Which greenhouse gas has the biggest effect? CO₂ is the one that comes to mind.
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Actually the largest greenhouse gas is water vapour, which accounts for about 60% of the greenhouse effect. CO₂ is about 20%, and clouds contribute too. But water vapour increases as the atmosphere warms, as a feedback, so in practice CO₂ is the main one we can control directly.
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I see. But halving CO₂ would not halve the temperature, would it?
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Right. The radiative forcing formula is logarithmic, $\Delta F = 5.35\ln(C/C_0)$, so each doubling of concentration adds about 3.7 W/m². The first 100 ppm increase has a large effect, but a 100 ppm increase from 600 to 700 ppm has less. The same applies in reverse.
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I heard the Earth would be −18°C without the greenhouse effect. Why so cold?
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If you consider only the balance between the energy coming from the Sun and the energy escaping to space, the Earth would be −18°C. But the atmosphere acts like a blanket, sending long-wave radiation back down to the surface, which keeps the current average at +15°C. That 33°C difference is what the greenhouse effect gives us.
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So the greenhouse effect itself is not a bad thing!
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That's right: the natural greenhouse effect is essential for life. The problem is that the human addition of CO₂ since the Industrial Revolution is upsetting the balance. Select the "Pre-industrial" scenario in this simulator and compare, and you will see it clearly.
Frequently Asked Questions
Q. What is the difference between the 1.5°C and 2°C targets?
A. The Paris Agreement's "1.5°C target" and "2°C target" are upper limits on warming relative to pre-industrial levels. According to the IPCC Special Report on 1.5°C, the 0.5°C difference changes sea-level rise by 2100 by about 0.1 m, and coral reefs are projected to decline by 70–90% at 1.5°C but by more than 99% at 2°C. Try adjusting CO₂ around 420 ppm in the simulator to compare.
Q. What does it mean that CH₄ (methane) is more potent than CO₂?
A. Over 100 years (GWP100), CH₄ has a greenhouse effect about 27–30 times that of CO₂. Its atmospheric lifetime is short, however, at about 12 years (CO₂ lasts centuries), so cutting methane is effective for curbing warming in the short term.
Q. What is climate sensitivity (ECS)?
A. It is the rise in global mean temperature when the atmospheric CO₂ concentration doubles. The IPCC AR6 central estimate is about 3°C, with a "likely" range of 2.5–4°C. It is the result of feedbacks added to the direct radiative forcing (about 1°C).
Q. How accurate is this model?
A. This tool uses a zero-dimensional energy balance model, a simplified model for education. Real climate models (GCMs) perform three-dimensional coupled calculations of the atmosphere, ocean, land surface and cryosphere. The radiative forcing formulas are the standard ones used in IPCC AR5.
What Is the Greenhouse Effect and Global Warming Simulator?
The physical model of the greenhouse effect and global warming simulator is based on the Earth's energy balance. The balance between short-wave radiation supplied by the Sun and long-wave radiation emitted by the Earth determines the surface temperature. Greenhouse gases in the atmosphere absorb and re-emit long-wave radiation, warming the surface. The radiative forcing \(\Delta F\) of CO₂ is logarithmic in the concentration \(C\): \(\Delta F = 5.35 \ln(C / C_0)\) (\(C_0\) = 280 ppm). In this tool, the total radiative forcing of CO₂, CH₄ and N₂O is converted into a change of the atmospheric absorptivity \(\varepsilon\), and the equilibrium surface temperature is found with the one-layer atmosphere model \(T = T_e / (1-\varepsilon/2)^{1/4}\). Changing each gas concentration or the albedo shows the change in radiative forcing and temperature in real time, giving an intuitive grasp of how the greenhouse effect works physically.
Real-World Applications
Industrial use In the climate policies of companies and local governments, the relationship between radiative forcing and temperature is used to link emission reductions to their effect on temperature. The idea of radiative forcing by gas is also the basis for comparing the effect of cutting emissions not only of CO₂ but of methane and nitrous oxide.
Research and education In university earth-science and environmental-science courses, it is used as a topic for understanding the physics of radiative forcing and energy balance. Calculating the temperature change for a doubling of CO₂ with a simple model like this and comparing it with the IPCC figures shows how large the feedbacks from water vapour, clouds and so on are.
Relation to CAE analysis and practical use This tool is a learning aid for quickly grasping the size of the relationship between radiative forcing and temperature before detailed projections with climate models (GCMs). For practical studies such as building air-conditioning loads or environmental impact assessments, use climate projection data from public institutions.
Common Misconceptions and Points to Note
It is tempting to imagine that "adding greenhouse gases raises the temperature right away", but in reality the large heat capacity of the oceans delays the warming by decades or more. This simulator shows the equilibrium temperature, so when you change a concentration the temperature jumps to its new value immediately; note that in reality it takes a long time to approach that value. It is also tempting to think that "cutting CO₂ alone will stop global warming", but methane (CH₄) and nitrous oxide (N₂O) are powerful greenhouse gases too; CH₄ in particular has a warming effect per unit mass tens of times that of CO₂, so cutting emissions of these gases is just as important. Furthermore, because radiative forcing is logarithmic in gas concentration, the warming from doubling CO₂ from the present level of about 420 ppm cannot be predicted by simple proportional calculation.
Set the CO₂ concentration slider in the range 180–1200 ppm to simulate changes in atmospheric composition from pre-industrial levels (280 ppm) to future scenarios
Adjust the CH₄ concentration in the range 700–4000 ppb and observe the increase in radiative forcing (W/m²) due to methane
Vary the N₂O concentration in the range 270–500 ppb to evaluate the contribution of nitrous oxide to the greenhouse effect
Change the albedo (0.10–0.60) to see how changes in reflectivity, such as from melting ice sheets, affect the global mean temperature
Check the relationship between the radiative forcing ΔF (W/m²) and the temperature rise ΔT (K) calculated in real time, and verify it with the energy balance equation
Worked Example
With CO₂ at 415 ppm, CH₄ at 1900 ppb and N₂O at 332 ppb, the radiative forcing is about 2.9 W/m² and the temperature rise relative to pre-industrial is about 1.3 K. Increasing CO₂ to 560 ppm raises the radiative forcing to about 4.5 W/m² and the temperature anomaly reaches about 2.0 K. Lowering the albedo from 0.30 to 0.29 (assuming melting ice sheets) adds further warming at the same gas concentrations, showing a positive feedback.
Practical Notes
Radiative forcing is calculated with the simplified formulas of Myhre et al. (1998) (CO₂: 5.35 ln(C/C₀); CH₄ and N₂O: differences of square roots of concentration) and converted to temperature with a one-layer atmosphere model. This model's climate sensitivity is about 1.6 K per doubling of CO₂, lower than the IPCC assessment (about 3 K), and it does not include feedbacks such as water vapour and clouds
The albedo parameter is a simplification of changes in sea-ice and snow-covered areas; the real, complex feedback mechanisms (water vapour, clouds, cryosphere) are not included
To evaluate or compare emissions, use official statistics such as the UNEP Emissions Gap Report or national greenhouse gas inventories rather than this simulator's results
This model finds the equilibrium temperature and does not include the time lag due to the heat capacity of the oceans. It cannot be used to predict how temperature will change year by year