Part 3 Blackbody radiation A body that emits

Part 3

Blackbody radiation • A body that emits radiation over all possible frequencies (remember inversely related to wavelength) • Wavelength distribution depends on the temperature of the blackbody • The Planck function describes the wavelength distribution relative to radiation intensity

UV Vis IR (heat) Energy flux and blackbody radiation temp. are related. Because it is hotter, sun emits more energy per unit area at all wavelengths. Changes in energy flux determine the spectrum of the EM radiation emitted. Shifts along the l axis. Blackbody emission curve for sun and earth – note higher energy shorter wavelength (higher frequency) emissions for the sun.

Energy output of the sun • Predicts a maximum of radiation emission in the visible region (0. 4 – 0. 7 mm) • Large component of high energy UV (remember this is damaging) • Assume Earth is a blackbody radiator • The Earth System responds to both the amount of solar radiation and its EM spectrum

Earth’s energy balance • If temperature is constant, the planet has to be in radiative balance

Energy input = energy output Some incoming solar radiation is lost before reaching the earth’s surface (30%). Some is returned to space as IR (70%) Fig. 3 -19 Earth’s globally averaged atmospheric energy budget.

O 3 Plus CO 2 and H 2 O Some incoming solar radiation is reflected by clouds Some is absorbed in the atmosphere (ozone at low l; CO 2 and H 2 O at high l)

UV O 3 Vis IR (heat) H 2 O, CO 2 Fig. 3 -8 Blackbody emission curves for the Sun and Earth.

O 3 Plus CO 2 and H 2 O Only 50% if incoming solar radiation gets through the atmosphere to the Earth’s surface

Blackbody radiation of the sun minus that “lost” in the atmosphere. Solar spectrum clipped at high and low ends Assumes Earth is a blackbody radiator. Energy loss shifts lmax to lower wavelengths when energy is re-radiated by the Earth. Reradiated as IR (heat). Shift in lmax controlled by Wein’s law Net effect of atmosphere on incoming and outgoing radiation

CO 2 and H 2 O Outgoing IR also absorbed by greenhouse gases.

Greenhouse gas absorption • Major role in warming atmosphere • Traps heat • Leads to re-distribution of radiation before it is re-radiated to space • Without greenhouse gases, planet would be much colder (~ -20 o. C)

Feedback loops • Feedback between lithosphere, hydrosphere, atmosphere and biosphere interact to maintain relatively constant, favorable temperatures over most of geologic history

Greenhouse effect • Planetary energy balance in the absence of a greenhouse • Simple model – – – Planet with no atmosphere and albedo that of Earth Energy emitted by Earth= energy absorbed by Earth From distance of sun, Earth is a circle (projected area) Energy absorbed = energy input – reflected light Albedo is the average color of the planet • Low albedo = dark color = absorbs heat (warms up) • High albedo = light color = high reflectance (cools down) – Assuming blackbody radiation, Stefan-Boltzman law predicts the temperature of the atmosphereless planet

Apply Stefan-Boltzman Law and simplify Where S is solar flux A is earth’s albedo We know how to calculate area of a circle Box Fig. 3 -1

Low albedo = low reflectivity (light is absorbed) High albedo = high reflectivity Fig. 2 -6

Box Fig. 3 -1

Box Fig. 3 -1

Treat atmosphere as black body T 4 e = (S/4)*(1 -A) Energy balance T 4 s = 2 T 4 e Fig. 3 -2 The greenhouse effect of a one-layer atmosphere. Provides 33 o. C of surface warming

Fig. 3 -19 Earth’s globally averaged atmospheric energy budget.

Earth’s climate is now warmer than at any time in the last 1000 years 2. 16

How can the atmosphere warm? 1. Increased solar input 2. Less reflected shortwave, less sulfate aerosols, darker surface of Earth (land-cover change) 3. More absorbed longwave more “greenhouse gases” 2. 2

Most major greenhouse gases are increasing in atmospheric concentrations 15. 3

Earth’s climate is now warmer than at any time in the last 1000 years 1. increased solar input (small warming effect) 2. Increased sulfate aerosols reflects radiation (small cooling effect) 3. Increased greenhouse gas concentrations (large warming effect) 4. Land-cover change creates a darker surface (large warming effect) 2. 16

Climate is warming most rapidly at high latitudes This warming is most pronounced in Siberia and western North America

In January… At 30º N & S, air descends more strongly over cold ocean than over land At 60 º N & S, air descends more strongly over cold land than over ocean These pressure gradients create geographic variation in prevailing winds