Memorandum by the International Policy
THE SCENARIOS UNDERLYING CLIMATE CHANGE "PREDICTIONS"
Scientists are in broad agreement that human
activities have some influence on global mean temperatures and
climate, but they disagree about the extent of this influence.
Absent the influence of humanity, the earth's climate would not
be stableit experiences extreme natural changes, and small
manmade temperature increases are likely not to be a huge problem.
It is almost as uncontroversial that uncontrolled
human emissions of greenhouse gases (GHGs) will result in an increase
in global mean temperaturesall other things being equal.
This does not in itself justify political action in general or
climate mitigation in particular. The earth's climate would not
be stable without human influence. In fact climate has always
changed and will always change. Therefore, small manmade temperature
increases are not a problem.
Some greenhouse gases are more effective per
molecule when it comes to trapping infrared radiation than others.
In order to simplify a matter slightly, the concentrations of
the different GHGs in the atmosphere are converted into CO2-equivalents
on the basis of their relative radiative potency. The CO2-equivalent
atmospheric concentration is now about 32 per cent higher than
the pre-industrial level. The concentration is measured in parts
per million (ppm) air molecules. The pre-industrial concentration
of CO2-equivalent was approximately 280 ppm while the current
level is approximately 370 ppm. The IPCC projects a concentration
in the year 2100 that ranges from 540 to 970 ppm.
Other things being equal, this development will
enhance the natural greenhouse effect and cause the global mean
temperature to rise. Most scientists agree on this. What is unresolved
is the question of how much temperatures will rise. That is not
just an academic issue. If human emissions of GHGs have a small
effect on climate, it may prove too costly to curb these emissions
compared to the benefits. The issue is further complicated by
the fact that small increases in temperatures may in fact be an
It is important to establish by how much temperatures
can be expected to rise over a relevant time scale because of
the influence of human activities. The Intergovernmental Panel
on Climate Change (IPCC) has attempted this exercise for the past
15 years. The IPCC was established in 1988 by the World Meteorological
Organization (WMO) and the United Nations Environment Programme
(UNEP). The role of the IPCC is to assess
the scientific, technical and socio-economic
information relevant to understanding the scientific basis of
risk of human-induced climate change, its potential impacts and
options for adaptation and mitigation. The IPCC does not carry
out research nor does it monitor climate related data or other
relevant parameters. It bases its assessment mainly on peer reviewed
and published scientific/technical literature.
In its third assessment report (2001), the IPCC
predicted that the global mean surface temperature would rise
by 1.4 to 5.8ºC
and came to the widely-quoted conclusion that
There is new and strong evidence that most
of the warming observed over the last 50 years is attributable
to human activities.
However, the IPCC's conclusions are highly controversial.
The predictions depend on two sets of computer models. One set
predicts how much CO2 and other greenhouse gasses (GHGs) that
humanity will emit into the atmosphere. The other set of models
predict how the climate system will react to such increases in
In other words, the quality of the predictions
depends completely on the ability of the models to produce reliable
results. In fact, there are problems with both sets of computer
models. These are the two main uncertainties with these models:
1. We don't know how sensitive the Earth's
climate system is to increased levels of CO2.
2. We don't know how much CO2 we will emitthe
subject of this paper.
The IPCC's climate models calculate the consequences
of increasing atmospheric GHG concentrationstypically at
a level equivalent of a doubling of atmospheric carbon dioxide
compared to the pre-industrial level. However, in order to predict
expected warming, modellers need estimates of the rate of CO2
increase in order to forecast how long it will take for concentration
to double. In other words, they need forecasts of human emissions
of CO2 through the 21st century.
The IPCC has made such long range forecasting
exercises for all of their three assessment reports since 1992.
The latest results were published in 2000 in the Special Report
on Emissions Scenarios (SRES).
The SRES working group produced 40 scenarios out of which six
were chosen as "marker scenarios" and fed into the climate
models. In other words the emissions scenario exercise is a crucial
step in creating the above mentioned temperature growth range
for 2100 of 1.3 to 5.8ºC. In fact, most of the span in this
temperature range is produced by the huge difference in emissions
across the marker scenarios.
The scenarios are based on parameters influencing
emissions such as:
GDP per capita growth on a country,
regional and global level.
Energy efficiency (how much GDP does
one unit of energy produce).
Composition of fossil fuel consumption
(coal, oil, gas).
Non-carbon fuel share of total energy
production (nuclear, wind, solar etc.)
Obviously the quality of the emissions forecasts
depend completely on the way these parameters are treated and
the values given to them in the different scenarios. Over a time
span of a century the degree of uncertainty for each parameter
is enormous and as explained below, some of these uncertainties
compound. However, this does not mean that anything should be
considered plausible. Unfortunately the SRES scenario group does
a very poor job of creating plausible scenarios.
For those who use scenario methods in their
professions, the SRES scenario effort is problematic. There are
three basic reasons for concern:
Scenarios being used as forecaststhe
most important criticism.
The criticism concerning sloppiness includes
treating the period 19902000 as part of the forecast period
without taking into consideration that we now have data for that
period. Apparently the SRES has been reusing data from previous
scenario exercises. The problem is that time and real world observations
have proven these projections wrong. For instance the scenario
figures used for the increase in world GDP between 1990 and 2000
vary between 20.6 per cent and 35.4 per cent. However, IMF data
for most of that period was available in 1999 and showed growth
of 36.5 per cent.
Probably this error doesn't have a substantial impact on model
results by 2100. Nevertheless, it's amazing that such sloppy practices
are allowed as part of an input to a modelling exercise that involves
the use of supercomputers and costs millions of Euros.
The IPCC also suffers from poor methodology.
One leading economic modeller, John Reilly of the MIT Joint Program
on the Science and Policy of Global Change calls the SRES approach
to scenario building an "insult to science."
According to Reilly, the scenario teams have worked backward from
a desired end result in terms of emissions and temperature increases.
In other words, the IPCC has allegedly started with an emission
projection then made an estimate of the relationship between emissions
and growth and finally calculated the growth rate needed to achieve
the desired emissions projection.
Another criticism of methods has been advanced
by Ian Castles, former President of the International Association
of Official Statistics, and David Henderson, former Chief Economist
of the OECD. They accuse the SRES team of using inappropriate
exchange rates which exaggerate current global economic inequality.
Since the scenarios assume that income equality will improve over
the course of the century, models overestimate growth rates in
low income countries.
While the SRES methods concerning growth projections
are technically unsound, the scenario models will have to be modified
before we can conclude if the exchange rate error should lead
to substantial reductions in total global growth rate projections.
However, the "end result" in terms of world GNP/GDP
does seem very high. Many of the scenarios give us a world economy
which by 2100 is up to approx. 25 times larger than it is today.
In order to achieve that multiplication the
world economy would have to grow 3 per cent a year. This does
not seem to be a realistic assumption, even for a high end scenario,
given the historical performance of the world economy. But then,
the SRES does not seem to take historic trends much into consideration.
As David Henderson points out:
a surprising feature of the SRES that in
a document surveying the long term future, which contains over
300 pages of main text and presents 40 different scenarios prepared
by six different modelling groups, there is no chapter which systematically
reviews the evidence of the past. The starting point for any such
quantitative future-oriented inquiry should be a clear and careful
survey of earlier developments and trends, going right up to the
What such a survey could have considered is
the fact that since 1975 world GDP per capita has grown at an
average 1.2 per cent per annum.
Extending this growth rate for 110 years would give us a world
per capita income which was 3.7 times larger than the present.
If we assume that population doubles over the same period (and
that would be a high estimate) we get a global economy which is
7.4 times bigger than the present. That's quite far from a 25
The only economy which has produced average
annual growth rates comparable to the IPCC assumption over a sustained
period is Japan. Between 1913 and 1996 Japan grew at an annual
rate of 3.36 per cent.
But it seems doubtful that the entire world economy would be able
to do the same for as long as a century.
The most serious objection is the IPCC's use
of scenarios. The scenarios are presented as an exercise in "free
thinking" about the future. The SRES states that they are
"images of the future or alternative futures" and should
not be seen as predictions or forecasts. They are in fact "computer
aided storylines". But at the end of the day the scenarios
end up presenting a very concrete result in terms of numbers (emissions).
These numbers are fed into a computer, which in turn produces
a temperature. The SRES tries to have it both ways: a non-committal
scenario process and a clear result in terms of a number.
The SRES claims that the scenarios "are
not assigned probabilities of occurrence, neither must they be
interpreted as policy recommendations."
But since each scenario gives a result which translates into a
number there is in fact an implicit bias in favour of extreme
scenarios. An extreme scenario extends the temperature range.
A less extreme scenario doesn't. Therefore, an extreme scenario
is implicitly given more importance. In the real world we would
normally treat an extreme outcome as less likely and therefore
assign less importance to it.
But the SRES scenarios are not assigned probabilities.
That's normal when working with scenarios. The whole point of
a scenario exercise is to cover the full range of possible futures.
But this also implies that scenarios should not be used as forecasts,
because a forecast doesn't make sense without a discussion of
probability. Meteorologists only make weather forecasts extending
3-5 days into the future. The reason why they don't make longer
forecasts is that the probability of being right gets too small.
The SRES scenarios are used to make forecasts. In other words,
the SRES team abuses the scenario method.
The following section shows that the high end
temperature projection (which is being used as a forecast) is
based on two scenarios which again are based on assumptions that
have a very low probability, especially combined.
The six marker scenarios were based on very
different assumptions about the parameters listed above. It is
therefore not surprising that they produce very different result
in terms of carbon emissions as shown in Figure 1.
At the low end, the B1 scenario produces cumulative
emissions by 2100 of 983 gigatons. At the high end the A1FI scenario
produces emissions of 2.189 gigatons, more than twice as much.
How do those results compare with historic emissions?
Figure 2 shows emissions per capita over the past half century.
Until the beginning of the seventies carbon
emissions per capita grew, but since then they have stabilised
and even declined slightly. This has happened despite considerable
Ross McKitrick, Associate Professor at the University
of Guelph, concludes that:
there is reason to believe that per capita
CO2 emissions are somewhat invariant to economic growth, at least
at a globally averaged level.
It would therefore seem reasonable that per
capita carbon consumption in at least some scenarios was near
the current level of 1.1 tonnes of carbon. Nevertheless, the SRES
scenarios are all above 1.2Ct per capita by 2020. Not one scenario
follows the current trend of no growth!
The high end scenarios produce some results
which are improbable. Figure 3 is based on two historic trend
lines of carbon consumption per capita. One is a projection of
the high growth trend from 1950-73. It's certain that no time
period in human history has had a higher growth rate in carbon
consumption per capita than this period. The other line shows
the lower growth trend for the whole period from 1950 to 1999.
Figure 3 shows how the A1FI scenario
overtakes even the fast growth trend by 2050. By 2100 , per capita
emissions in the A1FI scenario are more than four times the current
level, and 25 per cent above the high growth historic trend which
was only sustained for a couple of decades in reality.
Not surprisingly the scenario also arrives at
very high estimates of atmospheric concentration of CO2. Currently,
CO2-levels are increasing by approximately 0.4 per cent a year.
Figure 4 shows how, by 2100, atmospheric CO2 concentrations at
the high end of the A1FI scenario reach a level 2.27 times what
the current trend would imply.
The fact that the 1990s had both high global economic growth and
probably the highest nominal population growth ever (including
all of the 21st century)
suggests that annual CO2-growth may not increase much above the
current 0.4 per cent.
The current trend for the second most important
GHG, methane, is that the rate of growth has been decreasing and
the actual concentration is currently falling. Yet none of the
scenarios follow that trend. Figure 5 shows how the A1FI scenario
predicts a concentration more than double the current trend.
It is reasonable to conclude that the A1FI scenario
depicts an extremely unlikely future. Nevertheless, it is this
scenarioand this scenario alonewhich is responsible
for the high end of the IPCC's Third Assessment Report temperature
range of 1.3 to 5.8ºC for the year 2100. The 5.8º forecast
is arrived at by running the GHG concentration figures of the
A1FI scenario through the climate model with the highest sensitivity
to increased CO2. If it wasn't for this scenario, the top temperature
would be 1ºC lower.
If we ignore the 1ºC which is produced
by the A1FI scenario, we get a high temperature estimate of approx.
4.8ºC. This is generated by running the A2 scenario through
the most GHG sensitive climate model. The A2 scenario predicts
yearly CO2 emissions by 2100 which are almost as exorbitant as
the A1FI figures. But this scenario gets there in a different
way. One of the tricks of the A2 scenario is a very high population
Figure 6 shows the 2002 UN Population projection.
The medium population projection for 2050 is 8.9 billion. The
high estimate is 10.6 billion. Nevertheless, the A2 scenario has
11.3 billion people in 2050, a figure above the UN's high estimate.
Unfortunately, the latest UN projection doesn't
go beyond 2050. But the 2000 median projection estimated that
world population would be 10.4 billion in 2100. In the new projection,
the median figure for 2050 is revised downward from 9.3 to 8.9
which implies that the forecast for 2001 would also be lower today.
The A2 scenario assumes a world population in 2100 of 15.1 billion50
per cent above the median estimate. The A2 population is below
the UN high projection of 18 billion, but few people consider
that to be a likely outcome. It's not really a projection but
a kind of "business-as-usual" scenario.
Secondly, and much more importantly, as figure
7 illustrates, the A2 scenario has a much higher share of coal
than any of the other scenarios. This is significant since coal
emits more carbon per unit of energy than oil and especially gas.
On top of that A2 has a much lower share of zero carbon energy
(renewable sources, nuclear etc.).
In short, the A2 scenario assumes virtually no advances in energy
technology over the next century. In fact, the scenario assumes
that a trend towards less carbon intensive energy sourcesa
trend which has persisted for over a centuryis actually
It is interesting to compare the A2 scenario
with the A1T scenario. By 2100, the A2 has the second lowest world
GDP of all six marker scenarios: 243.000 billion (measured in
USD at 1990). In comparison the A1T scenario forecasts a world
economy of 550.000 billion (1990 USD)more than twice that
Still, the A2 world emits 28.9 Gt of carbon per year, which is
6.7 times more than the 4.3 Gt emissions that the A1T world produces.
In other words, the carbon intensity of the A2 economy is 15 times
higherthis world needs fifteen times more carbon per unit
This difference illustrates the importance of
expectations about future energy technology. The A2 and the A1T
scenarios represent two extremes in this respect. In A2 there
is very little technological development, so the world returns
to coal as the primary source of energy. In A1T there is rapid
technological development, and non-carbon energy technologies
begin to replace fossil fuels by the middle of the century. Are
both scenarios likely? The answer is probably no. The A1T scenario
is likely, whereas the A2 scenario is not. The reason for this
is that the A2 scenario reverses a very long term trend whereas
the A1T scenario merely continues (and maybe accelerates) an existing
long term trend.
Despite the fact that the share of coal in world
energy supply has been decreasing over the whole of the 20th century
and no reversal of that trend seems in sight, the share of coal
increases in the A2 scenario from the current level of 23.5 per
to 53 per cent.
For more than a hundred years the global carbon
dioxide intensity of energy has been decreasing steadily, by almost
The A2 scenario inexplicably reverses this trend of decarbonisation.
Most of the literature on the subject of energy
technology expects that one or more non-carbon technologies will
become competitive in the course of the 21st century. Both photovoltaic
sources (solar energy) and windmills show historic learning curves
with a learning rate of approximately 20 per cent. In other words,
a doubling of cumulative installed capacity gives a 20 per cent
reduction in costs. At that rate, wind energy will be competitive
within a decade or two and photovoltaic energy around 2050. Even
models which are less optimistic about the learning curve have
alternative non-carbon energy sources overtaking fossil fuels
before the end of the 21st century.
For the time being, gas is becoming an increasingly
attractive alternative in much of the world and the price of gas-generated
electricity is falling. This is another trend which is reversed
in the A2 scenario.
While nuclear energy has stumbled in Europe
for political reasons, other parts of the world are expanding
their nuclear energy base rapidly, particularly Asia. Nuclear
energy's per centage share of global energy supply has expanded
from 0.9 per cent in 1973 to 6.8 per cent in 2000. There is also
a learning curve for nuclear energy, so the expansion of this
energy source results in it becoming ever cheaper, so nuclear
energy could be expected to play a much larger role in the future.
To summarise, while both the A1FI and the A2
scenarios are possible scenarios and maybe even "good"
scenarios within a framework of six very different marker scenarios,
they are not suitable for the forecast exercise that the emissions
output constitutes. Both scenarios have a very low probability
since they are based on a number of assumptions which seem unlikely
given historical trends.
Table 1 summarises the basic steps that the
IPCC needs to undertake in order to achieve the expected warming
interval (left hand column). For each step it lists what are widely
considered to be the principal uncertainties given current scientific
understanding (right hand column).
|Create scenarios for future emissions of CO2
||World GDP growth per capita and its distribution among Low, Middle and High income countries.
|Composition of different fossil fuels in total consumption
|Technological change, including shifts to non-carbon or low- carbon energy sources, energy efficiency, carbon sinks etc.
|Convert emissions to atmospheric concentrations
||The life time of different GHGs in the atmosphere
|Model radiative forcing and convert this forcing to a projected temperature
||The sensitivity of the climate system to increased CO2 (feedback effects of clouds, aerosols etc).
| Natural climate effects enhancing or counterbalancing manmade effect
Through the forecast exercise, total uncertainty compounds
as each new uncertainty is added. Each of the parameters has a
factor 2 uncertainty. The highest estimate is double the lowest
estimate. But the result has a factor 4 uncertainty. The highest
estimate is four times higher than the lowest.
Adding a third parameter with a factor 2 uncertainty would
gives us a high estimate which was 8 times higher than the low
estimate. Uncertainty compounds.
The short answer is that we simply do not know how much warmer
climate will be in 2100. In fact, the degree of (compound) uncertainty
is so large that merely by providing temperature intervals, the
IPCC is extremely misleading. For many of the parameters even
the degree of uncertainty is controversial, despite the IPCC's
phoney confidence intervals. Climate science is not at a stage
where it is capable of providing confidence intervals, especially
not for the earth's climate almost one hundred years into the
future. In fact even the term "uncertain" is often misleading
when it comes to climate science. Many things are not uncertain
but simply unknown.
The emissions scenarios produce growth rates of carbon emissions
which are not in line with recent history. Especially the high
end scenarios, such as the A1F-scenario, seem completely unrealistic.
Nevertheless, the A1F-scenario provides the basis for the high
end of the IPCC temperature range. According to the guidelines
for scenario use that most forecasters and futurists refer and
adhere to, a scenario must be both possible and probable. While
the A1F-scenario is theoretically possible (and even that is debatable),
it simply isn't probable.
As mentioned, temperature intervals do not make sense given
current knowledge about the climate system. However, since this
is the game that the IPCC has forced upon us, we should examine
the interval of 1.55.8ºC temperature increase which
is the central IPCC projection. This interval is almost certainly
way too high. The upper estimate should be decreased by about
1ºC because it is based on the unrealistic emissions of the
A1F-scenario, and by a further 1ºC which results from the
unrealistic A2 scenario.
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IPCC (2000). Back
Henderson (2003). Back
Corcoran (2002). See Webster et.al. (2001) for a more in-depth
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The Economist (2003). Back
A spreadsheet with the scenario data is available at: http://sres.ciesin.org/ Back
Henderson (2003). Back
UNDP (2002), p 193. Back
Maddison (1991), David Landes (1999). Back
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Gregg Marland, Tom Boden Carbon Dioxide Information Analysis
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McKitrick (2003), p 14. Back
Own calculations based on tables SPM 1a and SPM 3a in the SRES
Summary for Policymakers. Back
McKitrick (2003). Back
Gray (2002). Back
Lomborg, Figure 13, p 47. Back
Gray (2002). Back
IPCC (2001), Technical Summary of Working Group I Report, Figure
22, p 70. Back
UNPP (2002). Back
IPCC (2000) Table SPM 1a. Back
Ibid., Table SPM 2a. Back
Ibid., Table SPM 1a. Back
IPCC (2000), SRES chapter 2.4.11 Carbon Intensity and Decarbonisation. Back
IEA (2002b). Back
IPCC (2000), SRES chapter 2.4.11 Carbon Intensity and Decarbonisation,
Figure 2-11. Back
Nakicenovic and Riahi (2002); Chakravorty and Tse (no date). Back
Magne« and Moreaux (2002). Back