Understanding the Greenhouse Effect:  Learning Objectives : 1) Relate strength of the greenhouse effect to common experience (i.e. its generally warmer on cloudy nights). 2) Apply physical principles to characterize response to increased greenhouse gases 3) Critically evaluate human-caused vs solar- caused hypotheses for recent warming.

Understanding the Greenhouse Effect:  Synposis: Students draw their expectations of diurnal temperature changes, then compare them to observations for both clear sky and cloudy sky conditions.

Understanding the Greenhouse Effect: Key Points 1.Students realize that Earth radiates energy, which is a challenging concept for them to really get. They are forced to see this when asked to explain why they draw temperature decreasing at night. Simply shutting off the sun would cause temperature to plateau. The known decrease at night can only come from a loss of energy at Earth’s surface. 2.The difference between clear and cloudy sky conditions, particularly at night in the winter, reveals the relative magnitude of the greenhouse effect due to cloud cover. 3.If the sun were causing global warming over the last 100 years, one might expect the signal would be strongest in summer, during the day, in the tropics. Instead, the signal of current warming is at high latitudes, at night, in the winter.

The Greenhouse Effect: Fundamental Questions & Context What is the Greenhouse Effect? How big is the effect? Why are cloudy nights warmer than clear nights?

The Greenhouse Effect  Electromagnetic (EM) radiation, radiation processes  “Clear Sky” Exercise  Earth-Sun System  Greenhouse Gases  “Cloudy Sky” Exercise

 Electromagnetic (EM) radiation  Solar and Terrestrial Emission  Radiation Processes and Interactions  Water Vapor is a Greenhouse Gas (selective absorpotion) Background Information

Central Project Question Knowing that water vapor is a potent greenhouse gas, what do you expect is the short-term effect of cloud cover on your local temperature?

The Greenhouse Effect  Electromagnetic (EM) radiation, radiation processes  “Clear Sky” Exercise  Earth-Sun System  Greenhouse Gases  “Cloudy Sky” Exercise  Earth vs Moon

Clear Sky Anticipation Questions For a weather station located at Des Moines, Iowa, consider the following questions regarding the diurnal cycle during both winter (January) and summer (July). 1)Why does temperature change through the day? 2) Assuming a cloudless day, plot your expectations for the diurnal cycle of temperature using the blank template figure.

Hour of Day

Diurnal cycle during both winter (January) and summer (July) at Des Moines Iowa. 3) Indicate the time of sunrise and sunset in your figure. Anticipation Questions (continued…) 5) How do summer and winter differ? Why? 4) What, specifically, causes temperature to increase or decrease?

Hour of Day

Diurnal cycle during both winter (January) and summer (July) at Des Moines Iowa.  Indicate the time of sunrise and sunset in your figure.  What, specifically, causes temperature to increase or decrease?  How do summer and winter differ? Why? Anticipation Questions (summary)

Data Source: Airport at Des Moines, IA from (NCDC Surface Airways) Observation Variables: Temperature and cloud ceiling height (the height of the lowest cloud layer, if present) Realization Methodology:  Sort data and select only “clear sky” conditions  Compute the average diurnal cycle  Repeat for both January and July data

Hour of Day

 Why does temperature change through the day?  How does temperature decrease?  How do summer and winter differ? Clear Sky Contemplation

The Greenhouse Effect  Electromagnetic (EM) radiation, radiation processes  “Clear Sky” Exercise  Earth-Sun System  Greenhouse Gases  “Cloudy Sky” Exercise  Earth vs Moon

As Temperature increases, wavelength of peak emission decreases All objects emit EM radiation; the wavelength and energy emitted depends on the object’s temperature Radiation: Fundamental Principles As Temperature increases, total emission increases

Solar and Terrestrial Radiation NOTE: Log Scale! UV | Visible | IR Sun Earth

Absorbed = Emitted = (No Atmosphere Case) Earth,

Solar Radiation Terrestrial Radiation “Radiative Equilibrium” Incoming = Outgoing Stable Temperature

The Greenhouse Effect  Electromagnetic (EM) radiation, radiation processes  “Clear Sky” Exercise  Earth-Sun System  Greenhouse Gases  “Cloudy Sky” Exercise  Earth vs Moon

Selective Absorption in the Atmosphere

Greenhouse Gases are…  …“transparent” for sunlight  …only partially transparent to Infrared Radiation (…from Earth’s Surface)  … “warmed” via radiation (However, sensible heat and convection also “warm” the atmosphere)

The Greenhouse Effect  Electromagnetic (EM) radiation, radiation processes  “Clear Sky” Exercise  Earth-Sun System  Greenhouse Gases  “Cloudy Sky” Exercise  Earth vs Moon

Diurnal cycle during both winter (January) and summer (July) at Des Moines Iowa. Cloudy Sky Anticipation Questions 1) How might temperature differ between clear and cloudy sky conditions? 2) Draw your expectations on the figure provided, labeling the clear and cloudy sky lines.

Hour of Day

 Why is the cloudy sky temperature relatively flat?  How does clear sky temperature get both hotter and colder than the cloudy sky temperature? Cloudy Sky Contemplation

Greenhouse Process Incoming Solar Earth’s Surface Emits Infrared Radiation

Greenhouse Process Greenhouse Gases 1)Absorb IR

Greenhouse Process Greenhouse Gases 1)Absorb IR 2)Emit IR

Greenhouse Process NET EFFECT: Earth’s surface Warmed by TWO Heating sources

Greenhouse ProcessTWO Heating sources What if we add more Greenhouse Gases??

The Greenhouse Effect  GHGs absorb IR emitted by Earth’s Surface  The gases also emit IR back to the surface, providing an additional heating source.  An example of this effect is the relative warmth of cloudy nights.  Additional GHGs will increase surface temperature in the same way as enhanced cloud cover. Summary