Download Radiative Forcing: negative

MET 12 Global Climate Change – Lecture 8
Radiative Forcing
Shaun Tanner
San Jose State University
Outline
 GHG/Aerosols
 Radiative Forcing
 Activity
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MET 112 Global Climate Change
Climate Change: how much and in
what direction?
 We have studied the various factors that contribute to
climate change.
 Some of these contribute to warming the climate
 Some of these contribute to cooling the climate
 It would be helpful if we could develop a tool for
measuring the strength of the various warming and
cooling factors.
 The Radiative Forcing calculation is a tool for measuring
how climate will change due to a particular forcing
mechanism.
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MET 112 Global Climate Change
Anthropogenic Forcing
 Without doubt, humans have altered the radiative balance of
the Earth system. The changes can be partitioned into the
following categories:
– Enhanced greenhouse gases
 CO2, CH4, N2O, CFC’s
– Ozone
 Tropospheric
 Stratospheric
– Aerosols (Natural and Anthropogenic)
First part of
 Sulfate
today’s lecture
 Carbon
 Biomass burning (black carbon)
– Land Use Changes
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Atmospheric Aerosols
 Microscopic liquid/solid particles
– Natural sources - examples:
 Volcanoes (sulfur)
 Fires, dust
– Dust, sulfate particles reflect incoming sunlight:
___________________
Cool atmosphere
– Smoky soot absorb incoming sunlight:
____________________
Warm atmosphere
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MET 112 Global Climate Change
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MET 112 Global Climate Change
Aerosols (II)
 Natural sources include:
– Dust, sea salt and volcanic emissions
 Anthropogenic sources include
– Automobiles, factories and biomass burning.
 Biomass burning:
– Anthropogenic portion: burning of large forests for
agriculture
 Aerosols have ‘short’ relative lifetimes
– They can ‘float’ around for a few days to a week or two.
 Aerosols affect the Earth’s energy balance by
– reflecting incoming energy and/or
– absorbing incoming shortwave and longwave radiation.
 Cooling influence is stronger
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Aerosols (III)
 There are three major types of aerosols
– Sulfate aerosols
– Black carbon aerosols
– Organic carbon aerosols
– All have been increasing in concentration
over the last 150 years – the industrial
revolution.
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Aerosol Observations from NASA Satellite
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MET 112 Global Climate Change
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Sulfate Aerosols




Natural source: volcanoes
Anthropogenic sources: burning of fossil fuels
90% of sulfur aerosols are anthropogenic
Example: SO2 (sulfur dioxide)
– From coal combustion
 Sulfate aerosols increasing globally
 Sulfate aerosols reflect incoming solar radiation.
 Total effect on Earth’s energy budget
– Cooling
– Radiative Forcing: negative
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Black Carbon (Aerosols)
 Natural source:
– Natural biomass burning
 Anthropogenic source
– incomplete combustion from coal and diesel
engines; biomass burning
 Also know as - ‘Elemental Carbon’ or ‘soot’
 Black carbon absorbs solar radiation
– It’s black so has a low albedo (can also affect
snow when it falls to the ground).
 Potentially harmful if inhaled.

Total effect on Earth’s energy budget
– Warming
– Radiative Forcing: positive
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Organic Carbon (Aerosols)
 Natural source
– Natural biomass burning
 Anthropogenic source
– Burning fuel
– Biomass burning
 Produced as a result of incomplete combustion.
 These aerosols are reflective
 Total effect on Earth’s energy budget
– Cooling
– Radiative Forcing: negative
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Indirect Effect due to aerosols (I)
 Certain aerosols may enhance cloud production and
character
 Recall how clouds form
– Water vapor condenses to liquid water
– This processes requires ‘cloud condensation
nuclei’
– Examples of cloud condensation nuclei
 Dust, salt, smoke (all of which are natural
aerosols)
 So, aerosols (with both natural and anthropogenic
origin)
– may serve as cloud condensation nuclei.
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Indirect Effect due to aerosols (II)
 More cloud condensation nuclei
– would enhance cloud production
 The question then is how would clouds change
– Current understanding is that
This processes would increase cloud albedo
 This idea of enhanced cloud formation by increases in aerosols
is termed
– The ‘indirect effect’ of aerosols
 Understanding of these processes is currently incomplete.
– But model results suggest more low clouds and thus

Total effect on Earth’s energy budget
– Cooling
– Radiative Forcing: negative
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Over the last 250 years, CO2
concentrations have increased. The
earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, organic carbon
aerosol concentrations have increased.
The earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, black carbon
aerosol concentrations have increased.
The earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, the earth’s albedo
has changed as a result of deforestation.
The earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, the amount of low
clouds in the atmosphere has increased.
The earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, the intensity of the
sun’s incoming radiation has increased.
The earth’s surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Over the last 250 years, aircraft induced
high clouds have increased. The earth’s
surface would therefore
1. Receive more energy
resulting in warming
2. Receive less energy
resulting in cooling
3. Stay the same
Radiative Forcing
 A change imposed upon the climate system
which modifies the Earth’s energy (radiative)
balance.
 Radiative forcing is usually given in units of
– watts per square meter (W/m2),
 Positive values of radiative forcing
– contribute to heating of the surface,
 Negative values of radiative forcing
– Contribute to cooling of surface.
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Figure 3.38 Earth’s annual energy budget
Radiative Forcing
 Examples of radiative forcing mechanisms
include
– Changes in solar intensity
– Volcanic activity
– Changes in atmospheric composition
 CO2
 Aerosols
 Ozone
– Changes in land surfaces
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Radiative Forcing
 Changes in these mechanisms
– produce changes to the earth energy
budget.
 The magnitude of the radiative forcing
determine how strong the effect is.
 Radiative forcing is computed by comparing
– Pre-industrial energy balance (1750) with
today’s energy balance (2000)
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Enhanced Greenhouse Gases
 Greenhouse gas concentrations have increased over
the last 150 years dramatically
 The main anthropogenic contribution to greenhouse
gases concentrations include:
 CO2, CH4, N2O, CFC’s (Halons)
 Increases in greenhouse gas concentrations are well
observed
 Radiative Forcing: positive
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Ozone
 Ozone exists in upper atmosphere
– Ozone layer (stratospheric ozone)
– Ozone layer protects the earth from harmful UV radiation
– Ozone layer responsible for heating the stratosphere
– Stratospheric ozone levels have been declining over last 20
years (ozone depletion)
– Radiative Forcing: negative
 Ozone exists in lower atmosphere
– Tropospheric ozone is another word for ‘smog’
– Tropospheric ozone levels have increased over last 50
years.
– Radiative Forcing: positive
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Ozone
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Land Use Change
 Changes in the land use have contributed to
– Albedo changes
 Deforestation has been largest contributor
 High latitudes have been most affected
– Pre Industrial: Snow covered forests (low
albedo)
– Current: Open snow covered areas (high
albedo)
 Radiative Forcing: negative
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Figure 4.27 Changes in global forcing in W m–2 by different factors from 1750 to 2005
Radiative Forcing from the IPCC
Radiative Forcing from the IPCC
1
2
3.0
3
4
5
-1.4
6
7
8
9
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Radiative Forcing from the IPCC
What does this part of the diagram mean?
Radiative Forcing from the IPCC
What does this part of the diagram mean?
Increases between 1750 through today have
caused a 1.66 Watts per meter squared increase
in the earth’s radiation budget.
This by itself would warm the earth’s surface.
Radiative forcing = 1.49 – 1.83 (average is
1.66)
Example

Imagine you and your partner get offers to work for a new progressive
company. They use a pay scale with ‘incentives’. You will get paid: $35,000
 5,000 depending on your performance and your partner will get paid
$75,000  60,000 . Calculate you and your partner’s total salary.
– The total (combined) salary is with no ‘incentives’:
– Your salary ranges from
– Your partner’s salary ranges from
 Maximum possible salary:
 Minimum possible salary:
 So, the total salary is between

 Big uncertainity!
Example





Imagine you and your partner get offers to work for a new progressive
company. They use a pay scale with ‘incentives’. You will get paid: $35,000
 5,000 depending on your performance and your partner will get paid
$75,000  60,000 . Calculate you and your partner’s total salary.
– The total (combined) salary is with no ‘incentives’:
= $110,000
– Your salary ranges from
$30,000 to 40,000
– Your partner’s salary ranges from
$15,000 to 135,000
Maximum possible salary:
$175,000
Minimum possible salary:
$45,000
So, the total salary is between
$45,000 – 175,000
 Big uncertainity!
Terms in the radiative forcing diagram
 Term 1-2: Increases in CO2, CH4, N2O and CFCs
– producing warming (positive)
 Term 3: Decreases in upper atmospheric ozone (ozone
depletion)
– provide less heating of upper atmosphere (negative)
Increases in lower atmosphere ozone
– produce warming (positive)
 Term 4: Increases in water vapor (due to extra CH4)
– produce warming (positive)
 Term 5: Changes in Albedo
– Increases in black carbon: produce warming (positive)
– Increase in surface albedo (deforestation): produce
cooling (negative)
 Term 6: Increases in sulfate aerosols
– produce cooling (negative)
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Terms in the radiative forcing diagram
 Term 7: Increases in aerosols altering cloud properties
(more low level clouds)
– produce cooling (negative)
 Term 8: Increases in aircraft induced high clouds
(contrails)
– produce warming (positive)
 Term 9: Increases in the strength of the sun
– produce warming (positive) Term 10-11: Increases in
aircraft induced high clouds (contrails and cirrus
clouds)
– produce warming (positive)
 Term 10: Total anthropogenic forcing
– Produce warming (positive)
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Activity
1. Calculate the total mean radiative forcing from the
provided figure of individual radiative forcing from
the IPCC. Please show your work!
2. Calculate the total range of possible values from the
above calculation.
3. What conclusions does the total mean radiative
forcing tell us about how the climate has changed?
4. How does the range of values (or uncertainties)
affect our above conclusions?
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What is the total radiative forcing?
1.
2.
3.
4.
5.
6.
7.
Between -5 and -3 W/m2
Between -3 and -1 W/m2
Between -1.0 and -0.1 W/m2
Between 0 and 1.0 W/m2
Between 1.0 and 2.0 W/m2
Between 2 and 4 W/m2
Between 4 and 6 W/m2
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Q3: What conclusions does the mean
forcing tell us
1. Warming forcing outweighs
cooling
2. Cooling forcing outweighs
warming
3. Human induced warming is much
stronger than naturally forced
warming.
4. Both 1 and 3
5. Both 2 and 3
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Q4: What conclusions does the range
of radiative forcing tell us
1.
2.
3.
4.
5.
6.
7.
8.
The warming forcing is more certain than the
cooling.
The cooling forcing is more certain than the
warming.
The earth should be warming, but by how
much is difficult to say because of the
uncertainty of the cooling factors.
The cooling is stronger than the warming and
thus the earth should cool.
Both 1 and 3
Both 1 and 4
Both 2 and 3
Both 2 and 4
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