MET 112 Global Climate Change

The Earth’s Energy BalanceDr. Craig Clements

San José State University

Review: Why is CO2 So Important?

Carbon Dioxide is a greenhouse gas. Greenhouse gases are those gases that cause

the greenhouse effect. The greenhouse effect makes a planet’s

surface temperature warmer than it would otherwise be.

The stronger the greenhouse effect, the warmer the surface (other factors being equal).

Consider the blanket analogy

Earth’s Energy BalanceEarth’s Energy Balance

Energy entering top of atmosphere

Energy entering the Earth’s surface

= Energy leaving top of atmosphere= Energy leaving top of atmosphere

= Energy leaving Earth’s surface= Energy leaving Earth’s surface

Conservation of EnergyConservation of Energy

There are three modes of energy There are three modes of energy transmission in the atmosphere.transmission in the atmosphere.

Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion.

Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy.

Radiation: the propagation of electromagnetic waves through space.

Energy transmissionEnergy transmission

ConductioConductionn

ConvectionConvection

Electromagnetic radiation

Radiation is the transfer of energy by rapid oscillations of electromagnetic fields.

The most important general characteristic is its wavelength (), ______________________________.

Radiation travels through space at the speed of light (3 x 108 m s-1).

Defined as the crest-to-crest distanceDefined as the crest-to-crest distance

Radiation What emits radiation?

– All objects with a temperature greater than 0°K emit some type of radiation (energy)

Examples:

Radiation laws:

– Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law)

– Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

Wien’s Law:

E = σT4

λ = w / Tλ = maximum wavelength (μm)w = constant = 0.2897 (μm K)T= temperature of the object (K)

Stefan-Boltzmann Law:

E = radiation emitted (W m-2)σ = Stefan-Boltzmann constant= 5.67 x 10-8 (W m-2 K-4)T= temperature of the object (K)

Solar Radiation (Sunlight)

Sunlight is primarily made up of the following:– Visible Light (44%)– Infrared Radiation (48%)– Ultraviolet Radiation (7%)

Unit: 1 m =

0.000001 m

Terrestrial or Longwave Radiation

Planets mainly emit infrared radiation Radiation emitted by

planets occurs mainly at wavelengths _____

than those contained in solar radiation

Solar Radiation (“Shortwave”)

Terrestrial Radiation (“Longwave”)

longer

Solar vs. Terrestrial Radiation

The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation;

Thus …

Review questions

Considering the previous discussion– Which object would emit more (intensity) radiation:

Earth or Sun?

– If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength?

– What wavelength radiation are you emitting right now?

Sun

Earth

infrared

Energy from the Sun

Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get?

Sun’s energy is either– Scattered (reflected away) or– Absorbed

Scattering happens by bouncing off – Particles in the atmosphere– Earth’s surface

Absorption happens when certain gases absorb the energy– The reality is the only certain gases absorb certain

wavelengths.

Absorption of radiation

Absorption of shortwave radiation by atmospheric gas molecules is fairly weak;

– most absorption of shortwave radiation occurs at the Earth’s surface.

Most gases do not interact strongly with longwave radiation, however

– Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

Incoming solar radiationIncoming solar radiation

Each ‘beam’ of incoming sunlight can be either:– Reflected back to space:

Clouds Atmosphere Surface

– Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface.

AlbedoAlbedo

Recap

______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation

__________________ radiation comes from the Earth and is composed of infrared radiation

Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength.

Shortwave or solar

Longwave or terrestrial

Review - sensors that measure radiation

A _________________ measures solar radiation.

A__________________ measures infrared radiation (terrestrial) that comes from the Earth.

Pyranometer

Pyrgeometer

Longwave radiation is emitted from surface.

Some surface radiation escapes to space

Most outgoing longwave is absorbed in atmosphere (by greenhouse gases)

Greenhouse gases emit longwave upward and downward

Some atmospheric radiation escapes to space

Some atmospheric radiation is absorbed at the surface

Greenhouse EffectGreenhouse Effect

Sequence of steps:

1. Solar radiation absorbed by earth’s surface.

2. Earth gives off infrared radiation.

3. Greenhouse gases absorb some of the Earth’s infrared radiation.

4. Greenhouse gases (water and CO2) give off infrared radiation in all directions.

5. Earth absorbs downward directed infrared radiation

Result: warmer surface temperature

Energy Balance

Assume that the Earth’s surface is in thermodynamic equilibrium:

Thermodynamic Equilibrium: – The flow of energy

away the surface equals the flow of energy toward the surface

Surface

Average surface temperature = 15°C

Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface.

Sudden Removal of all Greenhouse Gases

Removal of greenhouse gases would decrease downward flow of energy; now energy away from surface is greater than energy toward surface.

Thus, average surface temperature starts to decrease.

Sudden Removal of all Greenhouse Gases

As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops

Sudden Removal of all Greenhouse Gases

As surface cools, emission of radiation decreases until balance is restored. At this point, cooling stops and equilibrium is restored.

Average surface temperature = -18°C

Result: A Very Cold Planet!

Earth’s Greenhouse Effect

Without the greenhouse effect, the surface temperature of Earth would be – Way Cold (-18°C)

Greenhouse gases play an important role in shaping climate.– More GHGs – warmer climate– Less GHGs – cooler climate

Classwork 1-1

Start with the following diagram and assume the earth’s surface temperature is 15C and that the atmosphere has greenhouse gases.

Imagine that the concentrations of greenhouse gases were to increase by 50%. 1. Draw two more diagrams illustrating (with arrows) how the energy balance would change with the increase in greenhouse gases and explain why. 2. How would the average surface temperature change?

Surface

Average surface temperature = 15°C