What happens to sunlight and heat in the greenhouse effect?
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Earth & Space Science · Foundations
Climate Change
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In 30 seconds
Climate change The long-term shift in average climate conditions, including temperature, precipitation patterns, and sea level. Full entry → is the long-term shift in Earth's average conditions driven by a strengthening Greenhouse effect The warming of Earth's surface by atmospheric gases that absorb outgoing infrared heat and re-radiate some of it back downward. Full entry →. Sunlight passes through the atmosphere and warms the surface, which re-radiates heat; greenhouse gases absorb much of that heat and slow its escape. Human emissions have added more of these gases, and the climate has responded: the decade 2011-2020 was about 1.1 degrees Celsius warmer than 1850-1900, ice is retreating, and seas are rising. How much more warming occurs depends on emissions scenarios.
Why this matters
Climate change is one of the most consequential scientific findings of our time, and it is also one of the most misrepresented in public discussion. Knowing the underlying physics - how greenhouse gases warm the planet - and the evidence - thermometers, satellites, ice cores, tide gauges - lets a person tell a verified claim from a distorted one. The numbers matter: how much the planet has warmed, how fast seas are rising, and what range of future warming the emissions scenarios imply. Because greenhouse gases linger in the atmosphere for a long time, choices made now shape conditions for decades. This lesson gives you the science; what societies choose to do with it is a separate question.
The college version
The physics: sunlight in, heat trapped
Climate change starts with the greenhouse effect, well-established physics. Sunlight arriving at the top of the atmosphere is shortwave radiation: most of it passes through the air and clouds to the surface, where it is absorbed and warms the land and ocean. The warmed surface then radiates energy back toward space as longwave infrared heat, not visible light. Greenhouse gases - carbon dioxide, methane, nitrous oxide, water vapor, and others - absorb much of that outgoing infrared and emit it in every direction, so part of the energy returns toward the surface. NASA's framing: about half the light energy reaching Earth's atmosphere passes through to the surface, and about 90 percent of the heat the surface radiates is absorbed by greenhouse gases and re-radiated, slowing heat loss to space. The greenhouse effect is natural and essential: without greenhouse gases, Earth would be too cold to support life. The concern: human activities have added large amounts of greenhouse gases, mainly carbon dioxide from burning fossil fuels, strengthening the effect. The climate topic owns what climate is; this lesson covers the ongoing change itself.
What the observations show
The changes are measured, not modeled. The IPCC's Sixth Assessment Report states that global surface temperature was 1.09 [0.95 to 1.20] degrees Celsius higher in 2011-2020 than in 1850-1900, and that each of the last four decades has been successively warmer than any decade that preceded it since 1850. NASA's Goddard Institute for Space Studies reports that the 10 most recent years are the warmest on record: global temperatures in 2025 averaged 2.14 degrees Fahrenheit (1.19 degrees Celsius) above the 1951-1980 average, and the hottest year on record remains 2024. Atmospheric carbon dioxide measured 410 parts per million in 2019, higher than at any time in at least 2 million years, and NASA's most recent measurement is 429 ppm (July 2026). The cryosphere is responding: the IPCC assesses that human influence is very likely the main driver of the global retreat of glaciers since the 1990s and of the decrease in Arctic sea ice area, about 40 percent in September, between 1979-1988 and 2010-2019. The ocean is rising: global mean sea level increased by 0.20 [0.15 to 0.25] meters between 1901 and 2018, with the rate rising from 1.3 mm per year in 1901-1971 to 3.7 mm per year in 2006-2018. NOAA Climate.gov states it in everyday units: average sea level has risen about 8-9 inches since 1880, mostly from glacier and ice-sheet meltwater plus thermal expansion of seawater.
Attribution: what the IPCC actually says
Attribution The scientific determination of what caused an observed change, such as estimating how much of the warming comes from human activity. Full entry → is the scientific question of what caused an observed change, and the IPCC answers it in its own calibrated language. The Synthesis Report of the Sixth Assessment says: "It is unequivocal that human influence has warmed the atmosphere, ocean and land," and that human activities, principally through emissions of greenhouse gases, have unequivocally caused Global warming The long-term increase in Earth's average surface temperature, always measured relative to a stated reference period. Full entry →, with global surface temperature reaching 1.1 degrees Celsius above 1850-1900 in 2011-2020. It quantifies the human contribution: the likely range of total human-caused warming from 1850-1900 to 2010-2019 is 0.8 to 1.3 degrees Celsius, with a best estimate of 1.07 degrees. These statements use IPCC likelihood terms: virtually certain means 99-100 percent probability, extremely likely 95-100 percent, very likely 90-100 percent, and likely 66-100 percent; square brackets mark the assessed very likely (90 percent) interval. The same report assesses that it is virtually certain that hot extremes have become more frequent and more intense across most land regions since the 1950s, with high confidence that human-induced climate change is the main driver. NASA adds that the evidence shows the current warming cannot be explained by the Sun.
Projections: the future depends on emissions scenarios
Future warming is not a single predicted number; it depends on how much Greenhouse gas An atmospheric gas such as carbon dioxide, methane, or water vapor that absorbs infrared radiation emitted by the surface. Full entry → humanity emits, so the IPCC organizes projections around emissions scenarios. Global surface temperature will continue to increase until at least mid-century under all emissions scenarios considered. For 2081-2100, warming relative to 1850-1900 is very likely to be 1.0 to 1.8 degrees Celsius under the very low Emissions scenario A plausible future pathway of greenhouse gas emissions used to project how the climate will respond. Full entry → (SSP1-1.9), 2.1 to 3.5 degrees under the intermediate scenario (SSP2-4.5), and 3.3 to 5.7 degrees under the very high emissions scenario (SSP5-8.5). The Synthesis Report summarizes the same spread with best estimates of 1.4, 2.7, and 4.4 degrees. Sea level follows the same pattern: the likely rise by 2100, relative to 1995-2014, is 0.28-0.55 meters under SSP1-1.9 but 0.63-1.01 meters under SSP5-8.5. How much the planet warms this century tracks the emissions path the world follows.
Impacts, and two broad categories of response
The IPCC describes the impacts factually. Hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s, with human-induced climate change the main driver (high confidence). Heavy precipitation events have increased in frequency and intensity since the 1950s over most land area. Sea level rise will continue for a long time because the ocean takes up heat slowly and ice sheets respond on long timescales. These are general statements; patterns vary by region. Responses fall into two broad categories. Mitigation Actions that reduce greenhouse gas emissions or enhance their removal from the atmosphere. Full entry → means reducing greenhouse gas emissions or enhancing their removal from the atmosphere - for example, shifting energy production away from fossil fuels or protecting forests that absorb carbon dioxide. Adaptation Adjustments made in response to actual or expected climate change in order to reduce harm. Full entry → means adjusting to actual or expected climate change to reduce harm - for example, raising seawalls or planting different crops. The IPCC frames climate-resilient development as integrating adaptation and mitigation. Which mix of actions societies choose is a matter of policy and values, not a conclusion of this lesson.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Earth stays warm because of the greenhouse effect, and it is real physics, not an opinion. Sunlight is shortwave energy: it passes through the air, hits the ground and the sea, and warms them. The warm surface gives off longwave heat, like a stove gives off heat after you turn it off. Greenhouse gases in the air - carbon dioxide, methane, water vapor - catch a lot of that heat and send some of it back down, so it leaves space more slowly. That is how the planet stays warm enough for life. The catch: people have been burning fossil fuels for about two centuries, adding extra carbon dioxide, so the atmosphere now catches more heat than it used to. The planet has warmed about 1.1 degrees Celsius since the late 1800s, ice is melting, and the sea is rising. How much more it warms depends on how much more greenhouse gas we add.
Picture it like this
Imagine a winter jacket. Your body makes heat, and the jacket traps some of it so you stay warm. The natural greenhouse effect is the jacket Earth already wears - without it the planet would be freezing. Burning fossil fuels is like adding extra lining to the jacket: a little more insulation is fine, but too much, and the wearer overheats. The planet is that wearer, and the extra lining is the carbon dioxide we have added.
Where the picture stops working
The analogy has limits. A jacket only delays heat loss by minutes; greenhouse gases delay it for decades to centuries, which is why warming continues long after emissions change. A jacket is passive, while the climate system has feedbacks - for example, melting ice exposes darker water that absorbs more sunlight. And no one can simply take the jacket off: carbon dioxide already in the atmosphere stays there for a very long time. The analogy captures the trapping idea, not the timescales or feedbacks.
Worked example
A student reads a news headline: '2025 was 1.19 degrees Celsius warmer than the 1951-1980 average, and the 10 most recent years are the warmest on record.' The first number is an anomaly: each year's global average is compared with the average of the reference period 1951-1980, so 1.19 degrees is the amount by which 2025 exceeded that baseline, not the total temperature. The second statement places the decade in context: no earlier decade in the 1880-2025 record was as warm as the last ten years. To interpret responsibly, the student keeps the reference period attached to the number, because NASA and IPCC use different baselines (1951-1980 versus 1850-1900), and a number without its baseline is meaningless. The same discipline applies to projections: a 2081-2100 range is only interpretable alongside its scenario and reference period.
Key takeaway
The greenhouse effect is well-established physics, human influence on the climate is unequivocal, observed warming, ice loss, and sea level rise are measured and documented, and how much more the planet warms depends on the emissions scenario the world follows.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
According to the IPCC's Sixth Assessment Report, how confident are scientists that human influence has warmed the planet?
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NASA reports that global temperatures in 2025 averaged 1.19 degrees Celsius above the 1951-1980 average, and that the 10 most recent years are the warmest on record. What does this tell you?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Explain the greenhouse effect as well-established physics: shortwave sunlight passes through the atmosphere, and greenhouse gases absorb and re-radiate the longwave heat the surface emits.
- State the observed changes in global average temperature, ice, and sea level, using the IPCC, NASA, and NOAA figures with each source's own framing.
- Explain what the IPCC means by its statement that human influence on the climate system is unequivocal, and interpret its confidence language.
- Analyze how projected future warming depends on emissions scenarios, using the IPCC's scenario ranges for 2081-2100.
- Apply the distinction between mitigation and adaptation to concrete responses to climate change.
Common mistakes
Treating the greenhouse effect itself as a problem to be eliminated.
The greenhouse effect is natural and necessary; without greenhouse gases Earth would be too cold for life. The concern is the human-caused strengthening of the effect from added emissions.
Quoting a temperature number without its reference period.
NASA's anomaly is measured against 1951-1980 and the IPCC's against 1850-1900; a number without its baseline cannot be compared with another.
Reading 'very likely' as 'maybe' or as 'certain'.
In IPCC usage, very likely is a calibrated term meaning 90-100 percent probability, and virtually certain means 99-100 percent; these are precise statements, not hedging.
Confusing mitigation with adaptation.
Mitigation reduces greenhouse gas emissions or enhances their removal; adaptation adjusts to actual or expected climate change. Reducing fossil-fuel use is mitigation; building seawalls is adaptation.
Treating projections as a single predicted outcome.
Projected warming depends on emissions scenarios: the IPCC's 2081-2100 ranges differ by several degrees between the very low (SSP1-1.9) and very high (SSP5-8.5) scenarios, so there is no one 'predicted' temperature.
Easily confused
Weather vs. Climate change
Weather is the short-term state of the atmosphere at a time and place; climate change is the long-term shift in average conditions measured over decades, so a single cold winter neither confirms nor refutes it.
The natural greenhouse effect vs. The enhanced greenhouse effect
The natural effect, from water vapor and other gases, keeps Earth warm enough for life; the enhanced effect refers to the extra warming caused by human-added greenhouse gases.
Mitigation vs. Adaptation
Mitigation acts on the cause by reducing greenhouse gas emissions or enhancing their removal; adaptation acts on the consequences by adjusting to actual or expected change.
Emissions scenario vs. Forecast
A scenario is a plausible future emissions pathway used to explore possible climates, not a prediction of what will happen; a forecast is a statement about what is expected under given conditions.
Key vocabulary
- Greenhouse effect
- The warming of Earth's surface by atmospheric gases that absorb outgoing infrared heat and re-radiate some of it back downward.
- Greenhouse gas
- An atmospheric gas such as carbon dioxide, methane, or water vapor that absorbs infrared radiation emitted by the surface.
- Global warming
- The long-term increase in Earth's average surface temperature, always measured relative to a stated reference period.
- Climate change
- The long-term shift in average climate conditions, including temperature, precipitation patterns, and sea level.
- Attribution
- The scientific determination of what caused an observed change, such as estimating how much of the warming comes from human activity.
- Emissions scenario
- A plausible future pathway of greenhouse gas emissions used to project how the climate will respond.
- Mitigation
- Actions that reduce greenhouse gas emissions or enhance their removal from the atmosphere.
- Adaptation
- Adjustments made in response to actual or expected climate change in order to reduce harm.
Sources & references
- Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change — Summary for Policymakers — Intergovernmental Panel on Climate Change (IPCC)
- Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report — Summary for Policymakers — Intergovernmental Panel on Climate Change (IPCC)
- Global Temperature (Vital Signs of the Planet) — NASA Climate Change (Goddard Institute for Space Studies)
- The Causes of Climate Change — NASA Science (science.nasa.gov)
- Carbon Dioxide (Vital Signs of the Planet) — NASA Climate Change (citing NOAA Mauna Loa Observatory data)
- Climate Change: Global Sea Level — NOAA Climate.gov
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-21
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