AEF paper no. 1/08 – June 2008
What Are The Relative Importance of Shipping And Aviation
Emissions ?
1 Introduction
There have been claims in the past couple of years from the aviation industry and its supporters that
aviation has been unfairly singled out as a contributor to climate change. In particular, it has been
stated that shipping is growing faster than aviation. This paper compares the growth of emissions
from international shipping and aviation in order to investigate these claims.
The source of base data is the UK CO2 emissions inventory, compiled for UNFCCC and placed on
the DEFRA web site.1 The figures are compiled from bunkered fuel, ie fuel loaded into ships and
aircraft in the UK. Data is from 1970 to 2006.
Growth rates, ratios and cumulative values have been calculated from the data and selected results
are reproduced here (fuller data available on request).
2 UK trends
Fig 1 shows the emissions for both international shipping and aviation from 1970 to 2006. With the
exception of a few small ‘blips’ aviation has grown steadily. The picture for shipping is more
complex. It appears to have declined from around 15 million tonnes pa during the 1970’s and then
stayed in a band of 4 to 9 mtonnes since then. There have been considerable variations within this
period and there was a big rise in 2005 and 2006. However there is no evidence of a sustained
upward trend.
Uk aviation and shipping CO2 emissions
40.0
35.0
Mtonnes of CO2
30.0
25.0
Shipping
20.0
Aviation
15.0
10.0
5.0
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
0.0
Year
Fig 1
The diagram shows clearly that aviation now massively outstrips shipping as a source of CO2,
having been much less in 1970. The ratio of aviation to shipping is shown in Fig 2.
Aviation / shipping ratio - annual
7
5
4
Av/ship - annual
3
2
1
0
19
70
19
73
19
76
19
79
19
82
19
85
19
88
19
91
19
94
19
97
20
00
20
03
20
06
Av/ship - annual
6
Year
Fig 2
The pattern up to around 2000 was a rapid increase, but the ratio has been rather irregular since
then. However, it has always been over 4 since then, so aviation is clearly a much bigger source of
CO2.
These graphs are for CO2 only. For aviation there is thought to be a significantly higher climatic
impact due due mainly to water vapour and nitrogen oxides emitted at altitude. The UK
government accepts a Radiative Forcing Index (RFI) of 1.9 for aviation – that is, the total impacts
are 1.9 higher than for CO2 alone.
There are considerable annual fluctuations in the emission figures for both aviation and shipping.
To investigate this % changes from one year to the next were computed. These are shown in Fig 3.
Annual changes in emissions
40
30
20
-10
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
0
1970
% change in year
10
Shipping
Aviation
-20
-30
-40
-50
Year
Fig 3
The annual changes in aviation are usually positive (ie growth) but are very variable. They are
clearly far larger than changes in the amount of air travel. The annual changes in shipping are far
more variable.
The reason for these fluctuations is almost certainly due the method of estimating. As noted above,
the CO2 emissions are calculated from fuel loaded in the UK (bunkered). For ships, fuel can be
purchased and loaded at any convenient port of which there may be several on a round trips. The
port chosen is likely to be one where fuel happens to be cheapest at that particular time. For
aircraft, options are far more limited. Most trips are single hops and, except on short flights, weight
considerations may prevent any choice as to where fuel is loaded.
The fluctuations in annual reported emissions make it very difficult to draw conclusions from
annual figures whether shipping is now growing faster than aviation. Annual fluctuations can be
smoothed out by taking 5-year or other average. But as Fig 4 shows, there is still no clear evidence
of a pattern.
5-year change in emissions
80
60
20
Shipping
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
0
1970
% growth in 5 years
40
Aviation
-20
-40
-60
-80
Year
Fig 4
Long period averages can be calculated by cumulating data from the start date of the dataset,
namely 1970. If this is done, the aviation to shipping ratio shows a steady rise – see Fig 5.
Aviation / shipping - cumulative from 1970
Av/ship cumulative from 1970
2.5
2
1.5
Av/ship - cum from 1970
1
0.5
19
70
19
73
19
76
19
79
19
82
19
85
19
88
19
91
19
94
19
97
20
00
20
03
20
06
0
Year
Fig 5
3 International trends
If there are fluctuations in the UK data caused by the fuel bunkering effect, these should be
smoothed out by looking at worldwide trends. However, detailed worldwide figures do not appear
to be available.
4 Domestic aviation and shipping
The above figures are for international aviation and shipping. Domestic aviation and shipping
emission are much less than international, so the effect of including domestic is unlikely to make a
significant difference to the overall conclusions.
Fig 2, which shows the annual aviation/shipping ratio for international traffic only, has been redrawn to include domestic traffic as well. See Fig 5 below.
Aviation / shipping ratio - international + domestic
4.5
4
Av / ship ratio
3.5
3
2.5
Aviation / shipping ratio international + domestic
2
1.5
1
0.5
19
70
19
73
19
76
19
79
19
82
19
85
19
88
19
91
19
94
19
97
20
00
20
03
20
06
0
Year
The numbers are somewhat different, the ratio being lower in recent years if domestic traffic is
included. This is because the domestic aviation/shipping ratio is much lower than the international
ratio. However, the general pattern is the same.
Summary and conclusions
The conclusions that can be drawn from this analysis are as follows:
1. International aviation emissions have risen with only minor fluctuations from 1970 to 2006.
They rose by a factor of 5.4 between these dates.
2. The picture for shipping is more complex. It appears to have declined from around 15
million tonnes pa during the 1970s and then stayed in a band of 4 to 9 mtonnes since then.
There have been considerable variations within this period but there is no evidence of a
sustained upward trend.
3. The annual aviation/shipping ratio varies a good deal. It has increased a great deal since
1970 and it reached 4 for the first time in 2000.
4. There is some suggestion that the aviation/shipping ratio is no longer increasing. It has
remained in the 4 to 6 band since 2000 and but there is no indication that it will drop below
4 again.
5. If domestic aviation and shipping are included, the pattern is very similar but the ratio in
recent years is somewhat lower. It has been in the band 3 to 4 since 2000.
6. A good deal of the annual variation can be explained by the vagaries of the estimates, these
being based on fuel bunkered, rather than actual figures of fuel consumed. Variations due to
this are expected to be more significant in the case of shipping than aviation.
7. Worldwide figures would enable distortions in the figure due to fuel bunkering to be
removed. However, detailed worldwide figures do not appear to be available.
8. The analysis is for CO2 only. If other greenhouse gases are were considered, the ratios
would probably be much higher.
1.
http://www.defra.gov.uk/environment/statistics/globatmos/download/xls/gatb04.xls#'CO2'!A1