Geoscience Reference
In-Depth Information
12.6.3. Renewable Portfolio Standards
and the California Waiver
Despite the lack of commitment by the United States
to the Kyoto Protocol, several U.S. states legislated
renewable portfolio standards (RPS)
(also called
renewable electricity standards) in the 2000s, whereby
a certain fraction of electric power generation was
required to come from specified clean energy sources
by a certain date. The electric power devices allowed to
compete in the RPS market were limited to clean energy
technologies. However, the RPS market is a private
market and thus subject to free competition. By 2011,
RPS had been set in more than thirty U.S. states, the
UK, Italy, Belgium, and Chile. Worldwide, other policy
mechanisms such as the
feed-in-tariff
(Section 13.10)
have similarly spurred a conversion to clean energy and
thus a reduction in air pollution and climate-relevant
emissions.
Additional progress was made in the United States
in April 2007, when the U.S. Supreme Court ruled in
Massachusetts v. Environmental Protection Agency
that
carbon dioxide and other greenhouse gases were air pol-
lutants covered under the Clean Air Act Amendments of
1970. Thus, the U.S. EPA had the authority to
consider
regulating these gases. This prompted the U.S. EPA ulti-
mately to grant California a
waiver of Clean Air Act
preemption
(Section 8.1.12) on June 30, 2009. This
allowed the state to set carbon dioxide emission stan-
dards for 2009 model-year passenger vehicles, light-
duty trucks, and medium-duty passenger vehicles sold
in the state. Under the Clean Air Act Amendments of
1970, other states are permitted to set emission regula-
tions as stringent as California's standards. Despite this
regulation, the control of global warming is a process
still in its infancy. A proposed large-scale solution is
provided in Chapter 13.
Because greenhouse gases and aerosol particles have
different atmospheric
e
-folding lifetimes
(the time
required for an initial concentration of a pollutant to
decrease to 1
e
its initial value; Section 1.5), reducing
emissions of some pollutants results in a faster climate
response than reducing emissions of others. However,
even for the shortest-lived pollutants, complete climate
responses take longer than the time required to remove
the pollutant from the air because temperature changes
in the deep ocean take decades to equilibrate with those
in the surface ocean, which takes years to equilibrate
with those in the air.
The data-constrained overall
e
-folding lifetime of
carbon dioxide in the atmosphere against all loss pro-
cesses (primarily dissolution into oceans, photosynthe-
sis, and weathering) is 30 to 50 years (Section 3.6.2).
That of methane, due primarily to chemical oxida-
tion by the hydroxyl radical [OH(g)], is 8 to 12 years,
with a mean of 10 years. That of fossil fuel soot and
solid biofuel soot aerosol particles is approximately
4.5 days (Jacobson, 2010b). Fossil fuel soot is emit-
ted during diesel, jet fuel, kerosene, bunker fuel, oil,
gasoline, natural gas, and coal combustion. Solid bio-
fuel soot is emitted during the burning of wood, grass,
agricultural waste, and dung during home heating and
cooking, primarily in developing countries (Section
9.1.14). Soot particles contain black carbon and pri-
mary organic carbon (POC), along with other minor
constituents.
These
e
-folding lifetime data alone suggest that
con-
trolling soot aerosol particle emissions may be the
fastest method of slowing global warming
relative to
controlling either methane or carbon dioxide. This
result is also borne out by three-dimensional global
model computer simulations that account for emis-
sions and numerous physical processes and feedbacks
(Figure 12.33).
Figure 12.33 indicates that controlling continuous
anthropogenic emissions of CO
2
(g) causes a greater
reduction in globally averaged near-surface tempera-
tures after 100 years than does controlling either fossil
fuel soot (FS), FS plus biofuel soot and gas (BSG), or
anthropogenic CH
4
(g) emissions. Thus, although con-
trolling FS or FS
/
12.6.4. Fastest Methods of Slowing
Global Warming
Policies directed at controlling global warming need to
consider not only the total reduction in global temper-
ature from the policy, but also the speed at which the
temperature reduction is obtained. For example, Arc-
tic sea ice may disappear within a few decades, so a
control measure that results in a rapid reduction in tem-
perature may be desirable in addition to a control mea-
sure that results in the slower but larger reduction in
temperature.
BSG is the fastest method of slowing
global warming, FS
+
BSG is only the second leading
cause of global warming after CO
2
(g).
In the limit of infinite time, the magnitude of cool-
ing due to controlling the emissions of a chemical
depends on both the emission rate of the species and
the temperature change of the species per unit emission.
+
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