SCNC1112 Chap.7 Atmosphere, Climate and Earth in Space
Atmosphere, Climate and Earth in Space
The atmosphere and hydrosphere lecture is built on one sentence: how energy moves around a planet is what explains both a changing climate and the Earth's own history. Taken literally, that turns climate into an accounting problem. A planet's surface temperature settles where the energy leaving balances the energy arriving, so every climate argument is an argument about one of those two terms.
About thirty per cent of arriving sunlight is reflected straight back by bright surfaces and cloud, and the rest is absorbed. The warmed surface radiates at much longer infrared wavelengths, and several atmospheric gases, water vapour and carbon dioxide among them, absorb strongly at exactly those wavelengths and re-emit in all directions.
Part of the outgoing energy is therefore returned, and the surface has to run hotter before its output balances its input. The size of the effect is checkable: a body at the Earth's distance with the Earth's reflectivity and no atmosphere would settle near 255 kelvin, while the observed global mean is near 288 kelvin.
The 33-degree difference is the natural greenhouse effect, and the current argument concerns what adding to the absorbing gases does to the balance rather than whether the effect exists. Reflectivity is the other term and it is not fixed.
Snow, ice and thick cloud return sunlight before it can be absorbed, while open ocean and forest absorb nearly all of it, so anything that changes how bright the planet is changes the input side directly. That is the mechanism behind one of the clearest feedbacks in the system: warming melts reflective ice, the darker surface absorbs more, and the warming increases.
Averaged globally, energy in equals energy out; locally it does not. The tropics absorb more than they radiate and the polar regions radiate more than they absorb, so a surplus at low latitudes and a deficit at high ones must be settled by transport. Atmospheric circulation carries heat poleward and produces the belt of subtropical deserts where descending air is dry.
Ocean currents carry warm water poleward and return cold dense water at depth. Water vapour carries latent heat, absorbed where evaporation happens and released where condensation does. The chapter ends by returning to Chapter 1. The gravitation introduced for a falling apple is the same relation that holds the Earth in orbit, so the year, the day and the seasons need no new physics.
Seasons follow from the fixed tilt of the axis rather than from distance, which is why the two hemispheres have opposite seasons at the same moment and therefore at the same Earth to Sun distance.
What this chapter covers
- 01
The Radiation Budget as Accounting
- 02
Greenhouse Absorption and Re-Emission
- 03
Reflectivity and the Ice Feedback
- 04
Poleward Transport by Air and Ocean
- 05
The Carbon Cycle Across Reservoirs
- 06
Axial Tilt and the Seasons
Refuting the distance explanation for the seasons
- 2State the claim in a form that can be tested. If distance caused the seasons, the whole planet would experience summer at the same time, because there is only one Earth to Sun distance at any moment.
- 2Find the discriminating observation. When it is summer in the northern hemisphere it is winter in the southern one, simultaneously. A single distance cannot produce opposite outcomes, so the claim is refuted by observation rather than by authority.
- 2Supply the mechanism. The axis holds a fixed tilt of about 23.4 degrees as the Earth orbits, so each hemisphere is angled towards the Sun for part of the year and away for the rest. A steeper beam angle concentrates energy on less ground and daylight lasts longer, and the opposite hemisphere gets the reverse of both.
Key terms
- Radiation Budget
- The accounting of energy arriving at a planet against energy leaving it, whose balance point sets the surface temperature.
- Reflectivity
- The fraction of arriving sunlight returned to space before it is absorbed, which is high for ice and cloud and low for ocean and forest.
- Greenhouse Effect
- The warming produced when atmospheric gases absorb outgoing infrared radiation and re-emit part of it back towards the surface.
- Latent Heat
- Energy absorbed when water evaporates and released when it condenses, which transports energy from one place to another without a temperature change.
- Positive Feedback
- A loop in which the output of a process increases its own input, such as melting ice exposing a darker surface that absorbs more sunlight.
- Axial Tilt
- The fixed angle between a planet's rotation axis and the perpendicular to its orbit, which produces seasons by changing the angle of incoming sunlight.
Atmosphere, Climate and Earth in Space FAQ
Why is the greenhouse effect described as natural if it is also a problem?
Because the effect and the change in the effect are different claims. Without it the surface would sit around 33 degrees colder and the oceans would be frozen, so the natural effect is what makes the planet habitable. The concern is about the consequence of adding to the absorbing gases, which shifts where the energy balance settles.
Do the seasons have anything to do with distance from the Sun?
The distance does vary slightly over a year, and it is not what causes the seasons. The decisive evidence is that the northern and southern hemispheres have opposite seasons simultaneously, which one distance cannot explain. The cause is the fixed tilt of the axis changing both the angle of sunlight and the length of daylight.
How does carbon dioxide appear in two different parts of this course?
One added quantity drives two independent chains. In the radiation budget it absorbs outgoing infrared and raises the temperature at which the budget balances. In solution it forms carbonic acid and lowers ocean pH, which is chemistry from Module 1. Recognising that a question is joining those two chains is what the synthesis question type asks for.
Exam move
Draw the budget as three arrows and label which term each proposed change alters, then run three cases through it, including one that changes reflectivity rather than the atmosphere. For the seasons, rehearse the refutation until you can state the discriminating observation before the mechanism.
