243
Stratospheric Ozone and Corals
Early in the history of the Earth, more than 2.5 billion years ago, before photosynthesis by Cyanobacteria (i.e., blue-green algae) became prevalent, the atmosphere
and ocean waters contained no free oxygen (O 2 ). At that time, there was no stratospheric ozone (O 3 ) layer protecting the Earth from damaging UV radiation coming
from the Sun. Even today, some species of Cyanobacteria thrive in conditions of
elevated UV radiation, such as on snow on high mountains during summer. However,
the very Cyanobacteria that could live under high UV radiation, by capturing visible
light and using that energy to split the oxygen atom from the hydrogen atoms that
make up water (i.e., H 2 O), dramatically changed the atmosphere and oceans.
Photosynthesis combines the H atoms in water with carbon dioxide (CO 2 ) molecules to produce simple sugars (e.g., glucose, C 6 H 12 O 6 ), and produces O 2 as a
“waste” product. Over the next two billion years of Earth history, the Cyanobacteria
globally produced sufficient O 2 to enable multicellular life to evolve and thrive,
because there was O 2 for respiration, and also because O 2 in the stratosphere
absorbed UV-B radiation from sunlight that would have been too damaging for such
life to populate shallow water and ultimately the land.
When high-energy UV-B radiation enters the stratosphere, some of it is absorbed
by O 2 molecules. The energy gained splits the O 2 molecule into two O atoms, which
quickly unite with other O 2 molecules to produce ozone molecules (O 3 ). Ozone
molecules, in turn, are even more effective at absorbing UV-B radiation, causing
them to break apart, producing more free oxygen ions (O
−
), which again unite with
O 2 molecules to produce more ozone molecules. This ozone-oxygen cycle within
the stratosphere absorbs about 99% of the UV-C, 90% of the UV-B, and 50% of the
UV-A in sunlight reaching the Earth’s atmosphere.
Unfortunately, the stratospheric ozone layer is susceptible to depletion by both
natural processes and human activities. Certain man-made chemicals, which are
collectively called Ozone Depleting Substances (the most well-known are chlorofluorocarbons, also called CFCs), have caused depletion of stratospheric ozone
since the 1950s, most dramatically since the 1980s (Fig. 14.9). According to the
Scientific Assessment of Ozone Depletion: 2018, the stratospheric ozone layer
declined globally by more than 2% between 1980 and 1997, with no significant
recovery trend since 1997. That report also noted that the additional energy reaching
the Earth’s surface as a consequence of ozone depletion had increased temperatures
by approximately 0.2°C by 2016.
Volcanic eruptions that inject massive amounts of chlorine and bromine into the
stratosphere also cause stratospheric ozone depletion. The Scientific Assessment of
Ozone Depletion: 2018 noted that major low-latitude volcanic eruptions appear to
be the most important influence on stratospheric ozone over the tropics. For example, stratospheric ozone depletion following the May–June 1982 eruptions of El
Chichon in Mexico was estimated as 3–4%; following the November 1985 eruption
of Nevada del Ruiz in Colombia as about 2%, and following the Mt. Pinatubo eruption in the Philippines in May–June 1991 as about 5%. Overall, the steep decline in
14 What Is Happening to the World’s Coral Reefs?
Stratospheric Ozone and Corals
Early in the history of the Earth, more than 2.5 billion years ago, before photosynthesis by Cyanobacteria (i.e., blue-green algae) became prevalent, the atmosphere
and ocean waters contained no free oxygen (O 2 ). At that time, there was no stratospheric ozone (O 3 ) layer protecting the Earth from damaging UV radiation coming
from the Sun. Even today, some species of Cyanobacteria thrive in conditions of
elevated UV radiation, such as on snow on high mountains during summer. However,
the very Cyanobacteria that could live under high UV radiation, by capturing visible
light and using that energy to split the oxygen atom from the hydrogen atoms that
make up water (i.e., H 2 O), dramatically changed the atmosphere and oceans.
Photosynthesis combines the H atoms in water with carbon dioxide (CO 2 ) molecules to produce simple sugars (e.g., glucose, C 6 H 12 O 6 ), and produces O 2 as a
“waste” product. Over the next two billion years of Earth history, the Cyanobacteria
globally produced sufficient O 2 to enable multicellular life to evolve and thrive,
because there was O 2 for respiration, and also because O 2 in the stratosphere
absorbed UV-B radiation from sunlight that would have been too damaging for such
life to populate shallow water and ultimately the land.
When high-energy UV-B radiation enters the stratosphere, some of it is absorbed
by O 2 molecules. The energy gained splits the O 2 molecule into two O atoms, which
quickly unite with other O 2 molecules to produce ozone molecules (O 3 ). Ozone
molecules, in turn, are even more effective at absorbing UV-B radiation, causing
them to break apart, producing more free oxygen ions (O
−
), which again unite with
O 2 molecules to produce more ozone molecules. This ozone-oxygen cycle within
the stratosphere absorbs about 99% of the UV-C, 90% of the UV-B, and 50% of the
UV-A in sunlight reaching the Earth’s atmosphere.
Unfortunately, the stratospheric ozone layer is susceptible to depletion by both
natural processes and human activities. Certain man-made chemicals, which are
collectively called Ozone Depleting Substances (the most well-known are chlorofluorocarbons, also called CFCs), have caused depletion of stratospheric ozone
since the 1950s, most dramatically since the 1980s (Fig. 14.9). According to the
Scientific Assessment of Ozone Depletion: 2018, the stratospheric ozone layer
declined globally by more than 2% between 1980 and 1997, with no significant
recovery trend since 1997. That report also noted that the additional energy reaching
the Earth’s surface as a consequence of ozone depletion had increased temperatures
by approximately 0.2°C by 2016.
Volcanic eruptions that inject massive amounts of chlorine and bromine into the
stratosphere also cause stratospheric ozone depletion. The Scientific Assessment of
Ozone Depletion: 2018 noted that major low-latitude volcanic eruptions appear to
be the most important influence on stratospheric ozone over the tropics. For example, stratospheric ozone depletion following the May–June 1982 eruptions of El
Chichon in Mexico was estimated as 3–4%; following the November 1985 eruption
of Nevada del Ruiz in Colombia as about 2%, and following the Mt. Pinatubo eruption in the Philippines in May–June 1991 as about 5%. Overall, the steep decline in
14 What Is Happening to the World’s Coral Reefs?