Lesson 5 of 6 · Energy budget and greenhouse effect
Account for the energy entering and leaving Earth
10 minute lesson · 3 checks along the way
Your goal for this lessonCalculate a simple planetary energy imbalance and explain how infrared absorption changes the balance without creating energy.
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Draw the boundary before adding the arrows
Earth receives sunlight, reflects some of it, and emits thermal . To calculate a planetary budget, draw an imaginary boundary above the atmosphere and track energy crossing it. Energy exchanged between the surface and clouds stays inside that boundary until it escapes. Counting internal exchanges as extra sunlight would double-count the same energy. A surface budget and a whole-planet budget can both be useful, but their arrows refer to different boundaries.
The sunlight intercepted by Earth spans a circular cross-section, while the planetary average spreads that power over a sphere with four times the area. Thus an approximate solar irradiance of 1,360 watts per square meter becomes 340 watts per square meter averaged over the whole planet before reflection. This is not the sunlight reaching every location at every moment. Night, latitude, clouds, seasons, and viewing angle produce very different local conditions.
Follow infrared energy through the atmosphere
Earth’s surface emits infrared radiation. Greenhouse gases, including water vapor and carbon dioxide, absorb and emit radiation in particular wavelength ranges. Clouds also interact with outgoing infrared and incoming sunlight. Energy moves through the atmosphere by radiation, convection, and the water cycle. The resulting allows the surface to be warmer than it would be in a comparable simple calculation without infrared-absorbing air.
Increasing long-lived greenhouse gases changes how readily energy escapes to space at the existing temperatures. The system then gains energy until warming and other responses change the flows. Downward infrared radiation does not create energy or make the net heat flow run spontaneously from cold to hot. The surface and atmosphere exchange energy in both directions while the complete budget includes sunlight and emission to space. Water vapor commonly amplifies warming as a ; clouds require attention to competing effects.
Check 1 of 3
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Work through a decision
Balance an invented planet
Take an invented planet receiving a global average of 340 watts per square meter. Let its total reflected fraction be 0.30. Reflection is 340×0.30 = 102 watts per square meter, leaving 238 absorbed. If its outgoing thermal radiation is 235 watts per square meter, the net gain is 3 watts per square meter. Over one square meter for 1,000 seconds, that is 3,000 joules added to the system, assuming the imbalance stays constant.
We cannot convert those 3,000 joules into an exact temperature rise without specifying the amount of material heated, its , phase changes, and transfers. If outgoing radiation later rises to 238 watts per square meter while the other values stay fixed, the top-of-atmosphere budget balances. That does not mean the surface and atmosphere have equal temperatures or stop exchanging energy. These invented values illustrate accounting, not a current measurement of Earth’s imbalance.
Keep internal transfers inside the budget
Original hypothetical global-mean budget. Arrows identify terms, not proportional widths. Incoming 340 minus reflected 102 minus outgoing thermal 235 equals a gain of 3 W/m²; internal exchange is not an extra energy source.
Open full-size diagram in a new tab: Keep internal transfers inside the budgetCheck 2 of 3
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Measure small differences between large flows
The Clouds and the Earth’s Radiant Energy System instruments, known as CERES, have measured Earth’s reflected sunlight and emitted radiation from platforms including Terra and Aqua. Other observations help identify clouds, , and surface conditions within the broad radiation measurements. , sampling across the day, and consistency between instruments matter because a relatively small residual is calculated from much larger incoming and outgoing flows.
Ocean heat measurements provide an additional constraint because much of Earth’s accumulated excess energy enters the ocean. Comparing radiation changes with heat storage tests whether the picture is physically consistent. A bright visible cloud alone cannot determine its net effect: its reflection of sunlight must be considered together with infrared absorption and emission, height, location, and time. Energy accounting turns a visually simple scene into a measurable physical question.
Engineers clean EarthCARE at Airbus in Friedrichshafen, Germany, before shipment to the launch site; ESA published this photograph in February 2024. EarthCARE’s instruments investigate clouds, aerosols, and their roles in reflected sunlight and escaping infrared energy.
Look closely
What does this preparation photograph establish about the measurement process, and what evidence would still be needed to determine a cloud’s net effect on Earth’s energy budget?
See what to notice
It documents the spacecraft and preparation work. A cloud’s net effect requires calibrated radiation and cloud observations across relevant times and locations; neither the spacecraft’s appearance nor a bright cloud in a separate image supplies the full budget.
Check 3 of 3
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Put it together
Your lesson progress
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Go to the source
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