Fuel Use Scenarios
Overview ..............................................................................................................................1
The fuel use spreadsheet.....................................................................................................2
The fuel usage s cena rios .................................................................................................2
Res ource consumpti on ...................................................................................................2
CO2 Emissions..................................................................................................................3
CO2 Greenhouse forci ng and the resul ting equilibrium global wa rming........................3
Methane Emissions .........................................................................................................3
Greenhouse wa rmi ng associa ted with increases in a tmospheri c methane...................4
Ra te of fuel growth and decline......................................................................................4
Plots......................................................................................................................................5
Dis cussion.............................................................................................................................7
Overview
The global future energy consumption scenarios presented in the excel workbook we discuss here all
start with the global fuel consumption in 2011 as reported by the BP Statistical Review of World Energy
June 2013 table p 40. The table lists primary energy consumption in millions of tonnes of oil equivalent
(=1000 Gtoe, where Gtoe = billions of tonnes of oil equivalent). We convert this to exajoules (EJ) using a
conversion factor of 41.88 EJ/Gtoe, where the conversion factor is calculated (as indicated in N1-4) from
others given in the Statistical Review Approximate conversion factors appendix on p 44.
Future fuel use scenarios (Figure 1) are developed from this starting point by increasing the world
energy consumption by ~1.6% per year for 50 years and ~1.4% per year. Between 50 and 100 years in
the future the fossil fuels are phased out but the total energy, increasingly supplied by non-hydrocarbon
sources, continues to increase at ~1.4% per year. The result is that 100 years from now the total yearly
energy supply is ~2300 EJ. This energy supply is sufficient to supply every human with ~7 kW of energy,
about the amount of energy the typical Frenchman consumes today (e.g., 2300x1018 J yr-1/(10.5x109
p)(3.1 x107 s yr-1) = 7x103 J s-1 = 7 kW).
Figure 1. Fuel consumption scenarios, starting with world consumption in 2011.
The fuel use scenarios differ in how the mix of fuels changes with time. In the “Business as Usual”
scenario, the present mix of fuels maintained as fuel supply is escalated over the first 50 years, and
approximately maintained over the phase out period as shown in Figure 1a. In the first 50 y ears of the
“Substitute Gas” scenario, gas replaces coal so that the same amount of electrical energy is generated,
and gas substitutes for oil on an equal energy basis such that oil use does not increase. Gas and oil use
are the reduced over the final 50 years of the hydrocarbon transition (Figure 1b). In the “Low Carbon
Fast” scenario, the hydrocarbon fuels are phased out immediately (Figure 1c). All scenarios emit
approximately the same amounts of CO 2 per year 100 years from now in 2111.
The fuel use spreadsheet
The excel workbook provided here records these fuel use scenarios, calculates their resource
implications, and estimates their greenhouse impact in an approximate fashion. Although approximate
in ways we discuss, the spreadsheet illustrates important conclusions in a uniquely transparent fashion.
How the spreadsheet works and some significant aspects of it are described below. Entries with yellow
background are parameters, entries with red background are input parameters, and entries with grey
background area results. Results are summarized in the BasU tab.
The fuel usage scenarios
The primary input to the spreadsheet is the yearly use (consumption) or primary energy classified by the
fuel type (oil, natural gas, coal, and zero carbon renewables). These yearly inputs are prepared by a
separate program and are simply pasted into the spreadsheets in A16-E116 in the BasU, SubG, and LowC
tabs of the workbook. In the primary input the fuel consumption is cumulative (e.g., the first column is
the consumption of oil, the second oil plus gas, etc.), but the input is immediately separated into the
yearly consumption of each fuel in EJ yr-1 (G16-L116), and then to the yearly consumption of each fuel
in Gtoe yr-1 (billions or gigatons of oil equivalent) in N16-S116. The cumulative fuel consumption is
shown in U16-Z116.
Plots of the primary input of each fuel are presented in the Fuel Scenario Plots tab.
Resource consumption
The consumption of fuels as a percent of the resource base is shown in AB16-AE116. We use the
resource base estimates of Rogner (1997). They are shown in T1-X6. The resource base is a very
generous estimate of what might eventually be conceivably produced. For example, about 1 trillion bbls
of oil have been consumed by humanity to date, and most groups estimate oil reserves (the oil that can
be produced economically today) are about another trillion barrels. Rogner’s resource base is nearly 6
trillion, indicating that 5 trillion barrels might eventually be produced! This projection is based on an
economist perspective (which is supported historically) that technology will eventually allow recovery of
all the resources that could conceivably (e.g., and not too dispersed). Shale oil is included in Rogner’s
resource base estimate. Shale gas is included in the natural gas resource base estimate, but gas
hydrates are not. More discussion can be found in Cathles (2012), but the point to be emphasized here
is that the resource base estimates, by design, are very generous and should be thought of as the largest
quantity of a resource that could conceivably ever be produced.
AB16-AE116 show the percent of Rogner’s resource base depleted by each fuel consumption scenario.
The depletions for all scenarios, summarized in G6-J10 of the BasU tab, show that, except for coal,
hydrocarbon resources will be substantially depleted in the 100 year transitions to low carbon energy
sources we present! We will deplete available hydrocarbon resources in the scenarios presented, and
we will make a transition from oil and gas to low non-hydrocarbon energy sources (or coal) over the
next ~100 years whether we want to or not because oil and gas will be substantially depleted. The
resource extractions to 2011 in AH16-AJ16 were estimated by integrating by eye historical production
curves from the web.
The broadest context of the CO2 emissions of our scenarios of fossil fuel consumption is indicated by the
number of pre-industrial atmospheres (PAL) of CO2 they will release (G6-J10 BasU tab). Combusting the
full resource base of oil and gas will release ~2.1 PAL to the atmosphere, whereas combusting the
resource base of coal will release 6.6 PAL. The fossil fuel greenhouse threat comes from coal, not oil
and gas.
CO2 Emissions
AG16-AJ116 show the percent of PAL released by combustion over the various fossil fuel cycles,
assuming the leakage during natural gas production is 2% of consumption ( control AP4). These
emissions for all scenarios expressed as a percent of PAL are summarized in L6-P11 in the BasU tab. The
BasU scenario releases ~2, the SubG ~1.6, and the LowC ~1 PAL CO 2 to the atmosphere. The substitute
gas scenario reduces CO2 emissions by 46% of what could be achieved in the low carbon fast scenario.
AK16-116 shows the cumulative mass of CO2 introduced to the atmosphere by combustion of fossil fuels
expressed as GtC (giga tonnes or 109 tonnes of carbon). AL16-116 adds the CO2 produced when the
methane leaked to the atmosphere is oxidized (AU16-116) and multiplies the sum by the fraction
retained as specified in control cell AL6 to give the amount of CO2 retained in the atmosphere over the
transition period. The maximum increase in atmospheric CO 2 is shown in cell AL7, and a summary of
the CO2 emissions of all scenarios is shown B8-E8 of the BasU tab. The contribution of oxidized CO 2 to
the total CO2 is sumarized in BC7-BF11 of the BasU tab. The methane oxidation contribution is generally
quite small but can exceed 7% of the total in the SubG scenario when gas leakage is 9% of natural gas
consumption.
CO2 Greenhouse forcing and the resulting equilibrium global warming
AM16-116 in all the scenario tabs calculates the greenhouse forcing associated with the increase in
atmospheric CO2 in AL16-116 employing the standard IPCC methods as discussed here. AN16-116
calculates the equilibrium global warming that is expected to result from this forcing (see discussion
here) by multiplying by the equilibrium climate sensitivity specified in control cell AL7. The maximum
change in equilibrium global temperature caused by CO2 additions since 2011 in the BasU case is
2.55°C, and 2.11 and 1.44°C for the SubG and LowC cases respectively, as shown in cell AN7.
Methane Emissions
Methane emissions and emission rates are calculated in columns AT16-AW116. Methane generation is
related to leakage during natural gas production. This leakage is specified in control cell AP4 of the BasU
tab. We assume that 5 m3 of methane is released per tonne of coal mined, as discussed in Cathles
(2012). From the leakage rate per EJ (exajoule) and the fuel uses per year we calculate the GtCH4 y-1
(giga tonnes of methane pre year) released for the oil and gas consumed (AQ16-AR116), and these are
added to obtain the cumulative tonnes of methane emitted from gas and coal extraction (AS16-116).
The agricultural emissions in AT16-116 are added to obtain the total CH4 emissions in AU16-116.
The increased steady state concentration of methane in the atmosphere is calculated from the rate to
emission in AU16-116. The value depends on whether the atmosphere is in equilibrium with the
sources of methane at the start of the calculation or not. If it is in equilibrium, the rate of increase of
atmospheric methane is 0 in control cell AT5 and the initial steady state concentration of atmospheric
methane is zero in AW16. If methane is increasing in the atmosphere cell AT5 contains the rate of
increase in atmospheric methane in ppbv per year, and the initial steady state concentration of
atmospheric methane is not zero in AW16. For 2011 the rate of increase was about 6 ppbv/yr and the
steady state concentration of methane (e.g., the eventual equilibrium methane level under the
conditions that pertained in 2011) is 74.4 ppbv greater than the 2011 concentration of 1803 ppbv, as
shown in AW16. The oxidation sink for CH4 is computed from these same parameters and reported in
cell AV11 as discussed here. This oxidation rate is subtracted from the total (fossil fuel plus agricultural)
methane emission rate to obtain the net emission rate from which the steady state methane
concentration is computed.
Agricultural emissions are computed in the agricultural CH4 tab. Methane is released by animal
husbandry and rice and other agricultural crops, and these sources of methane are expected to grow in
proportion to the growth of the human population (see agricultural tab) by the amount estimated here.
The growth in methane emissions related to this source is estimate in the agricultural CH4 tab, and
added to the fossil fuel emissions, subject to a flag in control cell AV12. If cell AV12 contains a 1, the
agricultural emissions add to the fossil fuel methane emissions in AU16-116, and if cell AV12 contains a
0 they do not. This flag allows assessment of the importance of agricultural methane emissions to
future greenhouse warming.
Greenhouse warming associated with increases in atmospheric methane
AX16-116 gives the greenhouse forcing associated with this increase in atmospheric methane from fossil
fuel production and agriculture, and AZ16-116 gives the greenhouse warming associated with this
forcing. Unlike CO2, methane comes out of the atmosphere as fossil fuel use declines. AX8-AZ11
summarize minimum and maximum increases in atmospheric methane concentration and the
associated forcing and equilibrium change in global average temperatures. Total changes in CO2 and
CH4 for all scenarios, and the total equilibrium changes in average global temperature they could
induce, are given in B6-E10 of the BasU tab.
Rate of fuel growth and decline
Finally, columns BC16-BH116 show the rate of growth and decline in fuel usage assumed in the various
scenarios. These growth rates are important in assessing the feasibility of achieving the transitions
envisaged in the scenarios.
Plots
Plots are provided in tabs which change automatically as control parameters are changed in the
spreadsheet. The validity of the simpler spreadsheet calculations is established in these plots by
comparing them to more sophisticated convolution calculations described in Cathles (2012). The
convolution calculations do not make the simplifying approximations made in the spreadsheet, namely:
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that 55% of the emitted CO 2 is retained in the atmosphere, and
that the atmospheric CH4 concentration is in equilibrium with increased CH4 emission rates.
Figure 2 shows that agricultural emissions will prevent atmospheric methane concentrations from falling
as fossil fuels are replaced. The increases in methane concentrations predicted by the spreadsheet
match those predicted by the convolution program (Fig. 2b), but the assumption in the spreadsheet
methods that steady state concentrations are attained immediately causes the concentration curves to
track the fuel emission rate with more fidelity than is realistic.
Figure 3 shows that CO2 concentrations in the atmosphere can be predicted quite accurately by
assuming that 55% of CO2 emissions are retained in the atmosphere over a 100 year transition.
Figure 4 shows the climate forcing produced by CH 4 and CO2 emissions separately and combined.
Figure 2. (a) Only fossil fuel additions to the atmosphere. Methane concentrations drop relative to 2011 by about ~85 ppbv
as methane emissions decline. (b) Agricultural emissions are included and emissions no longer drop below 2011 levels.
Solid curves are computed by spreadsheet; dashed lines by convolution program with agricultural sources included. Gas
leakage is assumed to be 2% of natural gas consumption. Methane oxidation to CO2 is added to the CO2 emissions for all
curves. Note you need change the convolution curves by dragging the reference locations to shift from one plot to the next
on the spreadsheet.
Figure 3. Carbon dioxide introduced by the burning of fossil fuels according to the three scenarios in Figure 1. Solid curves
are computed from cumulative combustion emissions assuming 55% of the CO 2 remains in the atmosphere. The dashed
curves take the CO2 out according to the IPCC(2013) formula discussed here. The spreadsheet predicts the changes in
atmospheric CO2 concentrations quite well.
Figure 4. Climate forcing computed by spreadsheet (solid lines) and by convolution methods (dashed lines). Methane
leakage equal to 2% of natural gas consumption is assumed, agricultural methane sources are included, and methane
oxidized to CO2 in the atmosphere is included in the CO2 emissions.
Figure 5. Temperature change computed with the spreadsheet (solid lines in a) and with the convolution method (dashed
and dotted lines). 2% of consumption natural gas leakage and agricultural and fossil fuel methane forcings are assumed. (a)
Shows spreadsheet approximations capture warming predicted by more sophisticated methods well. The convolution
calculation comparison in (b) emphasizes the importance of ocean mixing in reducing predicted warming.
Discussion
The most important aspect of this spreadsheet is that it clearly illustrates simple physical relationships.
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CO2 atmospheric concentration results from ~55% of anthropogenic emissions being retained in
the atmosphere.
CH4 atmospheric concentrations can be estimated accurately from the methane emission rate.
Agricultural as well as fossil methane emissions are important.
Methane oxidation addition to CO2 sources is small (at most a 7% addition at a 9% of
consumption leakage rate in the substitute gas scenario; e.g. cell BE9 of the BasU tab).
References
Cathles, LM (2012) Assessing the greenhouse impact of natural gas, G 3, 13(6), 18 p.
http://www.geo.cornell.edu/eas/PeoplePlaces/Faculty/cathles/Natural%20Gas/Cathles%20Assessing%20GH%20Impact%20Natural%20Gas.pdf