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Title: Fossil-fuel constraints on global warming
Author: Antonio Zecca; Luca Chiari
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ARTICLE IN PRESS
Energy Policy 38 (2010) 1–3
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/enpol
Fossil-fuel constraints on global warming
Antonio Zecca , Luca Chiari
Physics Department, University of Trento, Via Sommarive 14, I-38050 Povo (TN), Italy
a r t i c l e in fo
Received 4 February 2009
Accepted 24 June 2009
Available online 31 July 2009
In 2008 and 2009 two papers by Kharecha and Hansen and by Nel and Cooper examined possible fossil
energy availability and energy consumption scenarios and consequences for future climate. The papers
yield somewhat similar results regarding atmospheric CO2 levels, but they reach substantially different
conclusions regarding future climate change. Here, we compare their methods and results. Our work
shows that Nel and Cooper’s paper signiﬁcantly underestimates future warming. Nel and Cooper
conclude that even if all the available fossil fuels would be burned at the maximum possible rate during
this century, the consequent warming would cap at less than 1 1C above the 2000 level. We ﬁnd that –
under Nel and Cooper’s assumption of an intensive exploitation of fossil fuels – the global temperature
in 2100 will likely reach levels which would lead to severely damaging long-term impacts.
& 2009 Elsevier Ltd. All rights reserved.
In recent years, we have witnessed a growing concern about
the likely impacts of the anthropogenic greenhouse gas emissions
on the climate system. The science of climate has made
tremendous progress in the last two decades both from the
observational side and in its forecasting capabilities. This allows
us to conﬁdently state that the Earth will warm during the next
centuries as a result of human fossil-fuel use. Predictions of the
extent of this warming are primarily limited by the unknown
future decisions of humankind in regards to fossil-fuel usage.
In parallel with the growth of climate science, research into
present and future availability of fossil fuels (and of mineral
resources) has made a huge jump forward. Obviously the two
ﬁelds intersect. We know now that the total fossil fuel availability
on the Earth is ﬁnite and that the annual production will likely
peak within this century. Here, we will investigate the intersection of the two research ﬁelds—speciﬁcally examining the extent
to which the ﬁniteness of the fossil-fuel resources could affect the
temperature rise during next two centuries.
Both the paper by Kharecha and Hansen (2008) (K+H from now
on), and by Nel and Cooper (2009) (N+C from now on) design a set
of possible scenarios regarding the anthropic energy consumption
during the 21st century. K+H outline two BAU scenarios plus four
CO2 emission reduction scenarios. The explicit goal of K+H is to
design emissions reduction scenarios that could limit the atmospheric CO2 concentration below 450 ppm by the end of this
century. The 450 ppm limit has been chosen as the concentration
Corresponding author. Tel.: +39 0461881553; fax: +39 0461881696.
E-mail address: firstname.lastname@example.org (A. Zecca).
0301-4215/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
value that limits 21st century temperature increase to 1 1C
(Hansen et al., 2006; Hansen and Sato, 2004).
N+C (Nel and Cooper, 2009, par. 2–5) calculate the cumulative
global fossil-fuel production until 2100 using a logistic analysis of
the historical production data (ASPO, 2009; Hubbert, 1956).
Projections are given also for non-renewable nuclear energy (Nel
and Cooper, 2009, par. 6) and for renewable sources (Nel and
Cooper, 2009, par. 7). These data are used as input for an energyeconomic model outlining possible growth trends of the world
economic system (Nel and Cooper, 2009, par. 8–10). This model
allows N+C to devise four world total energy supply scenarios plus
one scenario for fossil fuel availability up to 2100 (Nel and Cooper,
2009, Fig. 22) and compute the CO2 emissions in the same period.
Starting from the emission scenarios, both papers calculate the
CO2 atmospheric concentrations until 2100. Using these concentrations, N+C calculate the change in radiative forcing due to CO2
relative to the year 2000 and, as we will discuss, erroneously
convert these changes into global mean surface temperatures.
In addition to critically analyze the N+C’s results, one of the
goals of this contribution is to review three points of uncertainty
in the present understanding of the climate system which
inﬂuence our predictive capacity of future greenhouse warming.
2. Atmospheric CO2 lifetime
The ﬁrst point refers to the CO2 atmospheric lifetime. It is
generally accepted that CO2 added to the atmosphere will be
gradually captured by natural sinks, i e. terrestrial ecosystem
uptake and air–sea diffusion plus surface-deep ocean mixing. Each
sink has its own typical time constant spanning from the seasonal
ARTICLE IN PRESS
Atmospheric CO2 (ppm)
A. Zecca, L. Chiari / Energy Policy 38 (2010) 1–3
Fig. 1. CO2 concentration projections based on N+C’s emissions scenario obtained
by using different carbon cycle models: AH(0.03) by N+C’s, Bern (IPCC, 2007,
pp. 213, 790; Joos et al., 2001) and Archer (2005).
action of the vegetation to the century-to-millennia scale of the
A ﬁrst order description of the lifetime describes the absorption of a pulse injection of CO2 into the atmosphere as a single
exponential decay. N+C do not give an equation for their airborne
CO2 projection. Nevertheless, it is clear from their Fig. 25 that the
two models AH(0.03) and AL(0.022) yield an airborne fraction of
zero between 100 and 200 years after an emission pulse. This is
inconsistent with well-established ocean chemistry and physics
which dictate a millennial-scale airborne fraction of 20%
(Solomon et al., 2009, p. 1705).
A better approximation allows for the sum of three exponentials. The Bern carbon cycle model (IPCC, 2007, pp. 213, 790; Joos
et al., 2001) uses this scheme and predicts a CO2 airborne fraction
of the order of 14–19% after 1000 years (Kharecha and Hansen,
2008). The Bern model gives a realistic description since it reﬂects
the varying timescales of the relevant sinks.
More recently, Archer (2005) has investigated the behaviour of
sinks over geological times (up to 100 kyr) reaching the conclusion
that the residual concentration after a pulse injection will be of
the order of 17–33% after 1000 years. Archer’s results are possibly
an improvement of the Bern model since they factor in long-term
feedbacks (Archer, 2005).
The K+H’s paper also gives values for the CO2 concentration
according to the Bern model, but since their aim is to ﬁgure out
mitigation measures intended to keep the 2000–2100 warming
below 1 1C, they do not calculate the global temperature increase.
Our Fig. 1a shows the N+C’s curve (AH(0.03) model, Nel and
Cooper, 2009, Fig. 26) for the atmospheric CO2 concentration until
2200 compared with the Bern model results (as computed by
N+C). We also show a concentration curve computed starting from
a single lifetime value of 300 years, a simpliﬁcation suggested by
Archer (2005), by calculating the airborne fraction after a CO2
emission pulse as a function of time according to Eq. (1)
CO2 ðtÞ ¼ CO2 ðt0 Þe t=300
Here, we have assumed an atmospheric retention factor equal
3. Climate sensitivity
DT ¼ lDF
using values from the 2007 report of the Intergovernmental Panel
on Climate Change (IPCC, 2007): the 1850–2000 global temperature increase (DT ¼ 0.76 1C) and the corresponding DF value (DF is
the net change in the radiative forcing since 1750: DF 1.84 W/
m2). This way they obtain a value of l 0.413 1C/(W/m2). N+C aim
at estimating equilibrium climate sensitivity, but their choice
implies the wrong assumption that the climate system has
reached today a thermal equilibrium as a consequence of
increased GHG concentrations.
Using mid-values for DT and DF, the N+C’s paper does not
account for the known probability distribution functions (PDF)
(IPCC, 2007, p. 203) of the quantities involved in Eq. (3). This
leaves uncertainties—especially related to the negative aerosol
forcing value (IPCC, 2007, Fig. TS.5 and table therein). A detailed
discussion of the uncertainties in historical forcing (1880–2003) is
given by Hansen et al. (2007). In contradiction to the incorrect
calculation by N+C, they conclude that ‘‘it is fruitless to try to
obtain an accurate climate sensitivity from observed global
temperature change in the last century’’.
4. SO2 offsets greenhouse gases
The uncertainty about the aerosol negative forcing enters in a
second manner in the calculation of the future global temperature
increase. Any combustion produces simultaneously CO2 and SO2
emissions. It is also unanimously accepted that SO2 aerosols partly
offset the CO2 warming (IPCC, 2007, ch. 2). This compensation is
governed in time by the strongly different atmospheric permanence times of CO2 (decades: see paragraph 2) and SO2 (days).
This makes a big difference in the time evolution of the CO2
warming and SO2 cooling effects. Whenever the rate of fossil-fuels
combustions will decrease, the SO2 cooling will immediately
decrease in a proportional amount, while the CO2 warming will
decrease with a delay which can be qualitatively thought to be of
the order of the atmospheric lifetime.
This effect has been accounted for by N+C (Nel and Cooper,
2009, pp. 177–178) using the most probable value 1.1 W/m2 for
the 2006 aerosol forcing. No account has been taken for the
forcing PDF. Their temperature trajectories are subject to be
corrected upward/downward if more/less negative values of the
aerosol forcing are assumed.
5. Global energy supply vs. climate
From now on we will concentrate on CO2 emissions from fossil
fuels. Once the time evolution of the CO2 concentration is known,
it is possible to infer the corresponding contribute to the change
in equilibrium global surface temperature. This is made through
Eq. (2) (IPCC, 2001, pp. 354, 358)
DT ¼ l5:35 lnðC=C0 Þ
where DT is the temperature increase at a given year relative to a
reference year (2000 in N+C); C and C0 are the CO2 concentrations
at that given year and in 2000, respectively. The coefﬁcient l is the
climate sensitivity; l can assume values between 0.3 and 1.3 1C/
(W/m2). The lowest value can be achieved in a climate system
with no feedbacks, as a planet with an inert atmosphere, while the
highest one can be achieved in a system with very strong
feedbacks. In the present climate state of the Earth, l is believed
to have a value close to 0.7 1C/(W/m2) (Hansen et al., 2000; Hoffert
and Covey, 1992; IPCC, 2007, pp. 798–799; Seinfeld and Pandis,
N+C calculate the l value in an incorrect way: they use Eq. (3)
Starting from their invalid 0.8 1C maximum temperature
increase, N+C arrive at conclusions for world energy policy
through the use of an energy-based economic growth model.
We are not dealing with N+C’s evaluations about resources/
reserves of fossil fuels or their economic model, as our focus is on
their CO2 and temperature calculations.
ARTICLE IN PRESS
A. Zecca, L. Chiari / Energy Policy 38 (2010) 1–3
From the economic part of the N+C’s paper, we quote the
following conclusion: ‘‘The economic impact of the ERC (energy
scenario) is a signiﬁcant divergence from 20th century equilibrium
growth conditions. Stabilisation of human welfare is only achieved
under optimistic assumptions with respect to technology change
and human behaviour, demanding a paradigm shift in contemporary economic thought’’. We note here that the N+C’s assessment of
the global fossil-fuel production curve does not consider the
possibility of an extreme exploitation of /low quality/high cost/
difﬁcult extractionS unconventional fossil fuels. Such exploitation
will be ultimately limited by the energy return only. Nevertheless,
any degree of extreme exploitation will raise the N+C’s fossil
energy availability scenario, leading to higher CO2 emissions and to
a warming larger than the one presented in their paper and very
likely to ‘‘dangerous’’ climate change (Hansen et al., 2007).
We fully agree with their statement that the impacts of the
warming should not be underestimated and will ‘‘require
considerable adaptation to minimise environmental and socioeconomic impacts’’y ‘‘Although energy efﬁciency is of primary
importance, it must be supported by behavioural changes that
bring about energy conservation’’.
However, we stress that the N+C’s conclusion ‘‘that the extent
of global warming may be acceptable and preferable compared
with the socio-economic consequences of not exploiting fossilfuel reserves to their full technical potential’’ is demonstrably
wrong and could lead to grossly misguided policies. This assertion
would have been quite different if the temperature calculations in
the paper by N+C had been done properly.
Uncertainties in understanding of climate should not be used
as an excuse to minimise the grave risks of unchecked anthropogenic climate change. Indeed, it is increasingly apparent that
climate impacts are occurring even faster than models predicted.
We thank W.P. Nel and C.J. Cooper for supplying us with the
numerical data from Figs. 26 and 27 of their paper. The authors
would also like to thank Referee #2 for carefully examining our
paper and providing us a number of important comments.
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