1. MET 12 Global Climate Change – Lecture 8
Radiative Forcing
Shaun Tanner
San Jose State University
Outline
GHG/Aerosols
Radiative Forcing
Activity
The primary goal of the concept of radiative forcing is to quantify all the factors that have contributed to our changing climate including the increases in greenhouse gases, increases in aerosols, ozone increases in the troposphere and decreases in the stratosphere, and changes in the sun.
MET 112 Global Climate Change

2. 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.

3. MET 112 Global Climate Change
That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.
MET 112 Global Climate Change

4. 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

5. Atmospheric Aerosols Microscopic liquid/solid particles
Natural sources - examples
Volcanoes (sulfur)
Fires, dust
Dust, sulfate particles reflect incoming sunlight: ___________________
Smoky soot absorb incoming sunlight:
____________________

6. 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 atmosphere
Warm atmosphere

7. MET 112 Global Climate Change

8. MET 112 Global Climate Change

9. 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

10. Aerosols (III) There are three major types of aerosols
Sulfate aerosols
Black carbon aerosols
Organic carbon aerosols

11. 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.

12. Aerosol Observations from NASA Satellite
That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

13. MET 112 Global Climate Change
That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.
MET 112 Global Climate Change

14. That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

15. 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

16. 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

17. 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

18. 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.

19. 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

20. Over the last 250 years, CO2 concentrations have increased
Over the last 250 years, CO2 concentrations have increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

21. Over the last 250 years, organic carbon aerosol concentrations have increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

22. Over the last 250 years, black carbon aerosol concentrations have increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

23. Over the last 250 years, the earth’s albedo has changed as a result of deforestation. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

24. Over the last 250 years, the amount of low clouds in the atmosphere has increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

25. Over the last 250 years, the intensity of the sun’s incoming radiation has increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

26. Over the last 250 years, aircraft induced high clouds have increased
Over the last 250 years, aircraft induced high clouds have increased. The earth’s surface would therefore
Receive more energy resulting in warming
Receive less energy resulting in cooling
Stay the same

27. 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.

28. Figure 3.38 Earth’s annual energy budget
Bloom-Fig jpg

29. 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

30. 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)
So, the idea of radiative forcing is like this. We want to compare the Earth’s energy budget today (2000) with preindustrial times (1750) and account for the changes in greenhouse gases, aerosols, ozone amounts and solar variations.

31. 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

32. 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
If we just consider increases in greenhouse gases(GHGs), then the additional GHGs in the atmosphere today compared with the year 1750 produce additional warming or cause an increase in the radiative forcing (positive value). We will see how large this is in a bit.

33. Ozone Radiative Forcing: negative Radiative Forcing: positive
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
As outlined above, changes in ozone cause both a positive and negative forcing due to the differences between tropospheric ozone (which is ozone that exists in the troposphere and that is produced mainly by automobile and factory emissions) and stratospheric ozone (which is our ozone layer).

34. Ozone
As outlined above, changes in ozone cause both a positive and negative forcing due to the differences between tropospheric ozone (which is ozone that exists in the troposphere and that is produced mainly by automobile and factory emissions) and stratospheric ozone (which is our ozone layer). 34 34

35. 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

36. Figure 4.27 Changes in global forcing in W m–2 by different factors from 1750 to 2005
Bloom-Fig jpg

37. Radiative Forcing from the IPCC
This is the ‘big’ radiative forcing diagram! It’s really a great summary of the science climate change and it’s a widely cited and used tool for understanding the processes that influence today’s climate.
In this diagram, there are 13 separate forcing terms that are displayed. When we say forcing terms, we mean processes that influence (or force) the climate. The first one is due to increases in GHGs (CO2, N2O, CH4 and Halocarbons).

38. Radiative Forcing from the IPCC1 2
3.0 3 4 5
-1.4 6 7
This is the ‘big’ radiative forcing diagram! It’s really a great summary of the science climate change and it’s a widely cited and used tool for understanding the processes that influence today’s climate.
In this diagram, there are 13 separate forcing terms that are displayed. When we say forcing terms, we mean processes that influence (or force) the climate. The first one is due to increases in GHGs (CO2, N2O, CH4 and Halocarbons).
Avg is 1.6
Max is =3.4
Min is =-0.6 8 9 10

39. Radiative Forcing from the IPCC
What does this part of the diagram mean?

40. 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 – (average is 1.66)

41. 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, 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!
Simply take the GHG forcing and ozone depletion term and combine. First add the average values (the mean value (2.4+ (-0.1)) to get the combined radiative forcing, and then calculate the uncertainty or range of possible values. You can do this by adding the most positive value (2.6 and 0) with the most negative (2.2 and –0.2) to give you the range (2.0 to 2.6).
This now means that even though ozone depletion acts to cool the earth, combined with increases in GHGs will still cause the earth to warm. The enhanced GHGs are quite strong and overwhelm the cooling of ozone depletion.

42. 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, 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!
Simply take the GHG forcing and ozone depletion term and combine. First add the average values (the mean value (2.4+ (-0.1)) to get the combined radiative forcing, and then calculate the uncertainty or range of possible values. You can do this by adding the most positive value (2.6 and 0) with the most negative (2.2 and –0.2) to give you the range (2.0 to 2.6).
This now means that even though ozone depletion acts to cool the earth, combined with increases in GHGs will still cause the earth to warm. The enhanced GHGs are quite strong and overwhelm the cooling of ozone depletion.

43. Terms in the radiative forcing diagram
Term 1-2: Increases in CO2, CH4, N2O and CFCs
Term 3: Decreases in upper atmospheric ozone (ozone depletion)
Increases in lower atmosphere ozone
Term 4: Increases in water vapor (due to extra CH4)
Term 5: Changes in Albedo
Increases in black carbon:
Increase in surface albedo (deforestation):
Term 6: Increases in sulfate aerosols

44. 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)
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)

45. Terms in the radiative forcing diagram
Term 7: Increases in aerosols altering cloud properties (more low level clouds)
Term 8: Increases in aircraft induced high clouds (contrails)
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)
Term 10: Total anthropogenic forcing

46. 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)
Term 10: Total anthropogenic forcing
Produce warming (positive)

47. Activity
Calculate the total mean radiative forcing from the provided figure of individual radiative forcing from the IPCC. Please show your work!
Calculate the total range of possible values from the above calculation.
What conclusions does the total mean radiative forcing tell us about how the climate has changed?
How does the range of values (or uncertainties) affect our above conclusions?
So, the idea now is to add up all the terms and all the uncertainties to get the total radiative forcing. Your final answer for 1 and 2 will be something like:
Mean: 3.3 W/m2 with a range from –0.3W/m2 to +4.5W/m2.

48. What is the total radiative forcing?
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

49. Q2: What is the maximum radiative forcing
Between -5 and -3 W/m2
Between -3.0 and -1.0 W/m2
Between -1.0 and 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

50. Q2: What is the minimum radiative forcing
Between -5 and -3 W/m2
Between -3.0 and -1.0 W/m2
Between -1.0 and 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

51. Q3: What conclusions does the mean forcing tell us
Warming forcing outweighs cooling
Cooling forcing outweighs warming
Human induced warming is much stronger than naturally forced warming.
Both 1 and 3
Both 2 and 3

52. Q4: What conclusions does the range of radiative forcing tell us
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