Greenhouse Gases, Albedo, and Feedback Loops
Earth sits about 150 million km from the Sun, yet the average temperature at the surface is a comfortable 15 °C instead of a frozen −18 °C. Why? And why is the Arctic warming faster than almost anywhere else? Three ideas answer both questions: greenhouse gases, albedo, and feedback loops.
The greenhouse effect
Sunlight passes through the atmosphere and warms the surface. The warm surface sends energy back out as infrared (heat) radiation. Greenhouse gases absorb part of that infrared radiation and re-emit it in all directions, including back toward the ground. The result is a warmer surface than there would be without them.
The main greenhouse gases:
| Gas | Natural source | Human source |
|---|---|---|
| Water vapor (H₂O) | Evaporation from oceans | Little directly; rises as air warms |
| Carbon dioxide (CO₂) | Respiration, volcanoes | Burning coal, oil, and natural gas |
| Methane (CH₄) | Wetlands | Cattle, landfills, natural gas leaks |
| Nitrous oxide (N₂O) | Soil microbes | Fertilizers |
Nitrogen and oxygen make up about 99% of the air, but they do not absorb infrared radiation. Tiny amounts of greenhouse gases do the job.
Now put numbers on it. The Sun delivers the solar constant $S = 1{,}361\ \text{W/m}^2$ to a surface facing it. Earth is a sphere, so on average each square meter gets one-fourth of that. A fraction $\alpha$ (the albedo, next section) is reflected.
$$\text{absorbed} = \frac{S(1-\alpha)}{4}$$Name three greenhouse gases. For each one, give one natural source or one human source.
Name three greenhouse gases. For each one, give one natural source or one human source.
The average solar energy arriving at the top of the atmosphere (solar constant) is $1{,}361\ \text{W/m}^2$ on a surface facing the Sun. Because Earth is a sphere, the…
The average solar energy arriving at the top of the atmosphere (solar constant) is $1{,}361\ \text{W/m}^2$ on a surface facing the Sun. Because Earth is a sphere, the average over the whole planet is one-fourth of this. Earth's albedo is 0.30.
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Calculate the average absorbed solar energy per square meter, $\frac{S(1-\alpha)}{4}$.
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Without any greenhouse effect, Earth's average surface temperature would be about $-18\,^\circ\text{C}$. The actual average is about $15\,^\circ\text{C}$. Calculate the difference.
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Explain how greenhouse gases cause this difference.
A class models the greenhouse effect with two identical clear bottles under a lamp. Bottle A contains air. Bottle B contains air with extra CO$2$ produced by mixing…
A class models the greenhouse effect with two identical clear bottles under a lamp. Bottle A contains air. Bottle B contains air with extra CO$_2$ produced by mixing vinegar and baking soda in a separate container and transferring the gas. A thermometer in each bottle is read before the lamp turns on and after 20 minutes.
| Bottle | Start ($^\circ$C) | After 20 min ($^\circ$C) |
|---|---|---|
| A (air) | 24.1 | 26.0 |
| B (extra CO$_2$) | 24.0 | 28.2 |
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Calculate the temperature change in each bottle and the difference between them.
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Use the model to explain how greenhouse gases trap thermal energy.
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Identify at least two ways the model differs from Earth's real atmosphere.
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Suggest one change that would make the experiment more reliable.
Albedo
Albedo ($\alpha$) is the fraction of incoming sunlight that a surface reflects. It has no units and ranges from 0 (absorbs everything) to 1 (reflects everything).
| Surface | Albedo |
|---|---|
| Fresh snow | 0.85 |
| Desert sand | 0.40 |
| Forest | 0.15 |
| Open ocean | 0.06 |
Light surfaces reflect most sunlight and stay cool. Dark surfaces absorb it and warm up. Think of wearing a white shirt versus a black shirt at a football game in August.
The energy absorbed per square meter is
$$\text{absorbed} = I(1-\alpha)$$where $I$ is the sunlight arriving.
Define albedo. Which surface has a higher albedo: fresh snow or the open ocean? Explain what this implies about how much sunlight each surface absorbs.
Define albedo. Which surface has a higher albedo: fresh snow or the open ocean? Explain what this implies about how much sunlight each surface absorbs.
Albedo values for several surfaces: | Surface | Albedo | |---|---| | Fresh snow | 0.85 | | Desert sand | 0.40 | | Forest | 0.15 | | Open ocean | 0.06 | Sunlight reaches…
Albedo values for several surfaces:
| Surface | Albedo |
|---|---|
| Fresh snow | 0.85 |
| Desert sand | 0.40 |
| Forest | 0.15 |
| Open ocean | 0.06 |
Sunlight reaches each surface at $1{,}000\ \text{W/m}^2$.
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Calculate the energy absorbed per square meter by fresh snow and by open ocean.
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Calculate how much more energy a square meter absorbs when sea ice (assume albedo 0.85) melts and becomes open ocean.
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Explain what this means for the warming of the Arctic.
Feedback loops
A feedback loop is a cycle in which a change in the climate triggers other changes that either strengthen or weaken the original change.
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Positive (amplifying) feedback: the result pushes in the same direction as the original change.
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Negative (stabilizing) feedback: the result pushes in the opposite direction and brings the system back toward balance.
"Positive" does not mean "good." It means "more of the same."
Important climate feedbacks:
| Feedback | Type | Why |
|---|---|---|
| Ice-albedo | Positive | Less ice means lower albedo and more warming |
| Water vapor | Positive | Warm air holds more vapor, a greenhouse gas |
| Permafrost thaw | Positive | Thawing soil releases CH₄ and CO₂ |
| Plant growth | Negative (partly) | More CO₂ can boost photosynthesis, which removes CO₂ |
Classify each of the following as a positive (amplifying) or negative (stabilizing) feedback loop, and justify each answer: 1. Warming melts Arctic sea ice, which lowers…
Classify each of the following as a positive (amplifying) or negative (stabilizing) feedback loop, and justify each answer:
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Warming melts Arctic sea ice, which lowers albedo, which causes more warming.
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Warmer air holds more water vapor, a greenhouse gas, which causes more warming.
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Higher atmospheric CO$_2$ increases plant growth, which removes more CO$_2$ from the air.
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Warming thaws permafrost, which releases methane, which causes more warming.
Milankovitch cycles
Over tens of thousands of years, small changes in Earth's orbit shift how sunlight is spread over the planet. These are the Milankovitch cycles.
| Cycle | What changes | Period |
|---|---|---|
| Eccentricity | Shape of the orbit, from nearly circular to slightly oval | about 100,000 yr |
| Axial tilt (obliquity) | Tilt of Earth's axis, 22.1° to 24.5° | about 41,000 yr |
| Precession | Wobble of the axis, changing which season occurs nearest the Sun | about 26,000 yr |
A larger tilt makes summers warmer and winters colder at high latitudes, so more ice melts in summer. A smaller tilt gives milder seasons and cooler summers, so winter snow survives, ice sheets grow, and albedo rises.
Orbital changes alone only slightly change the total sunlight. They act as a trigger. Feedbacks then amplify them: ice-albedo (ice grows, reflects more), CO₂ release from or uptake by the ocean (colder water holds more CO₂, which cools further), and water vapor. Ice cores from Antarctica trap tiny bubbles of ancient air, and they show that CO₂ and temperature rose and fell together over ice-age cycles. Tree rings (thick rings mean good growing years), ocean sediments, and historical records add more evidence about past climate. Since 1958, direct measurements at NOAA's Mauna Loa Observatory in Hawaii show CO₂ climbing steadily, from about 315 ppm to over 420 ppm.
Name the three Milankovitch cycles. For each one, state what part of Earth's orbit or orientation changes (use eccentricity, axial tilt, and precession).
Name the three Milankovitch cycles. For each one, state what part of Earth's orbit or orientation changes (use eccentricity, axial tilt, and precession).
Earth's axial tilt (obliquity) varies between about $22.1^\circ$ and $24.5^\circ$ with a period of about 41,000 years. Since about 1850, atmospheric CO$2$ has risen from…
Earth's axial tilt (obliquity) varies between about $22.1^\circ$ and $24.5^\circ$ with a period of about 41,000 years. Since about 1850, atmospheric CO$_2$ has risen from roughly 280 ppm to roughly 420 ppm, over about 175 years.
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Calculate the percent increase in CO$_2$.
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Explain how a larger tilt changes the contrast between summer and winter at high latitudes, and how this could influence the growth or melting of ice sheets.
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Explain why a small change in solar energy from orbital cycles is not enough by itself to explain the size of past ice-age cycles, and identify at least two feedback loops that amplify it.
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Compare the time scale of Milankovitch cycles with the time scale of recent CO$_2$ increase, and state what this comparison implies about the cause of current warming.
Back to the start: the greenhouse effect explains why Earth is 33 °C warmer than it would be without greenhouse gases. Low-albedo surfaces and feedback loops explain why the Arctic, where white ice gives way to dark ocean, is warming so fast.