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Lecture 1. The “Greenhouse” Effect. EFFECTIVE TEMPERATURE OF THE EARTH SYSTEM First the Sun : 1. The spectrum of solar radiation measured outside the Earth’s atmosphere matches closely that of a blackbody at 5800 K. 2. The Sun’s radiation peaks in the visible range of wavelengths - PowerPoint PPT Presentation
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EFFECTIVE TEMPERATURE OF THE EARTH SYSTEM
First the Sun:
1. The spectrum of solar radiation measured outside the Earth’s atmosphere matches closely that of a blackbody at 5800 K.
2. The Sun’s radiation peaks in the visible range of wavelengths (l = 400 - 700 nm).
3. The total radiation ES emitted by the Sun (temperature TS = 5800 K) per unit time is given by the radiation fluxmultiplied by the area of the Sun:
where RS = 7x105 km is the Sun’s radius and is the Stefan-Boltzmann constant [5.67x10-8W m-2 K-4]
Lecture 1. The “Greenhouse” Effect
Now for the Earth, a distance d = 1.5x108 km from the Sun.
The solar radiation flux FS distributed uniformly over the sphere centered at the Sun and of radius d is:
Substituting numerical values we obtain FS = 1370 W m-2 , which we also called the solar constant.
The mean solar radiation flux absorbed per unit area of the Earth’s surface is FS(1-A)/4, where A (albedo = 0.28) is the fraction reflected back to space.
The energy balance equation for the Earth is then given by
which yields an effective temperature for the Earth, TE = 255 K.
TE is the temperature at which the Earth System must radiate to be in energy balance with the Sun.
Why are we not a frozen block of Ice?Answer: “greenhouse” gases
1. What do “greenhouse” gases do? They absorb radiation in the IR (5000-50,000nm)2. How does this help?
If then which gives as the effective radiating temperature. This is for a simple isothermal uniform absorption model.
[extracted from “Atmospheric Chemistry” by Daniel Jacob]
GREENHOUSE GAS CANDIDATES
1. What is the number 1 natural “greenhouse” gas? Hint, it is responsible for much of the albedo
2. What is the number 2 natural “greenhouse” gas? Hint, it has also been around as long as number 1.
3. What is the number 3 natural “greenhouse” gas. Hint, early bacteria ate it up.
Total Column Water Vapor Anomalies (1987-2004)
Held and Soden J.Clim. 2006
We have high confidence in the model projections of increased water vapor.
Jouzel, Lorius et al-- Vostok late 70’s-80’s Etheridge, et al -- Law Dome 1990’s
Ice cores + direct measurementsprovide a beautiful record of the history of atmosphericcarbon dioxide
www globalwarmingart.com.
IPCC 2007 Non-Technical Definition
• Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate, such as atmospheric composition or surface reflectivity, are altered.
• When radiative forcing is positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system.
• In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system.
Radiative forcing of climate (1750 to present):Important contributions from air pollutants
IPCC, 2007
Global ocean heat content
1955 20051980
Energy is going into ocean=>More energy is entering the atmosphere from space than isgoing out
Almost all parts of the Earth’ssurface have warmed over the past 100 years
IPCC 4th Assessment Report.
www.globalwarmingart.com