DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 1: MARCH 30, 2010 ATM S 111, Global...
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Transcript of DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 1: MARCH 30, 2010 ATM S 111, Global...
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
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