Comment Log Display

ARB Must Consider Radiative Forcing of Tropospheric Ozone and Black
Carbon 
Comments on California Air Resources Board Draft
Short-Lived Climate Pollutant Reduction Strategy by Jim Stewart,
PhD, DrJimStewart@gmail.com, 213-820-4345
ARB staff is well aware the Earth’s climate is determined by the
planetary radiation balance, which is the balance between energy
entering at the top of the atmosphere as sunlight and that leaving
the atmosphere as reflected light and heat back to space.  The
departure from the pre-industrial equilibrium is referred to as
radiative forcing, which is expressed in W/m2.  The Integrated
Assessment of Black Carbon and Tropospheric Ozone by UNEP and WMO
in 2011 reports the dramatic impact of black carbon (BC) and
tropospheric ozone (O3) on global warming: the best estimate (in
2010) for BC is 0.6 W/m2 and for O3 is 0.35 W/m2, which together
nearly equal the then radiative forcing for CO2 of about 1.5 W/m2.
Tropospheric Ozone 
The UNEP report states, “The increase of tropospheric ozone (O3) in
the past 100 years has made it one of the most important
contributors to human-induced global warming, together with CO2,
CH4 and BC. Even though its globally averaged tropospheric
human-derived concentration is about 10,000 times smaller than that
of CO2, O3’s current global radiative forcing is about one-sixth to
one-third that of CO2.” “Ozone is not an emitted pollutant and thus
for control purposes it is appropriate to attribute the radiative
forcing for O3 to the precursor emissions of methane (CH4), carbon
monoxide (CO), non-methane volatile organic compounds (NMVOCs) and
nitrogen oxides (NOx).”  
At the October 14 ARB workshop, Ryan McCarthy of the ARB staff
asserted that tropospheric ozone was nearly all caused by methane,
so the ARB would not consider it as a separate SLCP.  However, the
UNEP report states, “Two-thirds of the O3 radiative forcing to date
may be attributed to the increase in atmospheric CH4 over the last
century.”  This implies that the other components contribute about
a third, and thus should be considered by ARB as separate SLCPs.  
The ARB SLCP draft report uses GWP-20 values (which is to be
commended -- in contrast to the outmoded GWP-100 values used by ARB
in other reports and cap-and-trade calculations).  However, the
UNEP report states, “It is not possible to calculate GWPs or GTPs
for O3 since it is not emitted directly. Instead these metrics can
be calculated for emissions of O3 precursor species. These species
often have an impact on radiative forcing agents other than O3 and
their climate metrics need to take these into account (e.g. Collins
et al., 2002; Shindell et al., 2009 ).” 
The resulting implication is that CO, VOCs and NOx should all be
considered as GHGs for the AB 32 Scoping Plan, cap-and-trade, etc. 
Thus ARB should bring CO, VOCs and NOx under the cap and use
Greenhouse Gas Reduction Funds (GGRF) to pay for their reductions. 
If ARB does not do this, the ARB Environmental Assessment (EA) must
explain why not.
The complexity of methane’s effects is illustrated by Figure 3.14
from the UNEP report.  But the important thing to note is that the
RF of methane is about 1 W/m2, almost as much as carbon dioxide.
 
“Figure 3.14. Global mean pre-industrial to present-day RF by
emitted species from AR4 and Shindell et al., 2009. Numerical
values next to each bar give total forcing summed over all the
effects included in the calculations. Aerosol indirect effects are
not included, nor are interactions with ecosystems.” 
Black Carbon 
The UNEP report states, “Even though the globally averaged
ground-level concentration of black carbon (BC) is presently about
0.1 micrograms/m3 (Chapter 3), compared to about 200 milligrams/m3
of anthropogenic CO2, its instantaneous radiative forcing due to
absorption alone may be about half that of anthropogenic CO2.” 
“Globally averaged net forcing is likely to be positive and in the
range 0.0 to 1.0 W/m2, with a best estimate of 0.6 W/m2 (values
include the enhanced efficacy of BC forcing due to snow and ice
darkening).”
Thus all sources of black carbon (see list in the next figure) are
contributing to global warming and need to be considered by ARB as
GHGs for the AB 32 Scoping Plan, cap-and-trade, etc.  Thus ARB
should strictly regulate them, or bring them under the cap and use
Greenhouse Gas Reduction Funds to pay for their reductions.  If ARB
does not do this, the Environmental Assessment (EA) must explain
why not.
What is particularly appalling about the ARB Draft
Short-Lived Climate Pollutant Reduction Strategy is that no
reductions are scheduled for off-road vehicles by 2030. Thus
off-road vehicles will continue to be a major contributor to GHGs
as well as to health impacts. If ARB strategy does not include
off-road vehicles, the Environmental Assessment (EA) must explain
why not.


Impacts: Impacts of black carbon and tropospheric ozone are huge. 
A few of the ones mentioned in the UNEP report include:
1.	Temperature Rise: “The equilibrium warming that would result
from changes in burdens of black carbon (BC) and tropospheric ozone
(O3) from pre-industrial to 2005 is estimated at 0.0 to 0.8°C for
BC and 0.1 to 0.4°C for O3 as a global average, with larger
contributions in the northern hemisphere mid-latitudes. For
comparison, the equilibrium warming for the observed increase in
carbon dioxide (CO2) over the same time period is about 1.3ºC.” 
2.	Health Impacts: “Globally, premature deaths associated with O3
exposure are valued at US$400 billion.”
3.	Decreased Crop Yields: “Limited evidence suggests that crop
yields will be significantly affected by changes in regional
climate that are likely to be enhanced in areas with high BC and O3
pollution. Surface warming due to O3 acting as a greenhouse gas and
BC will affect temperatures, cloudiness, rainfall amounts and
patterns, and river flow, the latter through BC impacts on glacier
melting and evaporation; all of these factors will impact
agricultural production. Additionally, changes in the amount and
quality of photosynthetically active radiation caused by BC and
aerosols may also affect crop yields.” 
4.	Global Ecosystems: “Changes in global ecosystem net primary
productivity caused by an increase in O3 damage will have a
substantial impact on carbon sequestration in ecosystems and hence
on radiative forcing. A global modelling study has shown that
reduction in carbon sequestration caused by O3 impacts on
vegetation could double the effective radiative forcing
attributable to tropospheric O3 under a future 2100 SRES A2
scenario. 
Near-term Climate Protection Benefits from Controlling Short-Lived
Climate Forcers
The chart below sums up the climate benefits of controlling CH4 and
black carbon: even with no CO2 control measures, the 2040
temperature rise is limited to 1.4ºC, compared to 1.8ºC for only
CO2 measures.  The implication of this chart for ARB is that they
need to switch immediately from their current emphasis on CO2
measures (almost all GGRF funds go toward reducing CO2) to a
balance, perhaps 50-50. If ARB does not do this, the Environmental
Assessment (EA) must explain why not.

This report also has extensive data on the health benefits from
controlling Short-Lived Climate Forcers.