Download Broader perspectives for comparing different greenhouse gases

Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Phil. Trans. R. Soc. A (2011) 369, 1891–1905
doi:10.1098/rsta.2010.0349
Broader perspectives for comparing different
greenhouse gases
BY MARTIN MANNING*
AND
ANDY REISINGER
New Zealand Climate Change Research Institute, Victoria University of
Wellington, New Zealand
Over the last 20 years, different greenhouse gases have been compared, in the context of
climate change, primarily through the concept of global warming potentials (GWPs).
This considers the climate forcing caused by pulse emissions and integrated over a
fixed time horizon. Recent studies have shown that uncertainties in GWP values are
significantly larger than previously thought and, while past literature in this area has
raised alternative means of comparison, there is not yet any clear alternative. We
propose that a broader framework for comparing greenhouse gases has become necessary
and that this cannot be addressed by using simple fixed exchange rates. From a
policy perspective, the framework needs to be clearly aligned with the goal of climate
stabilization, and we show that comparisons between gases can be better addressed in this
context by the forcing equivalence index (FEI). From a science perspective, a framework
for comparing greenhouse gases should also consider the full range of processes that
affect atmospheric composition and how these may alter for climate stabilization at
different levels. We cover a basis for a broader approach to comparing greenhouse gases
by summarizing the uncertainties in GWPs, linking those to uncertainties in the FEIs
consistent with stabilization, and then to a framework for addressing uncertainties in the
corresponding biogeochemical processes.
Keywords: greenhouse gas; global warming potential; forcing equivalence index;
climate stabilization
1. Introduction
The concept of radiative forcing (RF) provides a robust way of comparing the
contributions of different greenhouse gases to climate change based on their
atmospheric concentrations at one particular point in time [1,2]. This creates
a basis for evaluating the current effects of past anthropogenic emissions of
greenhouse gases, as well as for developing approaches to different climate
mitigation strategies, and so it is an important scientific concept for policy
considerations.
However, greenhouse gases have different relative effects on the RF as well
as different lifetimes in the atmosphere. This creates the need for a way of
comparing the net effect of emissions of different greenhouse gases, which has
*Author for correspondence ([email protected]).
One contribution of 17 to a Discussion Meeting Issue: ‘Greenhouse gases in the Earth system:
setting the agenda to 2030’.
1891
This journal is © 2011 The Royal Society
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1892
M. Manning and A. Reisinger
led to a widespread use of the global warming potential (GWP) [3,4]. Continued
evaluation of this emissions metric has shown that the scientific uncertainties in
estimating it are significant [5], and that they have tended to increase, rather
than decrease, with further analysis [6]. This is coming from growing recognition
of uncertainties in the long-term response of the carbon cycle to future climate
change, but also from considering the full range of potential indirect effects that
can be caused by emissions of chemically reactive greenhouse gases [7].
Structural limitations in the GWP concept have also been recognized from
the outset [8], and there have been several proposed alternatives for comparing
greenhouse gases [9–11]. Some analyses have suggested that the GWP concept
does not lead to economically optimal greenhouse-gas mitigation strategies [12],
and the basis for comparisons between gases has also been dealt with more directly
in terms of the economic implications of managing future emissions [13].
In addition to the growing range of scientific issues that arise from comparisons
of different greenhouse gases, there are also issues from a policy perspective.
For example, it appears that there is now some resistance in policy circles
to continuing changes in the numeric values of GWPs, as has been done in
Intergovernmental Panel on Climate Change (IPCC) assessment reports [2,14,15].
It has been argued that values used in the context of international policy should
remain static at the 1995 values rather than become subject to continual revision
by scientific studies [16].
The complexity that underlies quantitative comparisons of emissions of
different greenhouse gases deserves broader consideration, and is an issue for both
the international policy framework and for scientific research. Science and policy
approach this issue from different perspectives, but both are directly linked to
considering the magnitude of potential effects of future greenhouse-gas emissions
on the environment. Furthermore, stabilizing anthropogenic forcing of climate
change is a common factor that sets up a broader context than just considering
pulse emissions into the present atmosphere.
In the context of climate stabilization, it is necessary to avoid the limitations
of a fixed and arbitrary time horizon when comparing different greenhouse gases.
Changes that keep the total GWP-weighted emissions constant are not sufficient
for keeping to a pathway for the long-term stabilization of RF. In particular,
stabilization requires an approach for comparing different greenhouse gases, which
is consistent with net anthropogenic emissions of CO2 having to be reduced to
zero or negative values, because a significant fraction of these remains in the
atmosphere for many thousands of years. Therefore, it is necessary to recognize
that permanent reductions in emissions of short-lived gases like methane, even
though associated with a high GWP, can only be treated as a basis for delaying,
not avoiding, permanent CO2 emission reductions.
In the context of scientific research, it is also necessary to go beyond considering
marginal changes to the present atmosphere and consider the full range of
potential changes in the physical, chemical and biological systems that affect
atmospheric greenhouse gases. For example, climate change can have significant
feedback effects on the global carbon cycle [17], leading to large changes in the
removal rates for anthropogenic CO2 , which was the primary cause. Similarly,
there are continuing large uncertainties in atmospheric oxidation rates, which
remove several greenhouse gases [18], as well as evidence for significant short-term
variations in those rates [19,20]. Future changes in atmospheric oxidation rates
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1893
could be caused by climate change and have a long-term significance, but at this
stage, neither the magnitude nor sign of such changes are clear [21]. Removal rates
for the major anthropogenic greenhouse gases depend on complex and potentially
nonlinear systems, and therefore need to be considered in a context that goes
beyond marginal changes to the present atmosphere.
In §2, we summarize concerns with the GWPs that are currently used to
compare greenhouse-gas emissions. This covers recent increases in uncertainty
for this metric, and raises questions about the scope of processes that should be
taken into account. It is also relevant to note that GWP values are currently
being continually revised to reflect changing concentrations of CO2 and other
greenhouse gases. Therefore, this does not provide the static framework for
comparing greenhouse gases that would be useful for a simple policy perspective.
In §3, we go beyond GWPs to cover the importance of a dynamic approach for
comparing changes in emissions of different greenhouse gases, which maintains
consistency with a specific pathway to future climate stabilization. This is
done by applying the forcing equivalence index (FEI), originally introduced by
Wigley [11], to stabilization trajectories. In the context of climate stabilization,
this provides a quantitative framework for considering fully compensating
changes in the emissions of different greenhouse gases, which could arise from
technological development leading to major structural changes in the options
available.
Section 4 considers a complementary framework for comparing the roles
of different greenhouse gases in climate change more broadly. It is based on
a comparison of the rates of natural removal of greenhouse gases from the
atmosphere in terms of the corresponding changes in RF. This provides a balanced
way of considering the relative importance of changes in the global carbon cycle
and in atmospheric chemistry, and can be relevant when considering focal points
for ongoing research.
Section 5 provides a short summary of the key points coming from these
complementary ways of comparing greenhouse gases and the processes that
control them. The approaches considered here go well beyond the concept of
GWPs as a basis for comparing different greenhouse gases. They are not intended
to produce alternative simple indices, but rather to provide a broader framework
for considering the time dimension and to recognize the significance of current
uncertainties when considering future pathways to climate stabilization.
2. Issues with global warming potentials
A clear basis for comparing the effect of different greenhouse gases on climate
is the concept of RF. This originated as ‘thermal trapping’ [1], but was rapidly
developed and extended into a well-defined measure of the change in radiative
balance caused by changes in the concentrations of greenhouse gases and aerosols,
and also by changes in surface albedo and solar radiation [2]. While the
relationship between RF and temperature change can vary somewhat across
different forcing agents, this relationship appears to be quite uniform for the wellmixed greenhouse gases [22]. Furthermore, the equilibrium global temperature
response to RF is expected to be linear over century time scales [14]; thus, this
measure of the change in the atmosphere provides a clear basis for aggregating
the effects caused by different greenhouse gases.
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1894
M. Manning and A. Reisinger
However, comparing the effects of emissions of greenhouse gases leads to a
more complex situation because it needs to take into account the rate at which
a gas is removed from the atmosphere, as well as its side effects on atmospheric
chemistry. In this context, the GWP is an index giving the ratio of the increases
in RF caused by pulse emissions of different greenhouse gases and integrated over
a fixed time horizon [3,4]. CO2 is used as the reference gas, and GWPs with a
time horizon of 100 years have become widely used in the policy framework.
There have been many research papers since 1990 on the uncertainties in
GWPs and on possible alternative indices for comparing greenhouse gases that
might have broader relevance [5,10,23,24]. The uncertainties in GWPs come
from three different factors: the RF caused by an increase in greenhouse-gas
concentration; the rate at which the concentration increase caused by a pulse
emission declines over time; and the indirect effects that an increase in the gas
may have on other radiatively active species. These processes, and their extension
across the time horizon being considered, mean that GWP values have far more
uncertainty than RF.
A recent study [6] has shown that these uncertainties in GWP estimates
are larger than previously reported. This arises primarily because of significant
uncertainties in the global carbon cycle that controls the rate at which CO2
from a pulse emission will decline, and because consistency with the full range
of carbon cycle and coupled ocean–atmosphere climate models used in the last
IPCC assessment [25] leads to significantly larger uncertainties in the GWP
values than was estimated in that assessment [2]. Reisinger et al. [6] also
show that these uncertainties increase with the time horizon being considered
because of fundamental questions involved in determining the details of longterm carbon-cycle responses to both the additional atmospheric CO2 and the
resulting climate change.
Because GWPs are used to compare all other greenhouse gases to CO2 , carboncycle uncertainties apply uniformly, but there are also specific uncertainties owing
to incomplete knowledge of the lifetimes of the species being compared with
CO2 , and the range of indirect effects that can be caused by their increase.
Recent analyses have shown a continuing uncertainty of about 10 per cent
(1 standard deviation) in the concentration of the hydroxyl radical (OH), which
is responsible for most of the removal of methane and some other greenhouse
gases [18]. The methane lifetime has additional uncertainty owing to its removal
by stratospheric processes and a soil sink. In addition, there is now strong
evidence, based on carbon isotopic measurements, for an additional sink similar
in magnitude to that of the stratospheric and soil chemistry processes and linked
to atmospheric chlorine [26–28]. Whether or not this fourth sink for methane
is variable or is undergoing a trend is still unknown, as are its implications for
long-term pathways to climate stabilization.
Additional uncertainties in the indirect GWP components for methane were
reflected in its significant increase between the last two IPCC Working Group I
assessments [2,14]. Since then, a new analysis of the interactions between methane
and aerosol chemistry suggests a case for a further significant increase in the GWP
value, but this also leads to a consequent increase in its uncertainty [7].
These continuing uncertainties could lead to ongoing changes in the best
estimates of GWPs and so to difficulties for climate policy based on this
form of greenhouse-gas comparison. This is complicated even further because
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1895
some GWP values are dependent on the continuing changes in atmospheric
concentrations of the gas concerned, and all are dependent on changing CO2
concentrations. RF is proportional to the logarithm of the concentration for
CO2 ; to the square root of concentration for methane and nitrous oxide; and
is linear in concentration for all other gases. This means that the incremental
effect of a pulse emission of CO2 decreases, as its concentration increases more
rapidly than for any other gas. It has been noted that this decreasing effect
of incremental changes in CO2 on RF could be compensated for by a similar
decrease in the rate of CO2 uptake by the ocean [29], if the GWP was based
on a non-steady state impulse response function. However, the present approach
keeps CO2 removal rates fixed, and so leads to increases in all GWPs as the CO2
concentration increases. The logarithmic dependence of RF on CO2 concentration
means that about one-third of the 19 per cent increase in the methane GWP
between the IPCC 2007 report [2] and the IPCC 1995 report [15] is due to
the increase in atmospheric CO2 concentrations that has taken place. The other
two-thirds is due to new estimates of the indirect effects of methane added to
the atmosphere.
So far, these incremental changes in GWP values owing to concentration
changes are small, however, continuing increases in CO2 will increase the
GWP for all the other greenhouse gases. If one considers pathways to climate
stabilization, such as recently given in the reference concentration pathways
(RCPs) [30,31], then it is clear that continuing updates of the GWP could have
quite significant implications in the future. Along the RCP3 pathway, which is
aimed at stabilizing global warming at 2◦ C, the methane GWP would increase by
about 33 per cent as the concentration of methane decreased to about 65 per cent
of present-day levels, and CO2 stabilized at levels about 10 per cent higher than its
present-day levels.1
Continual revision of the GWP value in the policy framework arising from
such adjustments would lead to progressive increases in the carbon price for
methane emissions, and so potentially towards a steadily shifting emphasis
on further reduction of methane rather than CO2 . This may have short-term
value, but is not consistent with long-term climate-change stabilization strategies.
Furthermore, the use of a fixed time horizon for the GWP concept does not cover
our understanding that about 20 per cent of CO2 emitted to the atmosphere
persists for tens of thousands of years, which is a critical factor when considering
climate stabilization [32,33].
Stability in a metric for comparing greenhouse gases has been recognized as
significant, and it has been proposed that the GWP values specified in the IPCC
1995 report [15] be retained in the policy framework rather than adjusting these
to cover the values given in the more recent assessments [16]. An alternative
approach that would allow for revisions based on scientific understanding, but not
on progressive changes in atmospheric composition, would be to redefine GWPs to
be based on atmospheric concentrations for a specific year, such as 2000. However,
in the next section, we show why comparing emissions of greenhouse gases in the
context of climate stabilization needs to follow a different approach.
1 This
is based on assuming that the indirect part of the methane GWP scales linearly with the
direct part, as has been used in the last two IPCC assessments, but that relationship is not yet
clearly established in studies of future atmospheric chemistry.
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1896
M. Manning and A. Reisinger
3. Comparing greenhouse gases for stabilization scenarios
An alternative framework for comparing emissions of different greenhouse gases
was introduced by Wigley [11] as the FEI. This is based on determining the
marginal changes in emissions of different greenhouse gases, which have exact
compensating effects on the total RF, and so keep to the same multi-gas scenario
over time. In this context, a constant change in the emissions of one greenhouse
gas over time is balanced by a time-varying change for another one. Thus, the
FEI specifically avoids the introduction of an arbitrary time horizon as used in
GWPs. Instead, it quantifies the time dependence of changes in emissions of
different greenhouse gases that keep to the same RF pathway.
Emission scenarios consistent with achieving climate stabilization have been
developed in some detail [30,31], but changes in the emissions of different
greenhouse gases that keep to the same total GWP-weighted emissions do not
keep to the same RF over time. For example, if technological development
led to faster reduction in methane emissions and there was a GWP-based
compensating increase in CO2 emissions, then the result would be inconsistent
with climate stabilization.
In this context, it is important to consider the FEI in the context of climate
stabilization. Here, we consider this as the basis for changes to a scenario for future
emissions that are intended to achieve stabilization of total RF at about 3 W m−2 .
Figure 1 shows the evolution of emissions and RF to the year 2300, as well
as constant increases or decreases in methane emissions and the compensating
changes in CO2 emissions that balance out to give exactly the same total RF. Our
results are illustrative, but are based on emulating 19 atmosphere–ocean general
circulation models (AOGCMs) and nine carbon-cycle models using a version
of the Model for the Assessment of Greenhouse-gas-induced Climate Change
(MAGICC), which has been designed so it can be tuned to reproduce the global
mean results for a very wide range of climate models [6,34]. Our approach is also
closely consistent with the recent analyses of uncertainties in GWPs [6,34].
The effects of methane emissions on atmospheric chemistry and the implications
for changes in RF are treated here in a way that is consistent with their inclusion
as an indirect effect in the GWP. This way of considering pathways to climate
stabilization is based directly on detailed models as mentioned above, but factors,
such as the recent increase in the release of methane from natural sources in the
Arctic region, suggest it may not be taking account of all the feedbacks that could
occur [35,36].
The key aspect of this approach is that it quantifies how, in the context
of climate stabilization, a permanent change in the emissions of a short-lived
greenhouse gas can only justify a steadily diminishing change in emissions of
CO2 . In the long run, emissions of CO2 would still have to decline to the
same level, regardless of the changes in methane emissions. This demonstrates
the limits of GWPs for comparing greenhouse gases in the context of climate
stabilization, which, as noted in §2, could result in an increase rather than
decrease in the exchange rate between CH4 and CO2 over time with perverse
long-term implications.
The time-varying ratio between exactly balancing changes in methane and CO2
emissions that maintain a specific stabilization scenario is shown in figure 2. The
average ratio reaches the values given by the methane GWPs with time horizons of
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1897
2000
2100
2200
2300
3
2
1
400
CO2 emissions (Gt yr–1)
300
200
30
CH4 emissions (Mt yr–1)
total radiative forcing (W m–2)
Comparing greenhouse gases
20
10
0
2000
2100
2200
2300
year
Figure 1. Comparison of changes from pre-industrial values in the emissions of CO2 and methane
that are consistent with stabilizing greenhouse gases with a total RF of 3.4 W m−2 . The upper graph
shows the total RF pathway that is followed with very minor variations between the three cases. The
solid (blue) lines in the lower two graphs show the average across 171 model runs for the evolution
of methane and CO2 emissions that are consistent with that. The dotted (red) and dashed (green)
lines show the averages for a fixed change in methane emissions and the compensating changes in
CO2 emissions that lead to the same RF. (Online version in colour.)
20, 100 and 500 years at times of 12, 54 and 172 years after the change in emissions
occurs. This comparison shows that there are significant differences between
considering pulse emissions in a stable environment and long-term changes in
emissions trajectories, which remain consistent with future climate stabilization.
Long-term changes in the balance of emissions of different greenhouse gases are
likely to become increasingly determined by technological changes, and GWPs can
not be used in that context to keep to the same climate-stabilization pathway on
any long-term basis.
Figure 2 also shows the corresponding uncertainties in the FEI approach for
considering marginal changes between emissions that keep to a pathway for
climate stabilization. This shows the time-dependent ratio of changes in emissions
of methane and CO2 , which are consistent with the stabilization pathway, and the
full range and standard deviation of these ratios for 171 different combinations of
carbon-cycle and climate-model parameters taken from the IPCC Working Group
I fourth assessment report [25]. We have followed the approach recently developed
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1898
M. Manning and A. Reisinger
120
ratio of CH4 to CO2 emission rate
(a)
100
80
60
40
(b)
relative error (%)
20
40
20
0
50
100
200
150
year after emission changes
250
Figure 2. The ratio of a change in methane emissions to the compensating change in CO2 emissions
that keeps to the stabilization scenario shown in figure 1. In (a), the central line is the median for
the wide range of values covering combinations of carbon-cycle and climate-model parameters; the
dark grey band shows the 1 standard deviation range around this; and the light grey band shows
the full range. Open circles show where the median ratio matches the values of the GWPs for the
25, 100 and 500 year time horizons. (b) The relative uncertainty in the FEI value based on its 1
standard deviation range.
for considering the uncertainties in GWPs [6], and used the simple climate
model (MAGICC) to emulate the results from complex climate and carbon-cycle
models used in the Coupled Model Intercomparison Project phase three (CMIP3)
and Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP)
intercomparisons [17,34]. The relative uncertainty in the ratios of emissions
changes keeping to a stabilization pathway increases from about 10 to 35 per
cent over the time frame from 0 to 250 years.
The FEI’s more explicit treatment of considering greenhouse-gas emissions
in the context of climate stabilization raises the need to consider structural
uncertainties in projections of future climate change. The uncertainties shown
in figure 2 do cover this to some extent, but a robust approach to considering
pathways to climate stabilization should also be extended to consider the
implications of nonlinear changes in atmospheric chemistry that might arise from
the combination of anthropogenic emissions and changes in global atmospheric
chemistry and biogeochemistry. Issues, such as the stability of atmospheric
oxidation rates, could be considered in the context of GWPs, but can be dealt
with far more directly when considering potential changes in FEIs through use
of coupled climate–atmospheric chemistry models.
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1899
4. Comparing the key processes controlling greenhouse gases in the atmosphere
As shown in the preceding two sections, there are significant uncertainties in
numeric comparisons of the effects of emissions of different greenhouse gases on
future climate change. Most of these uncertainties arise from the limits to our
current understanding of the processes controlling the removal of gases and how
these may change over the time scales that are relevant for considering the effects
of emissions in the context of climate stabilization. Furthermore, much of the
underlying research in this area has been showing growing evidence for changes
in the removal rates of CO2 and other greenhouse gases (e.g. [37,38]).
This section goes beyond considering marginal changes in emissions to
a complementary perspective for comparing the relative importance of the
natural processes that control different greenhouse gases and the importance of
understanding potential changes in these. The aim is to develop a balanced and
consistent framework for comparing the natural processes that control greenhouse
gases and, as with the GWPs or FEIs, use RF as a consistent way of doing this.
Thus, removal processes are represented as the corresponding rates of decrease in
RF. While greenhouse-gas sources are primarily anthropogenic, there is growing
evidence that climate change can also alter some natural sources [35,36], and
while the focus here is on the natural removal rates, this could be extended to
cover such climate-driven natural sources.
Using RF as a basis for comparing greenhouse gases raises the question as to
whether water vapour should be included in this framework as well. However,
the lack of any long-term trend in atmospheric relative humidity implies that
changes in water content are only in response to warming, and so are important
feedback effects, but not drivers of climate change [39]. From this perspective,
figure 3 shows the rates of change in RF since 1960 that are directly related
to concentration changes in the nine most significant greenhouse gases, CO2 ,
methane, nitrous oxide, CHClF2 (HCFC-22) and the now declining CCl3 F (CFC11), CCl2 F2 (CFC-12), CClF2 CCl2 F (CFC-13), carbon tetrachloride and methyl
chloroform. Figure 3a shows the rates of changes in RF owing to the annual
changes in concentrations, and so covers the net effect of both emissions and
removal of these gases.
Figure 3b then shows the rates of reduction in RF that is being caused by the
trace-gas removal processes. It is immediately clear that the ordering of gases by
the magnitude of their RF removal processes is very different from the ordering in
figure 3a based just on the relative changes in atmospheric concentrations. This
shows that the relative importance of removal processes is obscured when only the
net changes are considered. In addition, it is clear that the relative magnitudes
of removal rates have changed over time.
Data used in figure 3 are taken from the Scripps Institution of Oceanography
(http://scrippsco2.ucsd.edu/), Climate Monitoring and Diagnostics Laboratory
(http://www.esrl.noaa.gov/gmd/dv/ftpdata.html) and Carbon Dioxide Information Analysis Center (http://cdiac.esd.ornl.gov/home.html) web pages as
well as firn data for methane [40]. In some cases, the earlier values
within the time period shown here have been estimated from probable
rates of growth prior to the first reliable measurements. For example,
atmospheric concentrations of nitrous oxide used here combine results from
a review of ice-core data [41], for values prior to 1979, with global averages
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1900
M. Manning and A. Reisinger
(a)
1960
1970
1980
1990
2010
CO2
30
rate of change in RF (mW m–2 yr–1)
2000
20
10
methane
nitrous oxide
2
CFC-12
1
CFC-11
HCFC-22
CCl4 and CH3CCl3
CFC-113
0
(b)
methane
RF removal rates (mW m–2 yr–1)
CO2
10
HCFC-22
CFC-12
nitrous oxide
CFC-11
1
CCl4
CFC-113
CH3CCl3
0.1
1960
1970
1980
1990
2000
2010
year
Figure 3. (a) Rates of change in radiative forcing (RF) owing to the nine major greenhouse gases
over the period 1960–2008. The vertical axis is linear, but has a split in scale in order to show the
lower gases more clearly and the existence of negative annual changes in the RF owing to some
gases. (b) The magnitude of the removal rates of these gases converted to the corresponding rate
of decrease in RF. This uses a logarithmic axis to cover the wide range that is involved. (Online
version in colour.)
based on data from the Advanced Global Atmospheric Gases Experiment
(AGAGE) from 1979 onward (http://agage.eas.gatech.edu/data.htm). Estimates
of the removal rates of CO2 are based on budget closure with the
sources defined by Boden et al. [42] for fossil-fuel emissions and Le Quéré
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1901
(http://lgmacweb.env.uea.ac.uk/lequere/co2/carbon_budget.htm) for emissions
owing to land-use change. For the other gases, removal rates are determined by
using recent estimates of their lifetimes [2,21].
While it is not shown here, the sources of these greenhouse gases can also be
represented as the corresponding rate of input to RF. This rate was comparable
for both methane and CO2 over the period 1960–1980; however, the steadily
increasing emissions of CO2 have led to these driving a growing rate of increase in
RF from 1983 to 1991 and then again from 2001 to 2007. For CO2 , this source rate
has now become equivalent to an annual RF increase of about 65 mW m−2 yr−1 ,
while for methane, the source input to RF has remained almost steady at about
45 mW m−2 yr−1 since 1985.
Figure 3b shows that the magnitudes of annual removals of CO2 and methane
are about 10 times more significant than the other greenhouse gases in the
context of their control on RF. Furthermore, the rate of removal of methane by
atmospheric chemistry is having a greater effect in offsetting increases in RF than
the removal of CO2 by its land and ocean uptake. The emergence of HCFC-22 as
having the third most important removal rate for more than a decade now shows
another clear difference from the net effect seen in figure 3a.
Currently, for the nine greenhouse gases considered here, the total RF input
coming from emissions is about 120 mW m−2 yr−1 . Of this, about 90 mW m−2
yr−1 is being offset by the removal rates for the gases, leaving a net increase in
RF of about 30 mW m−2 yr−1 . Over the last 40 years, the total annual emissions
and removals of RF have each risen by about 80 per cent. The distribution of
this input to RF across the different greenhouse gases has changed, and, while
methane sources have contributed about 40 per cent of the total emissions RF
throughout, the CO2 contribution has risen above 50 per cent, while that of
the chlorofluorocarbons has dropped, but is being compensated by the growing
increase in HCFC-22.
About 85 per cent of the methane removal, as well as all the removal of
HCFC-22 and methyl chloroform, are due to the OH. This very significant role
that OH has on controlling RF is shown in more detail in figure 4 where the rate
of removal of all these nine gases is broken into: OH oxidation; carbon sinks for
CO2 ; and all other removal processes. OH oxidation is generally a more significant
control of anthropogenic forcing of the climate system than the global uptake of
atmospheric CO2 .
This perspective on the control of RF shows the significance of possible changes
in OH that might arise from the interactions between atmospheric chemistry and
climate change. At this stage, there is little evidence for trends in OH [18,43],
but several analyses indicate that a future reduction in OH can be caused by
the effects of climate change on atmospheric chemistry [21]. This raises questions
about the treatment of a feedback between the concentrations of OH and methane
when calculating its indirect GWP component. There is evidence for some large
fluctuations in OH that persist for several months or longer and are driven by
one or more of: volcanic eruptions; large emissions of gases from biomass burning;
or intense El Nino events [19,20]. There appears to have been a recovery of
OH each time, but because of the complex and nonlinear processes that are
involved, their role in limiting future climate change needs to be considered quite
specifically rather than just by focusing on possible marginal effects of specific
gases such as methane.
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1902
M. Manning and A. Reisinger
RF removal rate
by process (mW m–2 yr–1)
OH oxidation
40
CO2 removal
20
other removal
0
1960
1970
1980
1990
2000
2010
year
Figure 4. RF removal rates grouped into the main processes removing greenhouse gases from
the atmosphere as follows: oxidation by OH; removal of CO2 by vegetation and oceans; and the
other removal processes relevant for the nine greenhouse gases shown in figure 3. (Online version
in colour.)
Figure 4 also shows the large interannual variability in CO2 removal in the
context of RF, which can vary by up to a factor of two. The issue of whether or not
there is a long-term trend in the uptake of CO2 has recently led to some different
views [44–47]. However, the present analysis is not directly linked to this because
it is based on RF, which does not depend linearly on the CO2 concentration.
5. Conclusion
The strategic objective of climate stabilization is now a very important context for
studying the anthropogenic drivers of climate change. From an economic and longterm policy perspective, it needs to become recognized that the current framework
for comparing greenhouse gases using GWPs is not aligned with stabilization or
with economic optimization. In addition, it is becoming increasingly important
to develop ways of considering changes in emissions of different greenhouse gases
that are closely aligned with keeping to a climate-stabilization goal. However,
this raises the major issue of reducing scientific uncertainties that underlie all
comparisons of emissions of different gases.
The standard way of comparing emissions of different greenhouse gases is
through GWP values based on RF integrated over a fixed time horizon, but
this does not provide a way of equating emissions of short- and long-lived gases
in the context of climate stabilization. For this, it is necessary to cover the fact
that a permanent reduction in emissions of a short-lived greenhouse gas can only
justify a temporary increase in emissions of CO2 .
Furthermore, estimated uncertainties in GWP values have become larger
owing to the wide range of potential CO2 removal rates in the future and their
dependence on climate change [6]. Uncertainties in the removal rates of methane
[26–28] and a widening range of indirect effects for this gas [7] are also creating
growing issues for the use of the GWP in a policy framework.
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1903
Long-term changes in the emissions of different greenhouse gases can be
considered in a way that is directly consistent with climate stabilization through
use of the FEI concept [11]. This shows that a permanent decrease in emissions of
a short-lived gas, such as methane, is balanced by an increase in emissions of CO2 ,
which must steadily decrease over time. However, there are still large uncertainties
in this way of comparing the long-term effects of emissions of different greenhouse
gases. This raises the need to consider safety margins in long-term plans for a
pathway to climate stabilization to cover the uncertainty in changing atmospheric
processes, as well as continuing technological and economic development.
Addressing the uncertainties in a systematic way requires a clear perspective for
considering the relative roles of different removal processes for greenhouse gases
and how these may be affected by future climate change. We have summarized
how the natural removal processes can be considered in terms of their effects on
RF, and this shows that the oxidation of methane and other greenhouse gases
by OH is at least as important as the sum of oceanic and terrestrial vegetation
removal processes for atmospheric CO2 . It is clear that we still need to have a
better understanding of the way in which global changes may affect these two
quite different types of greenhouse-gas removals in order to reduce uncertainty in
how climate change can be managed in the long term.
Our research in this area received funding through recent research contracts with the New Zealand
Ministry for Agriculture and Forestry. It has also benefited considerably from use of the MAGICC
model provided to us by Dr Malte Meinshausen. We are also grateful for the very helpful comments
that were received from two reviewers.
References
1 Dickinson, R. E. & Cicerone, R. J. 1986 Future global warming from atmospheric trace gases.
Nature 319, 109–115. (doi:10.1038/319109a0)
2 Forster, P. et al. 2007 Changes in atmospheric constituents and in radiative forcing. In Climate
change 2007: the physical science basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change (eds S. Solomon, D. Qin,
M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller). Cambridge, UK:
Cambridge University Press.
3 Lashof, D. A. & Ahuja, D. R. 1990 Relative contributions of greenhouse gas emissions to global
warming. Nature 344, 529–531. (doi:10.1038/344529a0)
4 Rodhe, H. 1990 A comparison of the contribution of various gases to the greenhouse effect.
Science 248, 1217–1219. (doi:10.1126/science.248.4960.1217)
5 Fuglestvedt, J. S., Berntsen, T. K., Godal, O., Sausen, R., Shine, K. P. & Skodvin, T. 2003
Metrics of climate change: assessing radiative forcing and emission indices. Clim. Change 58,
267–331. (doi:10.1023/A:1023905326842)
6 Reisinger, A., Bodeker, G., Manning, M. & Meinshausen, M. 2010 Uncertainties of global
warming metrics: CO2 and CH4 . Geophys. Res. Lett. 37, L14707. (doi:10.1029/2010GL043803)
7 Shindell, D. T., Faluvegi, G., Koch, D. M., Schmidt, G. A., Unger, N. & Bauer, S. E. 2009
Improved attribution of climate forcing to emissions. Science 326, 716–718. (doi:10.1126/
science.1174760)
8 Harvey, L. D. D. 1993 A guide to global warming potentials (GWPs). Energy Policy 21, 24–34.
(doi:10.1016/0301-4215(93)90205-T)
9 Hammitt, J. K., Jain, A. K., Adams, J. L. and Wuebbles, D. J. 1996 A welfare based index for
assessing environmental effects of greenhouse-gas emissions. Nature 381, 301–303. (doi:10.1038/
381301a0)
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
1904
M. Manning and A. Reisinger
10 Shine, K. P., Fuglestvedt, J. S., Hailemariam, K. & Stuber, N. 2005 Alternatives to the global
warming potential for comparing climate impacts of emissions of greenhouse gases. Clim.
Change 68, 281–302. (doi:10.1007/s10584-005-1146-9)
11 Wigley, T. M. L. 1998 The Kyoto protocol: CO2 , CH4 and climate implications. Geophys. Res.
Lett. 25, 2285–2288. (doi:10.1029/98GL01855)
12 Johansson, D. J. A., Persson, U. M. & Azar, C. 2006 The cost of using global warming
potentials: analysing the trade off between CO2 , CH4 and N2 O. Clim. Change 77, 291–309.
(doi:10.1007/s10584-006-9054-1)
13 Manne, A. S. & Richels, R. G. 2001 An alternative approach to establishing trade-offs among
greenhouse gases. Nature 410, 675–677. (doi:10.1038/35070541)
14 Ramaswamy, V., Boucher, O., Haigh, J., Haglustaine, D., Haywood, J., Myhre, G., Nakajima,
T., Shi, G. Y. & Solomon, S. 2001 Radiative forcing of climate change. In Climate change
2001: the scientific basis. Contribution of Working Group I to the Third Assessment Report of
the Intergovernmental Panel on Climate Change (eds J. T. Houghton, Y. Ding, D. J. Griggs,
M. Noguer, P. J. van der Linden & D. Xiaosu), p. 881. Cambridge, UK: Cambridge University
Press.
15 Schimel, D. et al. 1996 Chapter 2: radiative forcing of climate change. In Climate change 1995 the
science of climate change (eds J. T. Houghton et al.), pp. 65–131. Cambridge, UK: Cambridge
University Press.
16 United Nations Framework Convention on Climate Change (UNFCC). 2009 Report of the Ad
Hoc Working Group on Further Commitments for Annex I Parties under the Kyoto Protocol on
its tenth session, held in Copenhagen from 7 to 15 December 2009. FCCC/KP/AWG/2009/17.
FCCC/KP/AWG/2009/17, UNFCC. See http://unfccc.int/essential_background/library/
items/3599.php?such=j&symbol=FCCC/KP/AWG/2009/17.
17 Friedlingstein, P. et al. 2006 Climate-carbon cycle feedback analysis: results from the c4mip
model intercomparison. J. Climate 19, 3337–3353. (doi:10.1175/JCLI3800.1)
18 Wang, J. S., McElroy, M. B., Logan, J. A., Palmer, P. I., Chameides, W. L., Wang, Y. &
Megretskaia, I. A. 2008 A quantitative assessment of uncertainties affecting estimates of global
mean OH derived from methyl chloroform observations. J. Geophys. Res. 113, D12302. (doi:10.
1029/2007JD008496)
19 Manning, M. R., Lowe, D. C., Moss, R. C., Bodeker, G. E. & Allan, W. 2005 Short term
variations in the oxidising power of the atmosphere. Nature 436, 1001–1004. (doi:10.1038/
nature03900)
20 Prinn, R. G. et al. 2005 Evidence for variability of atmospheric hydroxyl radicals over the past
quarter century. Geophys. Res. Lett. 32, L07809. (doi:10.1029/2004GL022228)
21 Denman, K. L. et al. 2007 Couplings between changes in the climate system and biogeochemistry
(chapter 7). In Climate change 2007: the physical science basis. Contribution of Working Group
I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds
S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L.
Miller). Cambridge, UK: Cambridge University Press.
22 Joshi, M., Shine, K., Ponater, M., Stuber, N., Sausen, R. & Li, L. 2003 A comparison of climate
response to different radiative forcings in three general circulation models: towards an improved
metric of climate change. Clim. Dynam. 20, 843–854. (doi:10.1007/s00382-003-0305-9)
23 Shine, K. P. 2009 The global warming potential: the need for an interdisciplinary retrial. Clim.
Change 96, 467–472. (doi:10.1007/s10584-009-9647-6)
24 Shine, K. P., Berntsen, T. K., Fuglestvedt, J. S., Skeie, R. B. & Stuber, N. 2007 Comparing
the climate effect of emissions of short- and long-lived climate agents. Phil. Trans. R. Soc. A
365, 1903–1914. (doi:10.1098/rsta.2007.2050)
25 Meehl, G. A. et al. 2007 Global climate projections (chapter 10). In Climate change 2007: the
physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change (eds S. Solomon, D. Qin, M. Manning, Z. Chen,
M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller). Cambridge, UK: Cambridge University
Press.
26 Allan, W., Struthers, H. & Lowe, D. C. 2007 Methane carbon isotope effects caused by
atomic chlorine in the marine boundary layer: global model results compared with Southern
Hemisphere measurements. J. Geophys. Res. 112, D04306. (doi:10.1029/2006JD007369)
Phil. Trans. R. Soc. A (2011)
Downloaded from http://rsta.royalsocietypublishing.org/ on May 4, 2017
Comparing greenhouse gases
1905
27 Allan, W., Struthers, H., Lowe, D. C. & Mikaloff Fletcher, S. E. 2010 Modeling the effects of
methane source changes on the seasonal cycles of methane mixing ratio and d13C in Southern
Hemisphere mid-latitudes. J. Geophys. Res. 115, D07301. (doi:10.1029/2009JD012924)
28 Thornton, J. A. et al. 2010 A large atomic chlorine source inferred from mid-continental reactive
nitrogen chemistry. Nature 464, 271–274. (doi:10.1038/nature08905)
29 Caldeira, K. & Kasting, J. F. 1993 Insensitivity of global warming potentials to carbon dioxide
emission scenarios. Nature 366, 251–253. (doi:10.1038/366251a0)
30 RCP Database Webmaster. 2009 RCP Database (version 1.0). International Institute for
Applied Systems Analysis. See http://www.iiasa. ac.at/web-apps/tnt/RcpDb.
31 Van Vuuren, D. P. et al. 2008 Temperature increase of 21st century mitigation scenarios. Proc.
Natl Acad. Sci. USA 105, 15 258–15 262. (doi:10.1073/pnas.0711129105)
32 Archer, D. 2005 Fate of fossil fuel CO2 in geologic time. J. Geophys. Res. 110, C09S05.
(doi:10.1029/2004JC002625)
33 Solomon, S., Plattner, G.-K., Knutti, R. & Friedlingstein, P. 2009 Irreversible climate change
due to carbon dioxide emissions. Proc. Natl Acad. Sci. USA 106, 1704–1709. (doi:10.1073/
pnas.0812721106)
34 Meinshausen, M., Raper, S. C. B. & Wigley, T. M. L. 2008 Emulating IPCC AR4 atmosphere–
ocean and carbon cycle models for projecting global-mean, hemispheric and land/ocean
temperatures: MAGICC 6.0. Atmos. Chem. Phys. 8, 6153–6272. (10.5194/acpd-8-6153-2008)
35 Dlugokencky, E. J. et al. 2009 Observational constraints on recent increases in the atmospheric
CH4 burden. Geophys. Res. Lett. 36, L18803. (doi:10.1029/2009GL039780)
36 Shakhova, N., Semiletov, I., Salyuk, A., Yusupov, V., Kosmach, D. & Gustafsson, Ö. 2010
Extensive methane venting to the atmosphere from sediments of the east siberian arctic shelf.
Science 327, 1246–1250. (doi:10.1126/science.1182221)
37 Le Quéré, C. et al. 2007 Saturation of the southern ocean CO2 sink due to recent climate
change. Science 316, 1735–1738. (doi:10.1126/science.1136188)
38 Watson, A. J. et al. 2009 Tracking the variable north Atlantic sink for atmospheric CO2 . Science
326, 1391–1393. (doi:10.1126/science.1177394)
39 Trenberth, K. E. et al. 2007 Observations: surface and atmospheric climate change. In Climate
change 2007: the physical science basis. Contribution of Working Group I to the Fourth
Assessment Report of the Intergovernmental Panel on Climate Change (eds S. Solomon, D. Qin,
M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller). Cambridge, UK:
Cambridge University Press.
40 Etheridge, D. M., Steele, L. P., Francey, R. J. & Langenfelds, R. L. 1998 Atmospheric methane
between 1000 AD and present: evidence of anthropogenic emissions and climatic variability.
J. Geophys. Res. 103, 15 979–15 993. (doi:10.1029/98JD00923)
41 Flückiger, J., Dällenbach, A., Blunier, T., Stauffer, B., Stocker, T. F., Raynaud, D. & Barnola,
J.-M. 1999 Variations in atmospheric N2 O concentration during abrupt climatic changes.
Science 285, 227–230. (doi:10.1126/science.285.5425.227)
42 Boden, T. A., Marland, G. & Andres, R. J. 2009 Global, regional, and national fossil-fuel CO2
emissions and preliminary 2007–08 global and national estimates. Carbon Dioxide Information
Analysis Center. See http://www.globalcarbonproject.org/carbonbudget/08/hl-full.htm.
43 Krol, M. & Lelieveld, J. 2003 Can the variability in tropospheric OH be deduced from
measurements of 1,1,1-trichloroethane (methyl chloroform)? J. Geophys. Res. 108, 4125. (doi:
10.1029/2002JD002423)
44 Gloor, M., Sarmiento, J. L. & Gruber, N. 2010 What can be learned about carbon
cycle climate feedbacks from CO2 airborne fraction? Atmos. Chem. Phys. 10, 9045–9075.
(10.5194/acpd-10-9045-2010)
45 Knorr, W. 2009 Is the airborne fraction of anthropogenic CO2 emissions increasing? Geophys.
Res. Lett. 36, L21710. (doi:10.1029/2009GL040613)
46 Le Quéré, C. et al. 2009 Trends in the sources and sinks of carbon dioxide. Nat. Geosci. 2,
831–836. (doi:10.1038/ngeo689)
47 Rafelski, L. E., Piper, S. C. & Keeling, R. F. 2009 Climate effects on atmospheric carbon dioxide
over the last century. Tellus B 61, 718–731. (doi:10.1111/j.1600-0889.2009.00439.x)
Phil. Trans. R. Soc. A (2011)