Global Stratospheric Ozone Reduction - Air Pollution and Global Warming: History, Science, and Solutions - page 256

is that the polar vortex around the Arctic is weaker
than around the Antarctic. The Arctic vortex is weaker
because the Earth at 60 ◦ Niscovered byland and
water, whereas 60 ◦ Siscovered by water. The rougher
land surface slows down surface winds, which in turn
slow down winds aloft at 60 ◦ N, weakening the vortex
there.

1% Cl (g), ClO(g) ClONO 2 (g)

HCl(g)

37%
ClONO 2 (g)

62%
HCl(g)

Cl(g), ClO(g)

11.9. Effects of Enhanced UV-B Radiation
on Life and Ecosystems
In the absence of the stratospheric ozone layer, most
UV-C radiation incident at the TOA would penetrate to
the surface of the Earth, destroying bacteria, protozoa,
algae, fungi, plants, and animals in a short time. For-
tunately, the ozone layer exists and absorbs almost all
UV-C radiation. Ozone also absorbs most UV-B radia-
tion, but some of this radiation penetrates to the surface.
UV-B radiation affects human and animal health, ter-
restrial ecosystems, aquatic ecosystems, biogeochem-
ical cycles, air quality, and materials (United Nations
Environmental Program, 1998).

Before PSC and photolysis
reactions

After PSC and photolysis
reactions

Figure 11.18. Pie chart showing conversion of
chlorine reservoirs to active chlorine. During chlorine
activation on polar stratospheric cloud surfaces,
HCl(g) and ClONO 2 (g) are converted to potentially
active forms of chlorine that are broken down by
sunlight in springtime to form Cl(g). Cl(g) forms
ClO(g), both of which react catalytically to destroy
ozone.

advects into the region. Ozone also regenerates chem-
ically, as in Figure 11.14, and chlorine reservoirs of
ClONO 2 (g) and HCl(g) reestablish themselves. Thus,
the Antarctic ozone hole is an annual, regional phe-
nomenon that is controlled primarily by the temperature
of the polar stratosphere and the presence of chlorine
and bromine.
The ozone dent over the Arctic is not nearly so large
or regular as the ozone hole over the Antarctic because
stratospheric temperatures over the Arctic do not drop
so low as they do over the Antarctic. Because tempera-
tures are higher in the Arctic stratosphere, PSC forma-
tion and subsequent chlorine activation over the Arctic
are less widespread than over the Antarctic.
Part of the reason that stratospheric temperatures are
higher over the Arctic than the Antarctic is that the Arc-
tic surface is an ocean covered with a thin layer of sea
ice, whereas the Antarctic is a mountainous continent
covered with thick ice sheets. Because the ocean tem-
perature below the Arctic sea ice cannot drop below
the freezing point of water, and because energy from
the ocean is conducted through the thin sea ice to its
surface, sea ice surface temperatures do not decrease
so much as do ice sheet surface temperatures over the
Antarctic. Because thermal-IR energy fluxes are pro-
portional to temperature to the fourth power (Equation
2.2), more energy is radiated from the Arctic surface to
the stratosphere than from the Antarctic surface to the
stratosphere.
The other reason Arctic stratospheric temperatures
are higher than Antarctic stratospheric temperatures

11.9.1. Effects on Humans
Increases in UV-B radiation affect the skin, eyes, and
immune system of humans. The layers of skin affected
by UV-B are the epidermis (the outer, nonvascular, pro-
tective layer of skin that covers the dermis) and the
stratum corneum (the top, horny layer of the epidermis,
made mainly of peeling or dead cells). The layers of the
eyeaffected by UV-B are the cornea, the iris, and the
lens. In the immune system, the Langerhans cells, which
migrate through the epidermis, are most susceptible to
damage.

11.9.1.1. Effects on Skin
The severity of effects of UV-B radiation on skin
depends on skin pigmentation. During the evolution
of Homo sapiens sapiens (the variant of Homo sapiens
to which every human belongs), humans who lived near
equatorial Africa developed a dark pigment, melanin ,
in their skin to protect their skin against harmful UV
radiation.
As humans migrated to higher latitudes, where sun-
light was less intense, dark pigmentation prevented
what little sunlight was available from catalyzing essen-
tial chemical reactions in the skin to produce vitamin D.
Vitamin D is important because it inhibits the softening
of bones (rickets), bone fracturing, and bow-leggedness
in humans. Thus, the skin of people who migrated