Manual for Refrigeration Servicing Technicians - UNEP - Division of ...

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Environmental Impact
Global warming
The earth’s temperature is maintained by a balance between
heating from solar radiation flowing in from the sun, and cooling
from infrared radiation emitted by the earth’s warm surface and
atmosphere escaping back to into space. The sun is the earth’s only
external source of heat. When solar radiation, in the form of visible
light, reaches the earth, some is absorbed by the atmosphere and
reflected from clouds and land (especially from deserts and snow).
CHAPITRE 1
The remainder is absorbed PAGE 24 by the surface which is heated and in
THE CONCEPT OF GLOBAL WARMING
turn warms the atmosphere. The warm surface and atmosphere
of the earth emit invisible infrared radiation. While the atmosphere
is relatively transparent to solar radiation, infrared radiation is
absorbed in the atmosphere by many less abundant gases. Though
present in small amounts, these trace gases act like a blanket,
preventing much of the infrared radiation from escaping directly
to space. By slowing
SUN
the release of cooling
SUN’S RADIATION
radiation, these gases
warm the earth’s surface.
This process is illustrated
here:
GREENHOUSE GASES
EARTH
TROPOSPHER
INFRARED RADIATION BEING TRAPPED
The concept of global warming
In a greenhouse, glass allows sunlight in but prevents some infrared
radiation from escaping. The gases in the earth’s atmosphere
which exert a similar effect are called “greenhouse gases” (GHGs).
Of the man-made greenhouse gases, the most important are
carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the
halocarbons (CFCs, HCFCs and HFCs).
Different gases absorb and trap varying amounts of infrared
radiation. They also persist in the atmosphere for differing time
periods and influence atmospheric chemistry (especially ozone) in
different ways. For example, a molecule of R12 has about the same
effect on radiation as 16,000 CO2 molecules. A molecule of methane
has approximately 21 times the effect of CO2; but its lifetime is far
shorter. The GWP is an index which compares the warming effect
over time of different gases relative to equal emissions of CO2
(by weight). A table of the ODP and GWP of various refrigerants
is included in Chapter 2. HFCs do not have chlorine, and in this
way, don’t destroy the ozone layer, but they do contribute to global
warming. For this reason, they are in the group of gases controlled
by Kyoto Protocol. These gases are: CO2, CH4, N2O, HFCs,
perfluorocarbons (PFCs) and sulphur hexafluoride (SF6).
Scientific measurements have shown that in the last century, the
Earth’s average near-surface atmospheric temperature rose 0.6 ± 0.2
°C, mostly attributable to human activities increasing the concentration
of CO2 and other greenhouse gases in the atmosphere. Moreover, a
global temperatures increase by between 1.4 and 5.8 °C between 1990
and 2100 has been predicted by the models and disseminated by the
Intergovernmental Panel on Climate Change (IPCC).
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