Radiative Forcing of Climate Change

TLDR: This article reviewed the current understanding of mechanisms that are, or may be, acting to cause climate change over the past century, with an emphasis on those due to human activity, and discussed the general level of confidence in these estimates and areas of remaining uncertainty.

Abstract: Our current understanding of mechanisms that are, or may be, acting to cause climate change over the past century is briefly reviewed, with an emphasis on those due to human activity. The paper discusses the general level of confidence in these estimates and areas of remaining uncertainty. The effects of increases in the so-called well-mixed greenhouse gases, and in particular carbon dioxide, appear to be the dominant mechanism. However, there are considerable uncertainties in our estimates of many other forcing mechanisms; those associated with the so-called indirect aerosol forcing (whereby changes in aerosols can impact on cloud properties) may be the most serious, as its climatic effect may be of a similar size as, but opposite sign to, that due to carbon dioxide. The possible role of volcanic eruptions as a natural climate change mechanism is also highlighted.

Figures

Table 2: Estimates of the Global Temperature-change Potential for three time 7 horizons for aviation emissions, relative to CO2 for the present-day aviation fleet for 8 each kg of fuel burnt. The bottom 4 rows show how much the CO2 effect, for each kg 9 of fuel burnt, should be multiplied to account for the non-CO2 effects. 10 11

Table 1: Estimates of the Global Warming Potential for three time horizons for 1 aviation emissions, relative to CO2 for the present-day aviation fleet for each kg of 2 fuel burnt. The bottom 4 rows show how much the CO2 effect, for each kg of fuel 3 burnt, should be multiplied to account for the non-CO2 effects. 4 5

Figure 2: Radiative forcing as a function of latitude for global aviation in 1992, 2 relative to pre-industrial times, for a number of aviation-induced forcings. Note that 3 the global-mean of these values will not correspond to those in Figure 1, as they are 4 for a different period and because of improvements in understanding. Reproduced 5 from IPCC (1999) © Cambridge University Press. 6 7 8

Figure 1. Global-mean radiative forcing components due to aviation for 2005, relative 2 to preindustrial times. Coloured bars represent best estimates (except for the case of 3 aviation-induced cloud changes (AIC) where a best-estimate cannot be given). Values 4 indicated by the white lines within the bars are from Forster et al. (2007). The induced 5 cloudiness (AIC) estimate includes linear contrails. Numerical values are given on the 6 right for both Forster et al. (2007) (in parentheses) and for Lee et al. (2009). Error 7 bars represent the 90% likelihood range for each estimate. The best estimate value of 8 total radiative forcing from aviation is shown with and without AIC. The spatial scale 9

Full text

Radiative forcing and climate change
Book or Report Section
Accepted Version
Shine, K. (2015) Radiative forcing and climate change. In:
Blockley, R. and Shyy, W. (eds.) Encyclopedia of Aerospace
Engineering. John Wiley & Sons, Ltd., pp. 1-11. ISBN
9780470686652 doi:
https://doi.org/10.1002/9780470686652.eae526.pub2
Available at https://centaur.reading.ac.uk/40552/
It is advisable to refer to the publisher’s version if you intend to cite from the
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Publisher: John Wiley & Sons, Ltd.
Publisher statement: This is the submitted version of K. Shine "Radiative Forcing
and Climate Change" in Encyclopedia of Aerospace Engineering, eds R. Blockley
and W. Shyy, John Wiley: Chichester. DOI: 10.1002/9780470686652.eae526.pub2
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Submitted 27 November 2014
1
Prepared for the Encylopedia of Aerospace Engineering
1
2
Reference: EAE526.PUB2
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Professor Keith P Shine FRS, Department of Meteorology, University of Reading,
5
Earley Gate, Reading RG6 6BB, UK
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email: k.p.shine@reading.ac.uk Tel: 0118 378 8405 Fax: 0118 378 8905
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Prepared in Microsoft Word 2007
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Figure files:
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aerospace_aea526_pub2_fig1.tif
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aerospace_aea526_fub2_fig2.gif
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This is the submitted version of
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K. Shine "Radiative Forcing and Climate Change" in Encyclopedia of
15
Aerospace Engineering, eds R. Blockley and W. Shyy, John Wiley: Chichester.
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DOI: 10.1002/9780470686652.eae526.pub2 which has been published in final form
17
on http://onlinelibrary.wiley.com/book/10.1002/9780470686652
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RADIATIVE FORCING AND CLIMATE CHANGE
20
21
Keith P. Shine
22
Department of Meteorology
23
University of Reading
24
Reading, United Kingdom
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26
Keywords: Radiative forcing, carbon dioxide, climate change, global warming
27
potential, global temperature-change potential
28
29
Abstract
30
31
Aviation causes climate change as a result of its emissions of CO
2
, oxides of nitrogen,
32
aerosols and water vapour. One simple method of quantifying the climate impact of
33
past emissions is radiative forcing. The radiative forcing due to changes in CO
2
is best
34
characterised, but there are formidable difficulties in estimating the non-CO
2
forcings
35
– this is particularly the case for possible aviation-induced changes in cloudiness
36
(AIC). The most recent comprehensive assessment gave a best-estimate of the 2005
37
total radiative forcing due to aviation of about 55 to 78 mW m
-2
depending on whether
38
AIC were included or not, with an uncertainty of at least a factor of two,. The aviation
39
CO
2
radiative forcing represents about 1.6% of the total CO
2
forcing from all human
40
activities. It is estimated that, including the non-CO
2
effects, aviation contributes
41
between 1.3 and 14% of the total radiative forcing due to all human activities.
42
Alternative methods for comparing the future impact of present-day aviation
43
emissions are presented – the perception of the relative importance of the non-CO
2
44
emissions, relative to CO
2
, depends considerably on the chosen method and the
45
parameters chosen within those methods.
46
47

Submitted 27 November 2014
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1. INTRODUCTION
1
2
The possibility that aviation could contribute to climate change was considered in the
3
earliest assessments of the climate impact of human activity (e.g., Matthews, Kellogg
4
and Washington, 1971). In the subsequent period, considerable attention focused on
5
examining the impact of possible future supersonic fleets on stratospheric ozone;
6
aviation’s impact on climate took a back seat. The European assessment by Brasseur
7
et al. (1998) gave renewed vigour to the appraising the climate influence. This was
8
followed by the wider-ranging report on “Aviation and the Global Atmosphere” by
9
the Intergovernmental Panel on Climate Change (IPCC, 1999), which remains a basic
10
reference. Lee et al. (2009, 2010) have provided updated assessments.
11
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2. CONCEPTUAL FRAMEWORK
13
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2.1 Radiative forcing
15
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The planetary energy balance can be characterised as a balance between two
17
components: solar radiation (which is mostly at wavelengths less than 4 m)
18
absorbed or reflected back to space by the Earth and its atmosphere, and longwave
19
(thermal-infrared) radiation (which is mostly at wavelengths greater than 4 m)
20
emitted and absorbed by the Earth’s surface and atmosphere.
21
22
At the top of the atmosphere, averaged over the globe and over a year, there is a close
23
balance between the absorbed solar radiation (ASR) by the Earth system and the
24
outgoing longwave radiation (OLR), so that the net radiation (NET)
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NET = ASR – OLR ≈ 0. (1)
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Satellite observations show that the global and annual mean ASR and OLR are both
29
about 240 W m
-2
(e.g., Hartmann, 1996).
30
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The main mechanisms which drive climate change perturb either or both the ASR or
32
the OLR, so that NET≠0. More precise definitions are available (e.g., Myhre et al.,
33
2013), but the size of the initial perturbation of NET, say following a change in CO
2
34
concentration, is a useful working definition of the radiative forcing (RF) of climate
35
change. RF provides a useful first-order indication of the size of different climate
36
change mechanisms and it will be a major focus here.
37
38
Unless otherwise stated, RF is taken here to refer to its global-average value. RF can
39
refer to the change in NET over any specified period of time. Forcing due to human
40
activity is often reported as the total change since some “pre-industrial” time, for
41
example since 1750. For aviation, the total present-day forcing has occurred over a
42
much shorter period, as emissions were negligible prior to 1940 (e.g., Lee et al.,
43
2009).
44
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2.2 Temperature response and climate sensitivity
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When RF is positive the planet is absorbing more energy than it is emitting (and vice
48
versa if RF is negative); the climate system responds by warming, leading to more
49
infrared emission and hence a higher OLR. Given sufficient time (and assuming that
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Submitted 27 November 2014
3
RF is not time varying), the system will reach a new equilibrium, where NET is once
1
again close to zero.
2
3
The simplest representation of the response of the climate system to a radiative
4
forcing is given by Hartmann (1996) and Fuglestvedt et al., (2010)
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6
( ) ( )
()
d T t T t
C RF t
dt



(2)
7
where t is time, C is the heat capacity of the climate system (most of which is
8
contained within the ocean) [J K
-1
m
-2
], ΔT is the departure of the global-mean surface
9
temperature [K] from its unperturbed value, and λ is the climate sensitivity parameter
10
[K (W m
-2
)
-1
].
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A useful special case is when RF is independent of time. The solution to Equation (2)
13
is then
14
15
)]exp(1[)(
C
t
RFtT



. (3)
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As t tends to infinity, the equilibrium surface temperature ΔT
eq
is given by
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19
ΔT
eq
=λRF. (4)
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Hence, Equation (4) tells us that ΔT
eq
resulting from a (constant) radiative forcing is
22
simply the product of that forcing and the climate sensitivity parameter. This equation
23
provides much of the justification for using RF as an indicator of climate change.
24
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Equation (3) shows that the product of λ and C defines a time constant for the climate
26
system to respond to an RF and is of order a few decades; a precise number cannot be
27
given, because of uncertainties in the value of λ (see below) and the value of C is not
28
well-defined, as it depends on the rate at which heat is transferred from the surface
29
layers of the ocean to the deep ocean.
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2.3 Climate feedbacks
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The value of λ is a chronic uncertainty in climate change science. If the Earth and
34
atmosphere emitted to space as a black-body, Stefan’s Law would give the OLR as
35
σT
e
4
where σ is the Stefan-Boltzmann constant and T
e
is an effective emitting
36
temperature of the climate system. In this case, the first derivative of Stefan’s Law is
37
4σT
e
3
, and its reciprocal would give λ. For an OLR of 240 W m
-2
, λ would be about
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0.3 K (W m
-2
)
-1
, indicating that the planet would warm up by 0.3 K for each W m
-2
of
39
forcing. This is sometimes referred to as the “black-body” or “no-feedback” response.
40
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However, as the Earth warms (or cools) a number of feedback processes occur that
42
alter the radiative properties of the atmosphere and surface. For example, in response
43
to a positive RF, a warmer atmosphere can contain more water vapour; water vapour
44
is a greenhouse gas and hence this further enhances the warming – giving a positive
45
feedback. Similarly, a warmer planet would be expected to have a decreased extent of
46
snow and ice – this would decrease the amount of solar radiation reflected back to
47
space leading to a further warming. These two feedbacks are believed to be relatively
48

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Radiative forcing and climate change" ?

The most recent comprehensive assessment gave a best-estimate of the 2005 total radiative forcing due to aviation of about 55 to 78 mW m −2 depending on whether AIC were included or not, with an uncertainty of at least a factor of two this paper. 

Q2. how long does the GWP decrease with time?

33 34 Table 1 shows that for all components, the GWP decreases with time horizon – this is 35 because the values are referenced to CO2, a significant component of which remains 36 in the atmosphere for much longer than 500 years; hence the part of the CO2 pulse 37 that remains in the atmosphere even at 500 years continues to contribute to (the 38 integral of) RF, whereas the RF from the short-lived components have long since 39 decayed to zero. 

Q3. How many years of time is the GWP given?

The bottom 4 lines in the Tables show how much the CO2 effect has to be 29 multiplied to incorporate the non-CO2 effects; it is given for the two NOx cases and 30 with and without AIC, because of the particularly high uncertainty. 

Q4. What is the current estimate of the global warming potential?

for example, the mass of 39 methane emitted in a given year can be cast in “CO2-equivalence” terms by 40 multiplying it by 28, the current estimate for methane’s100-year GWP (Myhre et al., 41 2013). 

Q5. How long would the GWP decay to zero?

19 The GTP is presented for 20, 50 and 100 years, as this is felt to be more appropriate 20 for such an index; in any case, for longer time periods, the non-CO2 emissions from 21 aviation would quickly decay to zero, as they are so short-lived compared to CO2. 

Q6. How long does the GWP change with time?

42 This clearly illustrates that the multiplier to be applied to CO2 to account for the non-43 CO2 emissions varies greatly depending on the chosen time horizon, and on which 44 effects are considered, from 1.1 to 4.9.