August 27, 2009-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.

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.

Co-authors of the study: left to
right: John Daniel, Ph.D., atmospheric physicist; A.R. Ravishankara, Ph.D.,
Director of the Chemical Sciences Division; and Robert Portmann, Ph.D.,
atmospheric physicist. (Credit: NOAA)

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

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