DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 1: APRIL 1, 2014 ATM S 111, Global Warming: Understanding the Forecast.

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