DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES

DAY 1 : MARCH 30 , 2010

ATM S 111, Global Warming: Understanding the Forecast

Reading Assignments

From last time: Make sure you’ve read Rough Guide p. 3-19 “Climate

Change: A Primer” The Big Picture The Outlook What Can We Do About It

Next reading assignment: Rough Guide p. 20-31 “The Greenhouse Effect” If it’s about hotels in Melbourne you might have

bought the wrong Rough Guide

Outline of This Lecture

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

What are the main greenhouse gases? And which are changed by human activity?

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!

We Are Small!

Fun facts (i.e., won’t be on the HW/tests): Sun has approximately:• 100 times the radius of the Earth• 1 million times the

volume of Earth• 300,000 times the

mass of Earth

The Sun is 10 light minutes away from Earth (100 Sun diameters away)

Fun Facts: Other Heat Sources?

The Sun is by far the main heat source for the atmosphere/ocean system

What are other ways the atmosphere and ocean can be heated? Heat from volcanoes or the solid Earth: 0.025% of the

Sun’s heating Direct heating of the atmosphere/ocean from burning of

fossil fuels or nuclear power: 0.007% of the Sun This is not the greenhouse effect

Tides: 0.002% of the Sun Caused by the moon, lead to motions which eventually turn to

heat from friction

So the Sun contributes over 99.97% of the energy input!

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 are

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 powerful 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: Winter is tilted away from the Sun,gets less direct light, and thus is colder

Next: When Solar Radiation Hits the Atmosphere

So, the average incoming solar radiation is 342 W/m2

What happens when this encounters the atmosphere?

Only about 20% gets absorbed within the atmosphere This includes absorption of dangerous UV by the ozone

layer50% is absorbed right at the surface

Meaning much of the sunlight makes it directly through the atmosphere!

30% is reflected away, back into 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 0.4Ocean 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

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

What are the Major Greenhouse Gases?

Our atmosphere is mostly nitrogen (N2, 78%), oxygen (O2, 21%), and argon (Ar, 0.9%) But these are not greenhouse gases

Molecules with 1 atom or 2 of the same atoms aren’t greenhouse gases though Just like the atmosphere is almost transparent to solar

radiation, the primary gases in our atmosphere are transparent to longwave radiation

If our atmosphere was only nitrogen, oxygen, and argon, this picture with no greenhouse effect would be accurate!

Greenhouse Gases

Polyatomic molecules are greenhouse gases Water vapor (H2O) Carbon dioxide (CO2) Methane (CH4) Nitrous oxide (N2O) Ozone (O3) Chlorofluorocarbons (the ozone depleting chemicals which

have been banned)

The fact that they can rotate and vibrate means they can absorb the right frequencies of longwave

Water Vapor

Gas form of water AKA humidity

The number one greenhouse gas! Powerful because there’s a lot of it

Not controlled by humans! It’s a feedback not a forcing (topic of the next

lecture) Observed to be increasing with global warming

Carbon Dioxide

CO2

It’s what we breathe out, what plants breathe inThe primary contributor to the anthropogenic

(human-caused) greenhouse effect 63% of the anthropogenic greenhouse effect so far

Increases primarily due to fossil fuel burning (80%) and biomass burning (e.g., forest fires; 20%) Preindustrial value: 280 ppm Current value: 386 ppm

Carbon Dioxide

CO2 will also be the main problem in the future

It’s extremely long-lived in the atmosphere 50% of what we emit quickly gets taken up by the

ocean or land We’ll discuss this more later

Most of the rest sticks around for over 100 years Some of what we emit will still be in the atmosphere

over 1000 years from now!

Methane

CH4

Natural gas like in stoves/heating systems

Much more potent on a per molecule basis than CO2

Only 1.7 ppm though – much smaller concentration than CO2

Natural sources from marshes (swamp gas) and other wetlands

Increases anthropogenically due to farm animals (cow burps), landfills, natural gas leakage, rice farming

Methane

The lifetime of CH4 is significantly shorter than carbon dioxide Breaks down in the atmosphere in chemical reactions Lifetime of methane is only 8 years

Methane concentrations have been leveling off in recent years, possibly due to drought in wetlandsat high latitudes

Global Warming Potential

CO2 lifetime > 100 yearsMethane lifetime = 8 years

But methane is a much stronger greenhouse gasHow to put these on similar terms? Global

warming potential (or GWP) Global warming potential is how much greenhouse

effect emissions of a given gas causes over a fixed amount of time (usually 100 years) Measured relative to CO2 (so CO2 = 1)

Methane’s global warming potential is 25 Much more potent than CO2 even though it doesn’t stay

as long

Nitrous Oxide

N2O Laughing gas

Also more potent on a per molecule basis than CO2

Global warming potential: 310Comes from agriculture, chemical industry,

deforestationSmall concentrations of

only 0.3 ppm

Ozone

Ozone or O3 occurs in two places in the atmosphere In the ozone layer very high up

This is “good ozone” which protects us from ultraviolet radiation & skin cancer

Remember ozone depletion is not global warming! Near the surface where it is caused by air pollution: “bad

ozone”Bad ozone is a greenhouse gas, and is more

potent on a per molecule basis than CO2

But very very short-lived Fun fact: Global warming potential for ozone is not usually

calculated – rather it’s wrapped into the GWPs of the other gases that lead to its chemical creation

CFCs

CFCs or chlorofluorocarbons are the ozone depleting chemicals Have been almost entirely phased out

CFCs are strong greenhouse gases Their reduction likely saved significant global

warming in addition to the ozone layer!Some replacements for CFCs (called HFCs)

are strong greenhouse gases thoughGlobal warmingpotentials of up to 11,000!

The Natural Greenhouse Effect

Contributions to the natural greenhouse effect: H2O (water vapor): 60% CO2 (carbon dioxide): 26% All others: 14%

These numbers are computed with a very accurate radiation model First running with all substances, then removing each

individual gas

The Unnatural Greenhouse Effect

Increasing levels of CO2 and other greenhouse gases leads to a stronger greenhouse effect With more greenhouse gases, it becomes harder for

outgoing radiation to escape to spaceIt’s like this picture from before, but more.

More radiation trapped before it gets out to space.

Longwave radiation is emitted from a higher (and colder) level on average.

The Unnatural Greenhouse Effect

Contributors to the “anthropogenic” greenhouse effect Numbers for the whole world up to this point:

Carbon dioxide: 63% Methane: 18% CFCs, HFCs: 12% Nitrous oxide: 6%

The Anthropogenic Greenhouse Effect

Contributors to the “anthropogenic” greenhouse effect Numbers for the US based on current (2008)emissions

CO2 is the big problem in the US currently.

Note how much lower the HFCs are than on the previous slide. This is b/c we basically don’t emit CFCs any more.

From US EPA 2010 report (draft)

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 radiationGreenhouse gases:

Number one is water vapor Number two is CO2

Global warming potential: key concept

Survey Question about Types of Radiation

Which of the following are types of electromagnetic radiation? A. X-rays B. Sonar C. Radio waves D. A + B E. A + C F. B + C G. All