Earth's climate.

Earth's energy balance depends on many factors, such as tiny particles in the air (aerosols), gases in the atmosphere that trap heat (greenhouse gases), how much sunlight is reflected by Earth's surface (surface albedo), clouds, and how land is used. When the energy Earth receives from the Sun equals the energy it sends back to space, Earth is in radiative equilibrium, and the climate system stays stable. Global warming happens when Earth receives more energy than it sends back, and global cooling occurs when Earth sends more energy back than it receives.

Measurements show that Earth has been gaining more energy than it sends back since at least 1970. This increase in energy is faster than any previous time in history. The main cause of changes in Earth's energy balance is human activities that change the atmosphere's composition. Between 2005 and 2019, Earth's energy imbalance averaged about 460 terawatts, or about 0.90 watts per square meter globally.

Changes in Earth's energy balance take time to affect the planet's surface temperature. This is because Earth's oceans, land, and frozen areas (cryosphere) absorb and release heat slowly. Most climate models accurately calculate how much heat is stored, how energy moves, and how long it takes for these changes to affect temperature.

Definition

Earth's energy budget describes the important ways energy moves that affect Earth's climate. These include how much energy enters and leaves Earth's atmosphere from space, how energy is exchanged between Earth's surface and the air above it, how energy is stored or used up around the world over time, and how energy moves between different parts of Earth's climate system, like oceans and land.

Earth's energy flows

Earth maintains a relatively constant temperature even though large amounts of energy move into and out of it. This happens because Earth sends back to space about the same amount of energy it receives from the Sun. Earth emits energy through radiation from the atmosphere and the surface, which has longer wavelengths than the sunlight it absorbs.

The main cause of changes in Earth's energy balance is human activities that change the atmosphere's composition. These changes are about 460 terawatts globally, or 0.90 plus or minus 0.15 watts per square meter.

The total energy Earth receives each second at the top of its atmosphere (TOA) is measured in watts. This is calculated by multiplying the solar constant by Earth's cross-sectional area (the area of a circle). Since Earth's total surface area is four times its cross-sectional area, the average energy received globally each year is one-fourth of the solar constant, or about 340 watts per square meter. These numbers are averages from satellite measurements over many years and account for differences in sunlight absorption due to location, time of day, seasons, and years.

Of the approximately 340 watts per square meter of sunlight Earth receives, about 77 watts per square meter is reflected back to space by clouds and the atmosphere, and about 23 watts per square meter is reflected by Earth's surface. This leaves about 240 watts per square meter of energy absorbed by Earth, called absorbed solar radiation (ASR). This means Earth's average net albedo (a measure of how much sunlight is reflected) is about 0.3, also known as its Bond albedo.

Energy leaves Earth as outgoing longwave radiation (OLR), which is thermal radiation emitted by Earth's surface and atmosphere. This radiation is in the infrared range, though infrared can also be shortwave radiation. A common threshold of 4 microns is used to separate longwave and shortwave radiation.

Absorbed solar energy is converted into heat energy. Some of this heat is emitted directly into space through the "atmospheric window," a range of wavelengths that pass through the atmosphere without being absorbed. The rest of the heat moves upward through the atmosphere via processes like evaporation, conduction, convection, or radiation. Eventually, all outgoing energy escapes into space as longwave radiation.

The movement of longwave radiation through Earth's atmosphere is described by equations like Schwarzschild's equation for radiative transfer and follows Kirchhoff's law of thermal radiation.

A simple one-layer model estimates OLR and predicts surface temperatures (T_s = 288 Kelvin) and temperatures in the middle of the troposphere (T_a = 242 K) close to observed values. In this model, σ is the Stefan–Boltzmann constant, and ε is the atmosphere's emissivity, which is less than 1 because the atmosphere does not emit radiation in the atmospheric window.

Aerosols, clouds, water vapor, and greenhouse gases contribute to an effective emissivity of about 0.78. Because energy flow is highly sensitive to temperature (fourth power), small temperature changes can balance incoming and outgoing energy flows.

From space, greenhouse gases affect Earth's atmospheric emissivity. Changes in atmospheric composition can shift the overall radiation balance. For example, more greenhouse gases trap more heat, reducing OLR and creating a warming imbalance. Eventually, surface temperatures adjust until absorbed solar radiation equals outgoing longwave radiation (ASR = OLR).

Geothermal heat from Earth's interior is about 47 terawatts, split equally between heat from Earth's formation and radiogenic heat. This is an average of 0.087 watts per square meter, or 0.027% of Earth's total energy budget, which is much smaller than the 173,000 terawatts of incoming solar energy.

Human energy production is about 18 terawatts, or 160,000 terawatt-hours per year in 2019. Energy use is growing rapidly, and burning fossil fuels increases greenhouse gases, creating a much larger imbalance in energy flows compared to natural processes.

Photosynthesis captures about 140 terawatts of energy, providing plants with energy to create biomass. A similar amount of energy is released when plants are used as food or fuel.

Other minor energy sources, like interplanetary dust, solar wind, and radiation from other stars, are usually ignored in calculations. Earlier, Joseph Fourier suggested that deep space radiation was significant in a paper on the greenhouse effect.

Budget analysis

Earth's energy budget is balanced when the amount of energy that comes into Earth equals the amount that leaves. Some of the incoming energy is reflected back into space, so the balance can also be described as the energy Earth absorbs from the sun being equal to the energy it sends back to space as heat.

Imagine the total sunlight reaching Earth's atmosphere as 100 units (equal to 340 W/m²), as shown in a diagram. About 35 units are reflected back to space: 27 from clouds, 2 from snow and ice, and 6 from other parts of the atmosphere. The remaining 65 units (220 W/m²) are absorbed: 14 by the atmosphere and 51 by Earth's surface.

The 51 units absorbed by Earth's surface are later released as heat. Of these, 17 units are directly sent back to space, while 34 units are absorbed by the atmosphere. These 34 units are absorbed through processes like evaporation (19 units), air movement (9 units), and heat trapped by greenhouse gases (6 units). The 48 units absorbed by the atmosphere (34 from Earth's surface and 14 from sunlight) are then released back to space. This example does not include all details about how heat is moved or stored.

In total, the 65 units of energy absorbed by Earth (17 from the surface and 48 from the atmosphere) are emitted as outgoing longwave radiation (OLR). This balances the 65 units of energy Earth absorbs from the sun, keeping Earth's overall energy gain at zero.

Earth's land, ice, and oceans, along with the atmosphere, are key parts of the climate system. These components have large mass and heat storage capacity, meaning they take longer to warm or cool. When energy is absorbed or surface temperatures change, heat moves into or out of these components through conduction or convection. The movement of water between solid, liquid, and gas forms also affects energy storage, as heat is released or absorbed during these changes. These processes help slow rapid temperature changes, making day-night temperature differences smaller. Similarly, Earth's climate system responds slowly to changes in energy balance.

The top few meters of Earth's oceans contain more energy than the entire atmosphere. Like air, ocean water moves energy across Earth's surface. Heat can also move into or out of deep ocean layers depending on conditions. Scientists study these movements by measuring changes in ocean heat content.

Since 1970, more than 90% of the extra energy from global warming has been stored in Earth's oceans. About one-third of this energy has reached depths below 700 meters. The rate of energy gain has increased in recent decades, reaching nearly 500 terawatts (1 W/m²) by 2020. This added energy equals about 14 zettajoules (ZJ) per year, which is more than 20 times the total energy used by humans globally.

Changes in Earth's energy balance are caused by external factors (natural and human-related, such as changes in radiation or other processes), feedbacks within Earth's systems, and natural variations. These changes are seen as shifts in temperature, clouds, water vapor, aerosols, greenhouse gases, surface reflectivity, and other factors. Earth's heating or cooling rate over time can be calculated by measuring the net energy changes linked to these factors.

When Earth's temperature rises, it emits more heat to space, which is called the Planck response. This process reduces the amount of heat Earth retains.

The increase in greenhouse gases since the 1970s has strengthened the greenhouse effect, adding more heat to Earth's system. In contrast, large volcanic eruptions, like those of Mount Pinatubo (1991) and El Chichón (1982), release sulfur compounds into the atmosphere. These compounds can reflect sunlight, temporarily reducing Earth's heat gain. Human-made aerosols can also affect heat balance, sometimes increasing or decreasing it depending on their type. Changes in the sun's activity have smaller effects compared to human-caused changes in greenhouse gases.

Climate changes are complex because they can create feedbacks that either strengthen or weaken the original cause. For example, rising temperatures increase water vapor, which enhances the greenhouse effect, leading to more warming. Another feedback is the ice-albedo effect: when ice melts, Earth's surface becomes darker, absorbing more heat and causing more ice to melt. These feedbacks, except for the Planck response, tend to increase global warming or cooling.

Clouds cover about half of Earth's surface and strongly influence how much sunlight is reflected. They also play a role in climate changes, either as feedbacks or direct causes. The effect of clouds on energy balance depends on their type and location. Scientists use satellite data and computer models to better understand clouds and reduce uncertainty in climate predictions.

Earth's energy imbalance (EEI)

The Earth's energy imbalance (EEI) is the difference between the energy the Earth receives from the Sun and the energy it sends back into space. This imbalance happens because greenhouse gases in the atmosphere trap some of the energy, causing the planet to warm over time.

If the energy the Earth absorbs (called absorbed solar radiation, or ASR) is greater than the energy it emits (called outgoing longwave radiation, or OLR), the planet gains heat and warms. If ASR is less than OLR, the planet loses heat and cools. This follows the law of energy conservation, which states that energy cannot be created or destroyed, only transferred.

A positive EEI means the Earth is gaining heat energy. This is usually measured in watts per square meter (W/m²). Between 2005 and 2019, the average EEI was about 460 terawatts (TW) globally, or 0.90 ± 0.15 W/m².

Large changes in EEI can be measured by satellites orbiting Earth. If EEI does not return to balance over time, it can cause long-term changes in temperature, sea level, ice, and other parts of the climate system. These changes also help scientists measure EEI.

The biggest changes in EEI come from human activities that alter the atmosphere, such as burning fossil fuels. These activities increase greenhouse gases like carbon dioxide, which trap heat and raise EEI. Pollution, such as tiny particles in the air called aerosols, can also affect EEI. Some aerosols absorb energy and increase warming, while others reflect energy and reduce warming.

Scientists cannot directly measure the exact size of EEI from space, but they can track changes over time using satellites. The best way to estimate EEI is by studying how energy is stored in the climate system. The largest energy reservoir is the ocean, which holds most of the extra heat from the imbalance.

To calculate how much heat is stored in the climate system, scientists use information about the heat capacity, density, and temperature of each part of the system, such as the ocean, land, and ice. Most areas are well monitored, but the deep ocean is less understood.

EEI has been positive for over 50 years because global temperatures have risen. Global surface temperature (GST) is calculated by averaging sea surface temperatures and land air temperatures. Reliable data since 1880 shows that GST has increased by about 0.18°C every decade since the 1970s.

Oceans absorb a lot of heat because they have a much larger heat capacity than the atmosphere. Before 1960, scientists measured ocean temperatures using research ships. Since 2000, thousands of robotic floats called Argo have measured ocean temperatures globally. Since the 1990s, ocean heat content (OHC) has increased steadily, and this change accounts for most of the EEI because oceans have absorbed over 90% of the extra energy entering the climate system.

Earth's surface and ice-covered areas absorb little heat compared to the ocean. Heat flows into these areas slowly through a process called thermal conduction. Much of the extra heat is used to melt ice, thaw permafrost, or evaporate water from the ground.

Satellites measure the energy Earth absorbs and emits, which helps scientists calculate EEI. NASA's Earth Radiation Budget Experiment (ERBE) used three satellites launched between 1984 and 1986 to study this.

NASA's Clouds and the Earth's Radiant Energy System (CERES) instruments, part of the Earth Observing System since 2000, measure both sunlight reflected by Earth and heat radiated back into space. CERES data show that EEI increased from +0.42 ± 0.48 W/m² in 2005 to +1.12 ± 0.48 W/m² in 2019. Factors like more water vapor, fewer clouds, and rising greenhouse gases contributed to this increase, while higher temperatures partially offset it.

Other studies using CERES and other satellite data found that radiative forcing (the net effect of energy changes) increased by +0.53 ± 0.11 W/m² between 2003 and 2018. About 80% of this increase was due to rising greenhouse gas concentrations, which reduced the amount of heat Earth emitted into space.

Additional satellite data, such as from TRMM and CALIPSO, show that increased precipitation is linked to more energy leaving Earth through evaporation. This process helps balance some of the heat trapped by greenhouse gases.

Satellite measurements are limited by uncertainties in how they detect energy, but changes in EEI over time can still be measured accurately.

Since 1994, ice has melted across Earth at an increasing rate, and global sea levels have risen due to melting ice and warmer oceans. These changes affect Earth's shape and gravity.

Scientists use satellite data from GRACE to track changes in water and ice distribution. These data match other measurements of ocean heat content and EEI, showing agreement within expected errors.

Climate scientists like Kevin Trenberth and James Hansen say monitoring EEI is important for helping policymakers address climate change. Because the climate system takes time to respond, long-term EEI trends can predict future changes.

Scientists say EEI is the most important measure of climate change. It reflects all the processes and feedbacks in the climate system. Understanding how extra energy affects weather and rainfall is key to predicting extreme weather events.

In 2012, NASA scientists said reducing atmospheric CO₂ to 350 ppm or less would stop global warming, assuming other factors stayed the same. By 2020, CO₂ levels reached 415 ppm, and long-lived greenhouse gases exceeded 500 ppm CO₂-equivalent due to human emissions.