Aerosols – The UNIPCC AR4 adjustment factor

Scientific effort should be dedicated towards resolving the biggest unknowns. After feedbacks, the largest area of uncertainty in forecasting future global warming is the measurement of radiative forcing components.

A quick analysis of the radiative forcing components table in the 2007 AR4 Summary Figure 2.4, page 17 (4.1MB pdf) , would suggest a number of fudge factors have been used to arrive at the results.

I have summarised the table below, less the fancy bars, but with the uncertainty spreads and some check totals.

Radiative Forcing Components
Derived from AR4 (accessed March 2012)

RF Effect (W m-2)

Forcing Component Mid-point

Low

High

Spread %

Carbon Dioxide

1.66

1.49

1.83

20%

Methane

0.48

0.43

0.53

21%

Nitrous Oxide

0.16

0.14

0.18

25%

Halocarbons

0.34

0.31

0.37

18%

Ozone – Stratospheric

-0.05

-0.15

0.05

400%

Ozone – Tropospheric

0.35

0.25

0.65

114%

Stratospheric water vapour from CH4

0.07

0.02

0.12

143%

Surface Albedo – Land Use

-0.20

-0.40

0.00

-200%

Surface Albedo – Black Carbon on Snow

0.10

0.00

0.20

200%

Aerosol – Direct effect

-0.50

-0.90

-0.10

-160%

Aerosol – Cloud lbedo effect

-0.70

-1.80

-0.30

-214%

Linear Contrails

0.01

0.003

0.03

270%

Net total

1.72

-0.61

3.56

242%

Of which:-
Positive Forcings

3.17

2.64

3.90

40%

Negative Forcings

-1.45

-3.25

-0.35

-200%

Assymetric Summing

1.72

0.65

2.29

96%

Total per the Report

1.60

0.60

2.40

113%

If these were financial figures, an external auditor might ask the following questions.

  1. Why do the columns not add up? The difference of 0.12 is the same as the figure for solar irradiance. I would guess that the error in the mid-point is due to someone having deducted this figure from the total, erroneously believing that they had previously included it.
  2. Given the breadth of uncertainty, is it more than a coincidence that the negative forcings almost exactly offset all the positive forcings with the exception of CO2? This conveniently reduces the language of the debate from discussing “anthropogenic greenhouse gases”, to “rising CO2″.
  3. Given the breadth of uncertainty, is it more than a coincidence that the range of negative forcings are exactly equal to 200% of the sum of the mid-points?
  4. Given the breadth of uncertainty, is it more than a coincidence that the range of postive forcings are almost exactly equal to 40% of the sum of the mid-points? Adjust any of the figures by .01, and the result becomes less exact.

This is an important issue, as this situation doubly increases the influence of CO2 on future warming. Firstly, it is the anthropogenic greenhouse gas that is consistently increasing. Others, like methane, levels, have stablised. Secondly, aerosols are likely to decrease in the future as countries develop and clean air legislation is enacted. Given the huge uncertainties in the other forcings, and possible fudge factors employed, it is possible that the significance of CO2 could be over-estimated a number of times. This is before water vapour feedbacks are considered.

Update June 3rd 2012.

Comparing with a paper published by James Hansen et al. in 2000, gives further circumstantial support to the fudge factors being employed.


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4 Comments

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    Reply
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