The greenhouse effect
What is it?
a natural process
works sort of like an actual greenhouse
(oversimplifying here …)
sun is the heat engine for both
Earth’s surface acts like the plants and
soil in the greenhouse
certain gases in Earth’s atmosphere act
like the glass of the greenhouse walls
Global Warming and
Sea Level Rise
Is it warm in here or is it just us (humans)?
Is it good or bad?
Venus - too much of a
good thing
The Venusian atmosphere
is comprised almost entirely
of CO2, an important
greenhouse gas.
Venus receives even more
solar energy than Earth.
So what would you expect
Venus’s surface to be like?
430oC
HOT! ~
(can melt lead)
“runaway greenhouse” effect
CO2 builds up in the atmosphere
atmosphere warms
no life can develop/survive
no hydrologic cycle (no liquid H2O)
no hydrosphere or biosphere to
take CO2 out of atmosphere
a lesson for Earthlings?
It is decidedly good - beneficial,
nay, essential, to life on Earth!
keeps Earth ~ 33oC (59oF) warmer
than it otherwise would be
15oC instead of –18oC (Brrr!!!)
Incoming!
The Sun emits radiant
energy at wavelengths
between 100-10,000 nm
of the electromagnetic
spectrum
reaches Earth’s atmosphere
as 1630 W/m 2 of energy
vast majority is between
wavelengths 200-2500 nm
200-400 nm – ultraviolet (UV)
400-700 nm – visible
700+ nm – infrared (IR)
~ 45% never reaches Earth’s surface
6% is reflected by atmosphere
20% is reflected by clouds
3% is absorbed by clouds
16% is absorbed by atmosphere
UV and visible wavelengths – ozone (O3)
IR wavelengths – water (H2O)
& carbon dioxide (CO2)
~ 55% reaches surface (yellow curve)
4% is reflected (mostly by ice)
51% is absorbed (by dark land & water)
peak wavelength is 500nm
(yellow-orange visible light)
relatively little infrared
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peak wavelength is 10 m
(= 10,000 nm) IR
no UV, very little visible light
emitted
but, together with clouds, they
absorb the 45% of outgoing
longwave radiation plus 19% of
incoming shortwave radiation.
Greenhouse gases (ghg’s) occur
as molecules of two or more
different atoms (note that N2 and
O2 are not ghg’s)
Which ghg contributes most to
the greenhouse effect?
Keeling curve
The famous “Keeling curve”
from Mauna Loa, Hawaii
far from big city influence
oscillations are seasonal
spring/summer – vegetation
takes up CO2
fall/winter – vegetation
releases CO2
≈
≈
H2O 60-70%
CO2 20-25%
O3 5-7%
others 3-8%
So why all the fuss about CO2?
Shouldn’t we be more worried
about water vapor?
answer: residence time (RT)
RTH2O 1 week
RTCO2 1 century (100 yrs)
RTCH4 1 decade (10 yrs)
choices (major contributors):
Global CO2 rise
≈
≈
CO2 0.035% of total
atmospheric gases
H2O 0.1-2.0% of total
(highly variable)
≈
6% escapes directly to space
45% is absorbed by gases in
the atmosphere
approximate contributions
≈
These are trace gases in the
atmosphere
≈
This energy is emitted from
Earth’s surface to Earth’s
atmosphere and to space.
Of the original 51%
≈
At night, Earth’s surface
emits the 51% of incoming
energy it absorbed from the
Sun during the day,
but at long wavelengths, in
the infrared portion of the
electromagnetic spectrum
The greenhouse gases
≈
Outgoing!
methane (CH4)
nitrous oxide (N2O)
carbon dioxide (CO2)
water vapor (H2O)
ozone (O3)
chlorofluorocarbons (CFC’s)
Global temperature rise
The famous “hockey stick” curve
of Mann et al., 1999
instrumental data since 1860
general increase over time
1998 was the warmest year on
record until 2005
natural forcings (volcanic and
solar) do not account for
observed warming after 1970
Ice core data from
Antarctica
actual atmospheric gas
bubbles trapped in ice
showed CO2 rose drastically
over last 1000 years
sharp increase at start of the
industrial revolution (~1800)
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Global sea level rise
Since 1993 global sea
level has risen 34 mm
(1.33 inches)
Sea level rise from
warming occurs via
two processes:
thermal expansion
water expands when
heated
accounts for ~2/3 of
total rise
melting glaciers
only glacial ice (landbased) contributes
melting/thinning sea
ice does not
contribute
accounts for ~1/3 of
total rise
change in mean sea level (mm)
Sea level future
Different climate models predict
different amounts of sea level change
in the next century (~0.2-0.6 m)
There is a great deal of uncertainty
as to how much melting will occur
It is possible that increased snowfall
will compensate for glacial melting in
a global warming scenario
It is equally possible that positive
feedbacks, such as basal melting
leading to higher glacial flow rates,
will speed up the glacial loss.
Sea level past
Sea level has risen more than
120 m since the peak of the last
ice age about 18000 years ago.
However, only 2-4 m of this
increase has occurred in the
last 6000 years.
From 3000 years ago to the
19th century the long term
change was roughly 0.5 meters
at a rate of 0.1 to 0.2 mm/yr.
Since 1850, sea level has
risen again at 1 to 2 mm/yr.
Since 1992 satellite altimetry
from TOPEX/Poseidon
suggests a rate of about
3 mm/yr.
This higher value may indicate
accelerating sea level change
from global warming.
Sea level in action
Melting or thinning sea ice does not
add to sea level because it is already
floating in the ocean.
The biggest concerns lie with melting
of the ice sheets on Greenland and
Antarctica
The Greenland ice sheet would
contribute about 7 m of sea level if it
melted completely.
The Antarctic ice sheets would
contribute about 60 m of sea level of
they melted!
This is possible, but it would take
several millenia.
But this is highly unlikely to ever occur.
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