Nitrous oxide has now become the largest ozone-depleting
substance emitted through human activities, and is expected to remain the
largest throughout the 21st century, NOAA scientists say in a new study.
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Layers of Earth's atmosphere.(Credit: NOAA) |
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For the first time, this study has evaluated nitrous oxide
emissions from human activities in terms of their potential impact on Earth’s
ozone layer. As chlorofluorocarbons (CFCs), which have been phased out by
international agreement, ebb in the atmosphere, nitrous oxide will remain a
significant ozone-destroyer, the study found. Today, nitrous oxide emissions
from human activities are more than twice as high as the next leading
ozone-depleting gas.
Nitrous oxide is emitted from natural sources and as a
byproduct of agricultural fertilization and other industrial processes.
Calculating the effect on the ozone layer now and in the future, NOAA
researchers found that emissions of nitrous oxide from human activities erode
the ozone layer and will continue to do so for many decades.
The study, authored by A.R. Ravishankara, J.S. Daniel and
Robert W. Portmann of NOAA's Earth
System Research Laboratory (ESRL) chemical sciences division, appears
online today in the journal Science. ESRL tracks the thickness of the ozone
layer, as well as the burden of ozone-depleting compounds in the atmosphere. It
maintains a large portion of the world air sampling and measurement network.
NOAA scientists also conduct fundamental studies of the atmosphere and
atmospheric processes to improve understanding of ozone depletion and of the
potential for recovery the ozone layer.
“The dramatic reduction in CFCs over the last 20 years is an
environmental success story. But manmade nitrous oxide is now the elephant in
the room among ozone-depleting substances,” said Ravishankara, lead author of
the study and director of the ESRL Chemical Sciences Division in Boulder, Colo.
The ozone layer serves to shield plants, animals and people
from excessive ultraviolet light from the sun. Thinning of the ozone layer
allows more ultraviolet light to reach the Earth’s surface where it can damage
crops and aquatic life and harm human health.
Though the role of nitrous oxide in ozone depletion has been
known for several decades, the new study is the first to explicitly calculate
that role using the same measures that have been applied to CFCs, halons and
other chlorine- and bromine-containing ozone-depleting substances.
With CFCs and certain other ozone-depleting gases coming in
check as a result of the 1987 Montreal Protocol, the international treaty that
phased out ozone-destroying compounds, manmade nitrous oxide is becoming
an increasingly larger fraction of the emissions of ozone-depleting substances.
Nitrous oxide is not regulated by the Montreal Protocol.
Nitrous oxide is also a greenhouse gas, so reducing its
emission from manmade sources would be good for both the ozone layer and
climate, the scientists said.
In addition to soil fertilization, nitrous oxide is emitted
from livestock manure, sewage treatment, combustion and certain other
industrial processes. Dentists use it as a sedative (so-called “laughing gas”).
In nature, bacteria in soil and the oceans break down nitrogen-containing
compounds, releasing nitrous oxide. About one-third of global nitrous oxide
emissions are from human activities. Nitrous oxide, like CFCs, is stable when
emitted at ground level, but breaks down when it reaches the stratosphere to
form other gases, called nitrogen oxides, that trigger ozone-destroying reactions.
NOAA understands and predicts changes in the Earth's
environment, from the depths of the ocean to the surface of the sun, and
conserves and manages our coastal and marine resources.
Original
article
SOURCE: NOAA
