Comment Log Display

To CARB,

The Compliance Offsets Protocol - Rice Cultivation Projects
currently relies upon a Methane GWP of 21, referenced through Table
A-1, p 52 of the Regulation for the Mandatory Reporting of
Greenhouse Gas Emissions.

The use of such a Methane GWP Coefficient does not accord with the
latest IPCC Methane GWP coefficients, which are 28 and 34 for a 100
year interval and 84 and 86 for a 20 year interval. Use of the
Methane GWP 21 grossly underestimates the global warming impact of
methane, and any cap and trade program needs to update the methane
GWP expeditiously to be legally and ethically tenable. I do not see
an intent to “update expeditiously” expressed in the document I
have reviewed today.

I am pasting a long chunk of text from Robert Howarth's seminal
2014 publication as support for my claims above. It includes some
material about natural gas as a fuel but then moves forcefully into
reasons for why shorter time frames and higher methane GWPs should
be considered, and used, in assessing methane's impact upon our
already rapidly-warming planet.

To conclude, I urge the CARB to address seriously the current
artificial deflation of methane GWP coefficients and methane global
warming impact that is currently reflected in this rule making
process for rice cultivation

Sincerely, 

Todd M Shuman, 2260 Camilar Dr. Camarillo, CA 93010 8095.987.8203


A bridge to nowhere: methane emissions and the greenhouse gas
footprint of natural gas
Robert W. Howarth Department of Ecology & Evolutionary Biology,
Cornell University, Ithaca, New York 14853

2014 The Author. Energy Science & Engineering published by the
Society of Chemical Industry and John Wiley & Sons Ltd.

Received: 4 March 2014; Revised: 18 April 2014; Accepted: 22 April
2014

The GWP of Methane

While methane is far more effective as a greenhouse gas than carbon
dioxide, methane has an atmospheric lifetime of only 12 years or
so, while carbon dioxide has an effective influence on atmospheric
chemistry for a century or longer [34]. The time frame over which
we compare the two gases is therefore critical, with methane
becoming relatively less important than carbon dioxide as the
timescale
increases. Of the major papers on methane and the GHG for
conventional natural gas published before our analysis for shale
gas, one modeled the relative radiative forcing by methane compared
to carbon dioxide continuously over a 100-year time period
following emission [2], and two used the global warming approach
(GWP) which compares how much larger the integrated global warming
from a given mass of methane is over a specified period of time
compared to the same mass of carbon dioxide.
 
Of the two that used the GWP approach, one showed both 20-year and
100-year GWP analyses [3] while another used only a 100-year GWP
time frame [4]. Both used GWP values from the Intergovernmental
Panel on Climate Change (IPCC) synthesis report from 1996 [35], the
most reliable estimates at the time their papers were published. In
subsequent reports from the IPCC in 2007 [36] and 2013 [34] and in
a paper in Science by workers at the NASA Goddard Space Institute
[37], these GWP values have been substantially increased, in part,
to account for the indirect effects of methane on other radiatively
active substances in the atmosphere such as ozone (Table 2). In
Howarth et al. [8], we used the GWP approach and closely followed
the work of Lelieveld and colleagues [3] in presenting both
integrated 20 and 100 year periods, and in giving equal credence
and interpretation to both timescales. 

We upgraded the approach by using the most recently published
values for GWP at that time [37]. These more recent GWP values
increased the relative warming of methane compared to carbon
dioxide by 1.9-fold for the 20-year time period (GWP of 105 vs. 56)
and by 1.6-fold for the 100-year time period (GWP of 33 vs. 21;
Table 2). Our conclusion was that for the 20-year time period,
shale gas had a larger GHG than coal or oil even at our low-end
estimates for methane emission (Fig. 1); conventional gas also had
a larger GHG than coal or oil at our mean or high-end methane
emission estimates, but not at the very low-end range for methane
emission (the best-case, low-emission scenario). At the 100-year
timescale, the influence of methane was much diminished, yet at our
high-end methane emissions, the
GHG of both shale gas and conventional gas still exceeded that of
coal and oil (Fig. 1). Of nine new reports on methane and natural
gas published in 9 months after our April 2011 paper [8], six only
considered the 100-year time frame for GWP, two used both a 20- and
100-year time frame, and one used a continuous modeling of
radiative forcing over the 0–100 time period (Table 2). 
Of the six papers that only examined the 100-year time frame, all
used the lower GWP value of 25 from the 2007 IPCC report rather
than the higher value of 33 published by Shindell and colleagues in
2009 that we had used; this higher value better accounts for the
indirect effects of methane on global warming.

Many of these six papers implied that the IPCC dictated a focus on
the 100-year time period, which is simply not the case: the IPCC
report from 2007 [36] presented both 20- and 100-year GWP values
for methane. 

And two of these six papers criticized our inclusion of the 20-year
time period as inappropriate [14, 17]. I strongly disagree with
this criticism. In the time since April 2011 I have come
increasingly to believe that it is essential to consider the role
of methane on timescales that are much shorter than 100 years, in
part, due to new science on methane and global warming presented
since then [34, 41, 42],
briefly summarized below. The most recent synthesis report from the
IPCC in 2013 on the physical science basis of global warming
highlights the role of methane in global warming at multiple
timescales, using GWP values for 10 years in addition to 20 and 100
years (GWP of 108, 86, and 34, respectively) in their analysis
[34]. The report states that “there is no scientific argument for
selecting 100 years compared with other choices,” and that “the
choice of time horizon . . .. depends on the relative weight
assigned to the effects at different times” [34].

The IPCC further concludes that at the 10-year timescale, the
current global release of methane from all anthropogenic sources
exceeds (slightly) all anthropogenic carbon dioxide emissions as
agents of global warming; that is, methane emissions are more
important (slightly) than carbon dioxide emissions for driving the
current rate of global warming. At the 20- year timescale, total
global emissions of methane are equivalent to over 80% of global
carbon dioxide emissions. And at the 100-year timescale, current
global methane emissions are equivalent to slightly less than 30%
of carbon dioxide emissions [34] (Fig. 3). This difference in the
time sensitivity of the climate system to methane and carbon
dioxide is critical, and not widely appreciated by the policy
community and even some climate scientists. While some note how the
longterm momentum of the climate system is driven by carbon dioxide
[15], the climate system is far more immediately responsive to
changes in methane (and other short-lived radiatively active
materials in the atmosphere, such as black carbon) [41]. 

The model published in 2012 by Shindell and colleagues [41] and
adopted by the United Nations [42] predicts that unless emissions
of methane and black carbon are reduced immediately, the Earth’s
average surface temperature will warm by 1.5°C by about 2030 and by
2.0°C by 2045 to 2050 whether or not carbon dioxide emissions are
reduced. Reducing methane and black carbon emissions, even if
carbon dioxide is not controlled, would significantly slow the rate
of global warming and postpone reaching the 1.5°C and 2.0°C marks
by 15–20 years. Controlling carbon dioxide as well as methane and
black carbon emissions further slows the rate of global warming
after 2045, through at least 2070 [41, 42] (Fig. 4). 

Why should we care about this warming over the next few decades? At
temperatures of 1.5–2.0°C above the 1890–1910 baseline, the risk of
a fundamental change in the Earth’s climate system becomes much
greater [41–43], possibly leading to runaway feedbacks and even
more global warming. Such a result would dwarf any possible benefit
from reductions in carbon dioxide emissions over the next few
decades (e.g., switching from coal to natural gas, which does
reduce carbon dioxide but also increases methane emissions). One of
many mechanisms for such catastrophic change is the melting of
methane clathrates in the oceans or melting of permafrost in the
Arctic. Hansen and his colleagues [43, 44] have suggested that
warming of the Earth by 1.8°C may trigger a large and rapid
increase in the release of such methane. While there is a wide
range in both the magnitude and timing of projected carbon release
from thawing permafrost and melting clathrates in the literature
[45], warming consistently leads to greater release. This release
can in turn cause a feedback of accelerated global warming [46]. 

To state the converse of the argument: the influence of today’s
emissions on global warming 200 or 300 years into the future will
largely reflect carbon dioxide, and not methane, unless the
emissions of methane lead to tipping points and a fundamental
change in the climate system. And that could happen as early as
within the next two to three decades. An increasing body of science
is developing rapidly that emphasizes the need to consider
methane’s influence over the decadal timescale, and the need to
reduce methane emissions. Unfortunately, some recent guidance for
life cycle assessments specify only the 100-year time frame [47,
48], and the EPA in 2014 still uses the GWP values from the IPCC
1996 assessment and only considers the 100-year time period when
assessing methane emissions [49]. In doing so, they underestimate
the global warming significance of methane by 1.6-fold compared to
more recent values for the 100-year time frame and by four to
fivefold compared to the 10- to 20-year time frames.