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MET 112 Global Climate ChangeMET 112 Global Climate Change
MET 12 Global Climate Change – Lecture 8
Radiative Forcing
Shaun TannerSan Jose State University
Outline GHG/Aerosols Radiative Forcing Activity
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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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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) Sulfate Carbon Biomass burning (black carbon)
– Land Use Changes
First part of today’s
lecture
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Atmospheric Aerosols
Microscopic liquid/solid particles– Natural sources - examples:
Volcanoes (sulfur) Fires, dust
– Dust, sulfate particles reflect incoming sunlight: ___________________
– Smoky soot absorb incoming sunlight: ____________________
Cool atmosphereCool atmosphere
Warm atmosphereWarm atmosphere
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MET 112 Global Climate ChangeMET 112 Global Climate Change
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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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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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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
Black Carbon (Aerosols)
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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
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
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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.
Figure 3.38 Earth’s annual energy budget
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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
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
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2
3
4
5
6
7
8
9
10
3.0
-1.4
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!
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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. Between -5 and -3 W/m2
2. Between -3 and -1 W/m2
3. Between -1.0 and -0.1 W/m2
4. Between 0 and 1.0 W/m2
5. Between 1.0 and 2.0 W/m2
6. Between 2 and 4 W/m2
7. 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. The warming forcing is more certain than the cooling.
2. The cooling forcing is more certain than the warming.
3. The earth should be warming, but by how much is difficult to say because of the uncertainty of the cooling factors.
4. The cooling is stronger than the warming and thus the earth should cool.
5. Both 1 and 3
6. Both 1 and 4
7. Both 2 and 3
8. Both 2 and 4
