PTYS 214 – Spring 2011  Homework #5 available for download at the class website DUE Thursday, Feb. 24  Reminder: Extra Credit Presentations (up to 10pts) Deadline: Thursday, Mar. 3 (must have selected a paper)  Class website:  Useful Reading: class website  “Reading Material” Announcements

Quiz #4  Total Students: 24  Class Average: 3.2  Low: 0  High: 4 Quizzes are worth 20% of the grade

Solar Spectrum at Earth’s Surface Greenhouse gases absorb IR radiation at specific wavelengths

Greenhouse gases and radiation  Solar radiation is absorbed on its way to the Earth’s surface  Terrestrial IR radiation is absorbed on its way out towards space

Effect of the Atmosphere Red reaches Earth’s surfaceBlue escapes to space (Earth and Solar spectra are NOT to scale)

Atmospheric Greenhouse Effect  The Greenhouse Effect increases the surface temperature by returning part of the outgoing IR radiation back to the surface  The outgoing IR radiation includes Earth’s radiation but also the IR part of the reflected solar spectrum  The magnitude of the greenhouse effect depends on the abundance of greenhouse gases (CO 2, H 2 O, O 3, CH 4, etc.)

Non-Greenhouse Gases  The molecules/atoms that constitute the bulk of the atmosphere: O 2, N 2 and Ar, do not interact with infrared radiation significantly (scattering)  While the oxygen and nitrogen molecules can vibrate, because of their symmetry these vibrations do not create any transient charge separation (dipole)  Without such a transient dipole moment, they can neither absorb nor emit infrared radiation

Water in the Earth’s Atmosphere  The water content of the atmosphere varies about 100- fold between the hot and humid tropics and the cold and dry polar ice deserts  Water vapor is the main absorber of radiation in the atmosphere, accounting for about 70% of all atmospheric absorption of radiation, mainly in the IR!  Liquid water and ice droplets are also present in the atmosphere as clouds  Clouds both reflect sunlight, which cools the Earth, and trap heat in the same way as greenhouse gases, and thus warm the Earth

Cumulus cloudCirrus clouds Stratus clouds puffy, white clouds grey, low-level clouds high, wispy clouds

Clouds and Radiation  Stratus Clouds reflect sunlight Cooling  Low thick clouds have a high albedo, reflecting more sunlight  Cirrus Clouds absorb and re-emit outgoing IR radiation Warming  High, thin clouds have a low albedo, letting most solar radiation through but absorbing and emitting IR

Atmospheric Greenhouse Effect Vis IR UV IR, UV all IR all IR

Activity: The Greenhouse Effect

The Greenhouse Effect 2) Does the Sun give off more UV or IR photons? IR photons – Why? 3) Does Earth’s surface emit radiation at night? Of course! 5) Which has an easier time getting through the atmosphere, Visible or IR? Visible 6) What about radiation emitted by Earth? It is IR, so it tends to be trapped by the atmosphere 8) What is the radiation heating Earth’s surface and atmosphere? Earth’s surface: mostly Visible and IR Earth’s atmosphere: IR, UV

Energy Flow WITHOUT Greenhouse Effect Earth without an atmosphere

Planet Emission Temperature Surface Temperature Venus282K740K Earth255K288K Mars210K Titan82K94K What about other solar system objects with an atmosphere? Difference between Emission and Surface Temperatures indicates the efficiency of the greenhouse effect

Back to the Habitable Zone Consider a planet with: –Earth’s atmospheric greenhouse warming (33 K) and –Earth’s planetary albedo (~ 0.3) Where would the boundaries of the Habitable Zone be for such planet?

Remember the Energy Balance Equation: E abs = E out E out E in aE in

The solar constant, S, at any given distance from the Sun, R, is determined by the Inverse Square Law: D is the distance of a planet from the star (the Sun for our Solar System)

We can substitute the formula for the Solar flux to the planetary energy balance equation and solve for the distance: The temperature in this equation is the effective emission temperature of the planet

The Habitable Zone The distance at which liquid water can be found on a planet’s surface varies with: - the Star’s Luminosity, L - the Planet’s Albedo, a - the Planet’s Effective Emission Temperature, T em But liquid water depends on the temperature on the surface…

The Habitable Zone  We want to find the region around the Sun where water could be in liquid form  For that assume for the surface temperature that 273K < T s < 373K How does the surface temperature relate to the emission temperature? T s = T em +  T GH

where: T em = T s -  T GH For an Earth-like planet:  T GH = 33K a = 0.3 The range of surface temperatures is limited by: Min: T s = 273K → T em ( min ) → D out Max: T s = 373K → T em ( max ) → D in The Solar System Habitable Zone

CHZ VI Habitable Zone Region around a star where a planetary body can maintain liquid water on its surface D in D out

Average surface temperature (T s ) The average surface temperature (T s ) depends on three main factors: a) Solar luminosity (energy emission from star) b) Planetary albedo (on Earth it is also affected by clouds) c) Greenhouse Effect (CO 2, H 2 O, CH 4, O 3 etc.) – this implies the presence of an atmosphere! Complication: The amount of atmospheric greenhouse warming (∆T g ) and the planetary albedo (a) can change as a function of surface temperature (T s ) through different feedbacks in the climate system

Climate System We can think about climate system as a number of components (atmosphere, ocean, land, ice cover, vegetation, etc.) which constantly interact with each other

Positive Coupling Car’s gas pedal Car’s speed A change in one component leads to a change of the same direction in the linked component (+) Negative Coupling Car’s break pedal Car’s speed (-) A change in one component leads to a change of the opposite direction in the linked component Coupling of System Components

Negative Coupling in Climate Earth’s albedo (reflectivity) Earth’s surface temperature An increase in Earth’s albedo causes a corresponding decrease in the Earth’s surface temperature by reflecting more sunlight back to space Conversely, a decrease in albedo causes an increase in surface temperature (-)

Positive Coupling in Climate Atmospheric CO 2 Greenhouse effect  An increase in atmospheric CO 2 causes a corresponding increase in the greenhouse effect, and thus in Earth’s surface temperature  Conversely, a decrease in atmospheric CO 2 causes a decrease in the greenhouse effect (+)

Feedbacks In nature component A affects component B but component B also affects component A This “two-way” interaction is called a feedback loop Loops can be stable or unstable B A

Unstable Loops Number of Births World Population Positive feedback loop: An unstable system which changes further following a perturbation positive coupling (+)

negative coupling positive coupling Negative feedback loop: A stable system which resists change following a perturbation Stable Loops (-) (+) Number of Predators Number of Preys

Odd numbers of negative couplings: Overall negative (stable) loop Even number of negative couplings: Overall positive (unstable) loop Multiple Feedback Systems