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DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 1 : APRIL 1 , 2014
ATM S 111, Global Warming: Understanding the Forecast
Outline
How exactly the Sun heats the Earth How strong? Important concept of “albedo”: reflectivity
How the greenhouse effect works How the Earth cools And how greenhouse gases lead to less cooling
From Before We Asked…
What factors influence climate at a given place? Sunshine (and latitude) Topography/mountains Proximity to oceans and large lakes Ocean currents Presence of trees/vegetation Etc.
But what are the main factors that control the global climate? We’ll study this next
The Sun
Driver of everything in the climate system!
How Does Energy Arrive From the Sun?
Energy from the Sun is “electromagnetic radiation” or just “radiation” for short Goes through space at the speed of light Radiation is absorbed or reflected once it gets to
EarthRadiation with shorter wavelengths is
more energetic And radiation is classified in terms of its wavelength
This has long wavelength and low energy
This has short wavelength and high energy
Types of Radiation
Types of electromagnetic radiation, from most to least powerful (or shortest wavelength to longest wavelength) Gamma rays X-rays Ultraviolet (UV) radiation Visible light Infrared radiation Microwaves Radio waves
Sun’s Radiation
The Sun emits: Visible light (duh) Also “near infrared” radiation (infrared with very
short wavelength) A small (but dangerous) amount of ultraviolet
radiation This is what makes us sunburn!
These three bands together we call “shortwave radiation”
How Strong is the Sun?
By the time it gets to the top of Earth’s atmosphere, the Sun shines at a strength of 1366 Watts per square meter
Watt (abbreviated as W): unit of power or energy per unit time
1366 W/m2 is roughly what’s experienced in the tropics when the sun is directly overhead
Average Solar Radiation
The average incoming solar radiation is not 1366 W/m2 though It’s only 342 W/m2 (exactly ¼ of this). Why?
Half the planet is dark at all times…
Here it’s nighttime
High latitudes get less direct radiation, which spreads out more
Reason for Seasons
Directness of solar radiation is key for seasons as well Winter is tilted away from the Sun, gets less direct
light, and thus is colder
Winter solsticeDecember 21-22
Summer solsticeJune 20-21
EquinoxMarch 20-21or Sept 22-23
South pole sky soon after equinox (October maybe? – Sun goes around low in the sky)Current sunlight
When Solar Radiation Hits the Atmosphere
Average incoming solar radiation = 342 W/m2
Only 20% gets absorbed in the atmosphere This includes absorption of dangerous
UV by the ozone layer50% is absorbed at the surface
Meaning much of the sunlight makes it directly through the atmosphere!
30% is reflected back to space What does the reflecting?
Key Concept for Climate: Albedo
Albedo: fraction of incident light that’s reflected away
Albedo ranges from 0 to 1: 0 = no reflection 1 = all reflection
Things that are white tend to reflect more (high albedo)
Darker things absorb more radiation (low albedo)
Albedo Values for Earth
Clouds, ice, and snow have high albedo Cloud albedo varies
from 0.2 to 0.7 Thicker clouds have
higher albedo (reflect more)
Snow has albedo ranging from 0.4 to 0.9 (depending on how old the snow is) and ice is approximately 0.4
Ocean is very dark (< 0.1), as are forests (0.15)Desert has albedo of 0.3
Relative Contributions to Earth Albedo
Remember we said 30% of incoming solar radiation is reflected away? 20% is from clouds 5% is by the surface 5% is by the atmosphere (things like dust from deserts and
air pollution are key players here)
Total Solar Input
Total absorbed solar radiation is 70% of the incoming solar radiation Because 30% is reflected away 70% of 341 W/m2 = 240 W/m2
Summary So Far
The Sun heats the Earth Some is reflected back, a bit is absorbed in the
atmosphere But other than that, the atmosphere is pretty much
transparent when it comes to solar radiation (half is absorbed right at the surface!)
Clouds and snow/ice are primary contributors to the albedo of Earth
Next, how energy escapes from Earth and the greenhouse effect
“Longwave Radiation”
The Sun is the energy input to the climate system
How does the Earth lose energy? Turns out it’s also by radiation! But it’s not visible light like
from the Sun, it’s infrared radiation AKA “longwave radiation”
Infrared satellite image
“Longwave Radiation”
Everything actually emits radiation Depends partly on the substance but mostly on
temperature
Infrared thermometerNeck = hotterHair = colder
Longwave Radiation
Higher temperature means more radiation
A WARM CAT….
IZ A HAPPY CAT
Eyes and inner ears are warmest: they radiate the most
Nose is the coldest: it radiates less
Thermal night vision technology detects longwave radiation
Longwave radiation
From my cat
Temperature & Radiation
Higher temperature = more radiation and more energetic radiation (shorter wavelengths)
Explains the Sun’s radiation too Sun is really hot
It emits much more radiation It emits shortwave radiation instead of longwave
radiation like the Earth
Energy Into and Out of the Earth
Heating/cooling of Earth The Earth is heated by the Sun (shortwave radiation) The Earth loses energy by longwave radiation (out to
space)
“Energy Balance”
If the energy into a system is greater than the energy out, the temperature will increase A temperature increase then results in an increase of
energy out Hotter things radiate more
This will happen until:
When energy in equals energy out, we call this “energy balance”
Energy in Energy out
Energy Balance on Earth
If the solar radiation into Earth is greater than the outgoing longwave radiation, the temperature will increase A temperature increase then results in an increase of the
longwave radiation out (hotter things radiate more) This will happen until:
Global warming upsets the energy balance of the planet
Shortwave in Longwave out
Energy Balance with No Atmosphere
If there was no atmosphere, for energy balance to occur, the mean temperature of Earth would be 0o F (-18o C)
Missing piece: the greenhouse effect All longwave radiation doesn’t escape directly to
space
-18o C (0o F)
The Greenhouse Effect
Greenhouse gases block longwave radiation from escaping directly to space These gases re-radiate both upward and downward The extra radiation causes additional warming of the
surface
Extra downward radiation due to greenhouse gases
15o C (59o F)
The Greenhouse Effect
Greenhouse gases cause the outgoing radiation to happen at higher levels (no longer from the surface) Air gets much colder as you go upward So the radiation to space is much less (colder less
emission)
15o C (59o F)
The Greenhouse Effect
Greenhouse effect is intuitive if you pay attention to the weather! Cloudy nights cool less quickly
In the desert, temperatures plunge at night! No clouds & little water vapor in the desert: little
greenhouse effect
Summary
The Earth is heated by the Sun This is shortwave radiation Albedo: key factor that determines how much
radiation is absorbed vs reflectedEarth loses energy due to longwave
radiation The greenhouse effect causes less heat loss due to
longwave radiationIf the Earth is pushed out of energy
balance, it warms or cools in response Warming can occur from increased solar radiation or
increased greenhouse effect