climate change and its consequences bill menke october 4, 2005
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Climate Change
and its consequences
Bill Menke
October 4, 2005
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1. The factors that influence global temperature
2. The role of greenhouse gasses
3. Anthropogenic emissions of greenhouse gasses
4. Natural Variability and Modeling efforts
5. Hazards of increased temperatures
Summary
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Sunlight – mostly visible light
Maximum = 1370 W/m2
Average* = 1370 / 4 = 343 W/m2
Aboput 30% reflected as visible light
The rest converted absorbed by earthEarth’s surface gets hot
and re-radiated energy as Invisible Infrared radiation343 W/m2
103 W/m2
240 W/m2
*average: surface area of earth is 4r2, but only a disk of diameter r2 faces the sun, hence the average is one fourth of the maximum
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1370 W/m2
One toaster per square meter
that’s a lot of heat …
1370 Watts
1 square meter
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Infrared energy radiated by a hot object depends
strongly
upon it temperature
1 square meter
temperature, T
E = c T4
With T in Kelvin andc = 5.6 x 10-8 W/m2K4
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So how hot is the earth’s surface
1 square meter
temperature, T
240 = 5.6 x 10-8 T4
240 Wattsdownward
Balanced by 240 Wattsupward
T = ( 240 / 5.6 x 10-8 )1/4
= 255 K = -17 deg C = 1 deg F
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Reality Check ! 255 K is too cold
We must be doing something wrong ...
New style of summer clothing
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We forgot the Atmosphere
Layer just thick enough to absorb allInfrared radiation that shines upon it
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Energy balance:IR shining into a layerMust be balanced by
energy radiated by layer
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Model with two layer atmosphereTop of Atmosphere
upward radiation must balance solar input
cT04 = 240 W/m2
T0 = 255K
Top surface240 up
Bottom surface240 down
Sun240 down
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Total energy leaving upper atmosphere2 x 240 W/m2
Must equal energy received from lower atmosphere
T0 = 255K
cT14 = 2 x 240 W/m2
T1 = 304K
Lower atmosphere must be hotter to balanceheat shining down from upper atmosphere
240 up
240 down
480 up
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Suppose you had three layers …
T0 = 255K
cT24 = (480 + 480 – 240) W/m2
T1 = 304K
Lower atmosphere must be hotter to balanceheat shining down from upper atmosphere
T2 = 337K
480 up
480 down
720 up
240 down
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The more layers, each just opaque enough to absorb IR radiation, the hotter the lower
atmosphere is
How many layers in the Earth’ atmosphere?
Somewhere between 2 and 3, probably closer to 2(well, this is a very simplified model …)
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What controls the IR opacity of the atmosphere
The concentration of greenhouse gasses:
Water vaporCarbon dioxide
MethaneHalocarbonsNitrous oxide
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How do you compare greenhouseGas concentrations ?
The effect on the IR opacity is what’s important
Not their actual concentration in the atmosphere
Standard practice – represent as an equivalent change in solar radiation,
“radiative forcing”
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Greenhouse gasses have been increasing due to anthropogenic causese.g. burningfossil fuels
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Can future rise in atmosphericCO2 be predicted?
Sources of atmospheric CO2:burning fossil fuelsdeforestation
Sinks of atmospheric CO2:absorption into oceanforest growth
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Spatial pattern of oceans absorption and emmission of CO2 is very variable
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But remember … water vapor is the major greenhouse gas
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That’s a problem for models of global warming
concentration of water vapor very variable
they are also temperature dependenthotter climate – more water vapor
but water vapor creates clouds, which increase the earth’s albedo
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The earth is getting hotter
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Is the increase in radiative forcing causing this global warming n?
And if so
Will temperatures continue to increase as we continue to add
greenhouse gasses to the atmosphere?
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Lesson from the Ice Age: global temperature has been very variable, soNatural variation needs to be seriously considered …
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Models of climate necessary to sort out causes
But
How do you build confidence that models are correctly predicting the climate system?
Traditional well-controlled experiments are impractical …
One approach … model the past
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An attempt to modelLast 150 yearTemperature variationsRequires bothNatural andAnthropogenicForcing
Note big increase inAnthropogenic forcingSince 1950
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EXEMPLARY IMPACT : SEA LEVEL RISE
Light grey: parts of MississippiDelta that are less than 5 metersbelow sea leavel.
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Why Sea Level Rise?
• Melting of Glaciers, especially in Greenland and West Antarctica
– Huge effect: 7 meters if you melt Greenland alone
• Thermal expansion of ocean waters as you heat them
– Huge effect: 1 meter per deg C, if you warmed the entire world ocean down to the sea floor
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A very large percentage ofpeople live at low elevations
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List of Physical ImpactsTemperature
hotter peak temperaturesfewer really cold dayshotter nights
Weathermore droughtsmore intense wind in hurricaneswarmer winters
Precipitationmore intense rain stormsmore evaporation, less runoff in rivers