Download Physics of the Greenhouse Effect

THE GREENHOUSE EFFECT
A PRIMER
Chapter 1
What is the Greenhouse Effect?
Earth’s Energy Sources
• The earth receives
most of it’s energy
from the sun.
• It also has leftover
warmth from when it
formed
• And it has radioactive
elements decaying in
it, which heats things
up.
What the Earth needs
• The Earth needs to hold on to the energy
from the sun and its interior for a little while
• And then it must get rid of it.
• Greenhouse gases are like a blanket, in that
they insulate the Earth from the extreme
heat of day and from the extreme cold of
night.
• A blanket too thin leaves a planet too cold
for life. One too thick boils all the water
away.
The moon lacks a greenhouse
effect
100ºC by
day
-173º C by
night
Venus suffers a runaway
greenhouse effect
Shrouded in CO2, Venus’ surface temp is about 450º
Read an interesting article here
The Earth’s Greenhouse Effect
is just right
Just right. Ahhhhhhhhh
T-t-t-t-oo
C-c-c-c-c-old
H-o-o-o-o-t. B-a-a-a-a-a-a-d
Earth’s Greenhouse Effect
It protects against heat AND
cold
Chapter 2
Earth’s Energy Budget and the
Global warming
Radiation Balance
• Averaged over several years, the amount of
incoming solar radiation (insolation) that
strikes the Earth (240 W/m2) exactly
balances what the Earth emits back to space
(terrestrial radiation, 240 W/m2 ).
• This balance results in the average
temperature of Earth, which supports life as
we know it.
Radiation Balance: What goes in
must come out
Global Warming
• The key to understanding our life-giving
climate is in understanding the rate at which
the Earth sheds it energy.
• That’s what’s special about the greenhouse
effect; it moderates how quickly incoming
energy is lost to space.
Global Warming - natural and
good!
• If the atmosphere had no greenhouse effect,
the Earth would be a brisk -18º C!! Too
cold!!
• Over the last 500,000 years, the Earth’s
temperature has fluctuated between 19ºC
and 27ºC - nice and warm and good for life.
Surface temperature vs Planet
temperature
• Earth’s average surface temperature ~15C
• A Reasonable assumption:
– Surface of earth radiates heat with an average
temperature of 15C
• However, satellite data indicate Earth
radiating heat average temperature ~-16C
• Why the discrepancy?
– What accounts for the 31C heating?
Global Warming
• The Earth reflects some light immediately.
The rest is absorbed by the surface of the
Earth, which gets warm.
• Warm things glow. Which is to say, the
warm Earth radiates this energy as infrared
radiation into the air.
• Greenhouse gases absorb the 95% of this IR
and give some of it back to Earth – and
what stays in the atmosphere keeps it warm.
“Trapped” energy
• The portion of IR energy that is reradiated
down to Earth’s surface accounts for the
31C heating
• Energy radiated from the Earth(called
terrestrial radiation) from elevation of
about 5 km (top of clouds) averages 240 W
m-2 - the same amount as comes in
– Keeps Earth’s temperature in balance
– Source: www.soest.hawaii.edu/GG/FACULTY/POPP/Lecture2.ppt
• The following are graphics that show how
the daily insolation is shared among the
various parts of the surface and atmosphere;
albedo reflects 30% of it, the rest is
absorbed by the air and the surface and reradiated as IR light.
• The energy that is re-radiated as IR light is
the energy that is responsible for global
warming. Pay attention to it!
The cause of
Global
Warming
http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
Energy Balance
Chapter 3
How Greenhouses and Greenhouse
Gases work
Blackbody Radiation
• To understand how greenhouse gases work, we
need to understand how greenhouses work.
• All warm things RADIATE energy. All radiation
is the form of electromagnetic waves.
• High-temperature object radiate short waves
(visible light), low temp objects radiate long
waves (infrared light).
• Low-frequency, long-wavelength radiation is
called blackbody radiation. (Black because the
things don’t glow in the ranges we can see - they
are dark to us)
Transparency and Opacity
• When light hits atoms of some material, the
energy is absorbed.
• From there, the energy can take one of two paths.
• Either it is transmitted as light energy, in which
case the material is transparent
• Or, the light energy RESONATES with the atoms,
which causes the atoms of the material to vibrate
with a very large amplitude (via constructive
interference). In this case, the atoms will bash
against each other and heat up; the light energy
becomes “heat energy.” The material is opaque.
How greenhouses work
• Glass has a natural frequency that matches that of
IR and UV light, but not visible light.
• IR and UV light cause glass atoms to vibrate a
LOT, and the light energy is turned to heat.
• Glass is transparent to visible (shortwave) light
waves which pass right through but opaque to
longer infrared waves, which cause the glass to
warm up.
Greenhouse physics
• The sun is very hot; in fact, it is glowing
hot. It radiates short waves in the visible
part of the spectrum which easily pass
through glass (and also air molecules).
• The light is then absorbed by ground and
plants and such, and they become warm.
• These warm things reradiate that energy as
blackbody radiation, but, being much colder
than the sun, they radiate at a longer
wavelength - in the infrared part of the
spectrum.
Greenhouse physics, two
• The IR light from the ground resonates with
the glass atoms; glass is OPAQUE to IR
radiation. As the glass warms, it will radiate
that energy in all directions - some to the
outside, but some energy is reradiated back
inside.
• This is why greenhouses and cars and
anything with glass windows will heat up
inside on a sunny day.
Glass absorbs and radiates
Infrared light; then reradiates part back inside and
part to the outside
Glass is
transparent to
Short-wave
visible light
The Earth’s greenhouse effect
depends on the transparency of
Air
• Mostly, air is transparent to both infrared
(long) waves and visible (short) waves
• However, some gasses in the atmosphere
are OPAQUE to infrared radiation.
• They act just like glass - letting the visible
light in, but only letting some of the IR light
out.
By Day
Energy Escapes to
space
Greenhouse gases
absorb IR light
energy, then re-emit
it in all directions
Energy comes back
to Earth
Warm Earth radiates IR light to air
By Night
Escapes to space
All night long, IR
light energy slowly
leaks out of the Earth
system
Comes back to Earth
Warm Earth radiates energy to air
Greenhouse gases
• Any gas that is transparent to visible light
but opaque to infrared light is a greenhouse
gas.
• Carbon dioxide and water vapor are two
such gasses. They constitute a TINY portion of
the air, but they play a BIG role.
ATMOSPHERIC GASES
This is where all the GHGs are
NITROGEN
OXYGEN
TRACE GASES
The problem
• We know we are tinkering with the natural
balance by increasing the levels of GHGs in
the atmosphere.
• We don’t know what the outcome will be.
Maybe hotter. Maybe colder.
• You can’t change just one thing because
Nature is a complicated place and
everything’s connected.
• What we DO know is that now is good for
humans.
Enhanced Greenhouse Effect Scary!
• What we are experiencing now is an enhanced
greenhouse effect due to recent human activity.
This fact is not in question.
• The vast majority of climate scientists believe that
the enhanced greenhouse effect (EGE) is due to
the release of additional CO2 into the atmosphere
as a result of burning fossil fuels.
Chapter 2
Factors affecting the Earth’s Mean
Temperature
Temperature and Radiation
Balance
• The equation for determining how the temperature
of the earth will change (or not) is given by:
• (incoming solar radiation minus outgoing
terrestrial radiation) x time / surface heat capacity,
or
Insolation
• How much energy we get from the sun
varies due to natural cycles
• The sun goes through an 11-year
sunspot cycle. Increased solar flare
activity decreases the sun’s light
output.
• Cyclical changes in the Earth’s orbit
can also change how much insolation
we receive.
Incoming Solar Radiation
• Radiation at top of Earth’s
atmosphere = 1368 W m-2
• If Earth flat disk with no
atmosphere, average
radiation = 1368 W m-2
• Earth 3-dimensional
rotating sphere,
– Area = 4r2
– Average solar heating =
1368  4 = 342 W m-2
Albedo
• How much sun energy is absorbed by a surface
depends largely on how shiny, or reflective, the
surface is.
• Albedo is the percentage of incoming solar
radiation that is reflected rather than absorbed;
reflected solar radiation divided by the incoming
solar radiation = albedo. The shinier the surface,
the higher the albedo.
• Snow, ice, and low-level clouds increase Earth’s
albedo. Vegetation, bare earth, and cirrus clouds
decrease Earth’s albedo.
Areas of
low abledo
Areas of high
albedo
Average Albedo
Click for more info on Albedo
• daily changes in surface albeo
• General info about albedo
Greenhouse Gases
• Major GHGs include, but are not limited to,
CO2, H2O, CH4, and N2O
• All of these have both natural and manmade origins.
• Of course, we’re pumping powerful,
unnatural GHGs into the atmosphere, too.
Factors Affecting Radiation
Balance: Volcanoes
• By far, the biggest polluter in the state is
Madame Pele.
• The volcano spews TONS of CO2 into the
air each day!
• Remember: if there weren’t natural sources
of GHGs, then we wouldn’t be having this
discussion because there would be no life
on Earth.
How all these add up
• Climate models are tough to develop
because of the interconnectedness of the
parts. For example:
• Warmer air and ocean leads to more cloud
cover, increasing albedo, and more
snowfall, increasing albedo, and
temperatures drop to iceage levels. Gobal
warming MIGHT lead to global cooling.
Or,
• Warming oceans release vast amounts of
CO2, and global temps rise suddenly and
dramatically; land ice melts, albedo
decreases; temps rise even further, melting
more ice; coasts are lost rising sea levels
and also to hurricanes, which gain in
strength and numbers; people move inland
en masse and clear forest for new cities and
agriculture, releasing even more carbon into
the air . . . . Etc……
Chapter 3
What makes a gas a Greenhouse Gas?
Resonance
• Infrared light from terrestrial radiation just
happens to match the natural frequency of
some molecules, now known as Greenhouse
Gases. GHGs resonate at IR frequencies.
• When IR light hits one of these molecules,
the molecules absorb the energy and begin
to bend and stretch; this light energy is
turned into kinetic energy (temp goes up!).
Click Link for original article
• The greenhouse gas molecules are
shown in the next series of figures along
with the IR spectra and the bending and
vibrations caused by absorbing the IR
radiation. The arrows on the molecules
indicate the direction of the bends and
vibrations of the bonds.
• The IR spectra graphs show the
wavelengths (on x axis) and the
transmittance (on y axis). At 100%
transmittance, no light is absorbed; the
molecule is transparent and the light passes
through. But at crucial wavelengths,
some/most of the energy is absorbed; thus,
transmittance falls.
Chapter 4
Evidence of Global Warming
Why do we think the globe is
warming up?
• The following are examples of
SYMPTOMS that indicate that the Earth is,
in fact, warming up.
• You will be asked to do some research of
your own on this topic.
Forests are drying out, dying
• Click link to see entire article published in
the Christian Science Monitor
• http://features.csmonitor.com/environment/
2009/01/22/us-forests-hold-new-evidenceof-global-warming/
Link to article from Lawrence Livermore Lab
re: Sea Level Increases
• New research suggests that ocean
temperature and associated sea level
increases between 1961 and 2003 were 50
percent larger than estimated in the 2007
Intergovernmental Panel on Climate Change
report. Observations that show sea levels
rose by 1.5 millimeters per year in the period
from 1961-2003. That equates to an
approximately 2.5-inch increase in ocean
levels in a 42-year span.
Your turn!
• Do a bit of research and provide further
symptoms of global warming. These might
include changes in natural or human
environments.
• What all these symptoms add up to is called
“climate change.”
Climate Change
• “Climate” refers to the long-term weather scheme
of an area. When the climate changes,
precipitation (rain, snow, etc) changes. This,
eventually, changes everything.
• Some areas are likely to become hot and dry,
others cool and wet, and still others may become
entirely frozen.
• If climates in food-growing regions become
seriously disrupted, expect hunger and famine.
Chapter 5
Scientific speculation as to the
underlying causes of these symptoms
Ice Cores - click link for full
article from Scientific American
• Researchers have recovered a nearly twomile-long cylinder of ice from eastern
Antarctica that contains a record of
atmospheric concentrations of carbon dioxide
(CO2) and methane--two potent and
ubiquitous greenhouse gases--spanning the
last two glacial periods. Analysis of this core
shows that current atmospheric
concentrations of CO2--380 parts per million
(ppm)--are 27 percent higher than the highest
levels found in the last 650,000 years.
Ice cores also show the connection
between CO2 and average global
temperature (click link for details)
• Average global average
temperatures have indeed been
rising while atmospheric CO2
increases at a rate of
approximately 1.6ppm per year
Your turn!
• Where are scientists looking to uncover the
mystery of climate change?
• Hint: lake sediments store pollen samples,
seabed sediments store all kinds of
information, tree rings tell about rainfall
patterns, amber preserves air samples from
ancient times.
Chapter 6
Lights! Cameras!
ACTION!
Human’s role in the Abundance
of Carbon in the Atmosphere
• Currently, much of Earth’s carbon is locked
up as fossil fuels and standing trees and
biomatter. These “standing carbon” sources
are called carbon sinks.
• When this carbon is burned or digested, it
becomes part of the air, as CO2 or as CH4
• CO2 is also stored in rocks, soil, and the
ocean. These are also considered sinks.
These release CO2 when heated.
Humans and Atmospheric
Carbon, continued
• Burning wood, burning fossil fuels, and
burning forests to create grasslands for
farting cows all transform carbon sinks into
atmospheric carbon.
• CO2 is a weak GHG, but we are pumping
LOTS of it into the air. CH4 (methane) is a
much more powerful GHG, and we are
creating a lot of it through ranching and
deforestation practices.
Begin with the
CO2
Turn it into
Amazon Rainforest wood
Burn to clear
land
CARBON SINK
Raise cattle on it
CH4
Transport the meat
CO2
Mmmmm
CO2
International Efforts to curb
Global Warming
• Kyoto Protocol
• Intergovernmental Panel on Climate
Change (IPCC)
• Asia-Pacific Partnership on Clean
Development and Climate
Industry Efforts
• Increased efficiency of power production
• Replacing the use of coal with cleaner fossil
fuels (oil, natural gas)
• Use of combined heating and power
systems (CHP)
• Increased use of nuclear and renewable,
alternative energy sources
• CO2 capture and storage
• Hybrid/alternative cars & transportation
Individual Efforts
• Walk, bike, bus, carpool. Live near to
where you work.
• Wash clothes with cold water. Take shorter
showers. Line dry your clothes.
• Use CFLs instead of regular light bulbs.
• Have fewer children.
• Eat less feedlot beef and pork. Grow your
own food. Buy from local farmers.