Aerosols, Global Climate, and the Human Health Co-Benefits of Climate Change Mitigation - Aerosols: Measurement, Dosimetry, and Health Effects - page 346

Natural
processes

Forcing agents

Emissions of greenhouse gases and precursors, aerosols and
precursors, and biogeochemically active gases

Sun, orbit, volcanoes

Solar irradiance and insolation changes
Land-cover changes

Human
activities

Nonradiative
forcing

Change in climate
system components

Fuel usage
Industrial practices
Agricultural practices

Atmospheric lapse rate

Atmospheric composition

Evapotranspiration flux

Direct
radiative
forcing

Indirect
radiative
forcing

Societal
impacts

Feedbacks

Climate response

Temperature, precipitation, vegetation, etc.

FIGURE 13.1 Framework of climate change inluences. (From National Academy of Sciences (NAS),
Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties , National
Research Council, Committee on Radiative Forcing Effects on Climate, Climate Research Committee,
Washington, DC, 2005.)

13.2 AEROSOLS AND CLIMATE CHANGE
Most climate change efforts have focused on controlling carbon dioxide (CO 2 ) emissions, but
there is need to control the shorter-lived greenhouse pollutants (SLGHPs) because these SLGHPs
can collectively inluence climate change more than CO 2 , and because many of the SLGHPs,
such as PM, have human health impacts as well as climate impacts. Moreover, since aerosols and
other SLGHPs are shorter-lived than CO 2 , aerosol atmospheric concentrations (and their associ-
ated climate effects) will respond much more quickly to reductions in PM emissions than will
reductions in CO 2 .
Factors that affect climate change are usefully separated into two categories: forcings and feed-
backs (see Figure 13.1). A climate forcing , as deined by the National Academy of Sciences (2005),
is an energy imbalance imposed on the climate system either externally or by human activities.
Examples include changes in solar energy output, volcanic emissions, deliberate land modiication,
or anthropogenic emissions of GHGs, PM, and their precursors. A climate feedback is an internal
climate process that ampliies or dampens the climate response to a speciic forcing.
As noted in Figure 13.1, aerosols are among the SLGHP forcing agents that result in changes in
climate system components, as well as direct radiative forcings. These SLGHP aerosol components
include sulfate, organic carbon (OC), and black carbon (BC). However, as depicted in Figure 13.2,
despite uncertainties about the exact size of the climate impacts, these different types of particles
have very different climate implications; the irst two, sulfates and OC are cooling forcings, but the
third (elemental BC soot) is warming. Thus, air pollution and climate change policies that reduce
elemental BC soot particles will have larger beneicial effects than many other pollutants, from a
climate change perspective.

13.3 PARTICULATE MATTER HEALTH EFFECTS
Tropospheric aerosols that affect climate change also have signiicant human health implica-
tions. A wealth of scientiic literature clearly links PM with numerous adverse health effects.
Indeed, a U.S. Environmental Protection Agency (USEPA) assessment of human health effects
beneits of the Clean Air Act attributed nearly 90% of the estimated monetary valuation of the
human health effects beneits to be derived from the Act during 1990-2010 to reductions in PM
(USEPA, 1999).