The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 290

12.4.2.8. Water Vapor-Low Cloud
Negative Feedback
If air temperatures initially increase, water evaporates
or sublimates from ocean or ice surfaces, increasing the
cover of low clouds (made of small liquid water drops,
which reflect solar radiation), increasing the effective
Earth-atmosphere albedo, and decreasing downward
solar radiation and temperatures.

Absorbing aerosol particles can either be emitted
with a coating or obtain a coating by internal mixing as
they age. An example of an emitted coated particle is a
jet fuel particle, which contains black carbon coated by
lubricating oil, unburned fuel oil, sulfate, and some met-
als. Internal mixing occurs after emissions due either
to the collision of two particles followed by their coa-
lescence (coagulation) or to the diffusional transfer of
agas onto a particle surface followed by a change of
state (condensation). Internal mixing of black carbon
is important because internally mixed black carbon can
cause twice as much global heating as externally mixed
(uncoated) black carbon (Jacobson, 2000, 2001b).
All aerosol particle components absorb thermal-IR
radiation, but thermal-IR absorption is weaker than
solar absorption for those components that absorb solar
radiation. Thermal-IR absorption by cooling particles
is also weaker than is solar scattering by such parti-
cles. Thermal-IR absorption is most important when
the size of a particle approaches the wavelength of
thermal-IR radiation (e.g., 10

12.4.2.9. Plant-Carbon Dioxide
Negative Feedback
If air temperatures initially increase, plants and trees
flourish and photosynthesize more, decreasing the
quantity of carbon dioxide in the air, offsetting some
of the original temperature increases.
The only practical way to elucidate the relative
importance of the different feedback mechanisms is
through computer modeling of the climate. To date,
climate models accounting for feedbacks have found
that increases in the atmospheric greenhouse gases
increase global near-surface temperatures and reduce
stratospheric temperatures, consistent with theory and
observations described in Section 12.3.1.

m). Thus, good thermal-
IR-absorbing particles are in the coarse aerosol size
mode (Section 5.1). They primarily include soil dust,
sea spray, and nitrate-containing and volcanic particles.
In sum, warming aerosol particles reduce sunlight to
the ground by absorbing and converting it to heat that is
released into the air around them. Cooling aerosol parti-
cles also reduce sunlight reaching the ground by reflect-
ing incoming solar radiation back to space. Because
both absorbing and scattering aerosol particles reduce
solar radiation reaching the surface of the Earth, they
all cause a dimming of the sunlight reaching the Earth.
Amajor issue is the extent to which warming from
warming aerosol particles offsets cooling from cool-
ing particles in the global and regional average. If only
instantaneous radiative effects are considered and time-
dependent effects are ignored, cooling caused by cool-
ing particles exceeds warming by warming particles.
Instantaneous radiative effects, however, tell only part
of the story. A true estimate of the effect of aerosol
particles on climate requires the consideration of time-
dependent climate responses ,summarized next.

12.4.3. Effects of Aerosol Particles
on Climate
Although all greenhouse gases warm near-surface air,
some aerosol particle components warm the air but
most cool the air. The main particle components of
warming particles are black carbon (BC), brown carbon
(BrC), and soil dust components. Constituents in BrC
that cause warming include primarily aromatic organic
compounds, nitrated organic compounds, and tar balls.
Constituents of soil dust that cause warming include
iron and aluminum (Section 7.1.3.1). Cooling particles
contain primarily sulfate, nitrate, ammonium, and liq-
uid water, among other compounds (Section 12.1).
Warming aerosol particles absorb solar radiation,
convert the sunlight to heat, and then reradiate the
heat (thermal-IR radiation) to the air around them.
Absorbing aerosol particles that become coated by other
aerosol material (either nonabsorbing or absorbing) heat
the air more than do uncoated absorbing particles. This
enhancement is due to the optical focusing effect .
When an absorbing aerosol particle becomes coated,
the overall particle becomes larger, so more sunlight
hits and refracts into the particle. The additional pho-
tons reflect internally within the particle multiple times,
and many are ultimately absorbed by the absorbing core
in the particle.

12.4.3.1. Daytime Stability Effect and Aerosol
Reduction of Wind
Aerosol particles that absorb solar radiation heat the
air by absorbing sunlight and converting the light into
thermal-IR (heat) radiation that is emitted back to the
air around it. Although the absorbing particles are short
lived, the heated air molecules last longer and are