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THANK GOODNESS FOR
THE GREENHOUSE EFFECT
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An interstellar space traveller would notice that the exterior temperature is about 270°C. Closer to Earth, it would rise by about
30 degrees thanks to the heat released by the planet’s radioactivity. The Sun’s rays would cause the thermometer to climb to 18°C.
At that temperature, however, the traveller would discover an Earth without life, completely frozen over. Fortunately, by keeping
heat in around the Earth, the greenhouse effect ensures an average temperature of +15°C.
What is the greenhouse effect?
The Main Greenhouse Gases
The greenhouse effect is a natural heat capture phenomenon
involving solar radiation and certain gases that make up the
atmosphere. The Sun emits radiations of various wavelengths. The
shortest are gamma rays and x-rays. Potentially dangerous, they
are absorbed by the ionosphere, which is the upper layer of the
atmosphere. Longer wavelengths, i.e. ultraviolet rays, are absorbed
by the stratospheric ozone layer, causing the temperature to rise
in this zone. The ozone layer protects living beings from the harmful effects of ultraviolet rays. When the Sun’s rays reach the Earth,
a third of them are reflected by clouds and aerosols. The rest of the
solar energy is absorbed by the Earth’s surface which, bathed in
sunlight, re-emits heat towards space in the form of long-wave
infrared rays. This is where greenhouse gases (GHGs) come into
play. They act like the glass walls of a greenhouse, letting light in
and retaining heat. The greenhouse effect is therefore a heat trap
that allows the globe to maintain a viable average temperature, or,
more specifically, allows water to remain in liquid form.
Greenhouse gases include all gases that, when present in the
atmosphere, absorb infrared radiation and redirect a portion of it
towards the Earth. These GHGs are composed of molecules of three
atoms and more, which excludes nitrogen (N2), oxygen (O2) and
hydrogen (H). Water vapour (H2O) is the most abundant greenhouse gas. It is a major warming factor locally—with desert nights
being particularly cold due to the lack of cloud cover—as well as
globally, since it represents 65% of the natural greenhouse effect
caused by the planet’s climate system. Carbon dioxide (CO2), with
a relatively low absorption power, is, on a global scale, the GHG that
plays the biggest role in amplifying the man-made greenhouse
effect. It is released mainly by volcanoes, the respiration of living
beings and the burning of fuel and other materials. Although less
present in the atmosphere than CO2, methane (CH4) has a global
warming potential that is 25 times greater than that of CO2. Produced from the decomposition of certain materials in the absence
of oxygen, it is naturally released by wetlands, rice paddies, animals’
digestion, as well as the melting of permafrost. Its increasing presence in the atmosphere is a major cause for concern to many
scientists. The ozone, particularly the tropospheric ozone formed
near the ground by photochemical reactions between nitrogen
oxides (NOx) and volatile organic compounds (VOCs), also contributes to the greenhouse effect. The same can be said for nitrous
oxide (N2O), naturally produced by the ground and the oceans.
Although not very abundant, they still have an absorption capacity
almost 300 times higher than that of CO2 and can remain in the
atmosphere for over a century.
The Albedo Effect
The term albedo comes from the Latin word for whiteness, and
the albedo effect is the capacity of a given material to absorb or
reflect light. A movie theatre screen is white because this is the
colour that reflects the most light; the albedo effect is therefore
at its maximum. In contrast, black material, such as a freshly
asphalted road, has a very low albedo, around 5 to 10%, absorbing heat and reflecting very little light. The clouds, whose albedo
can vary from 80% when they are thick to less than 50% when
they are thin, reflect light very efficiently. Snow, whose albedo
can vary between 80 and 95%, also reflects a considerable
amount of light. Sand (20-30%), grass (20-25%), a forest (5-10%)
or any other substance therefore contributes to the climatic
balance of the planet by reflecting or absorbing solar radiation. The
Earth’s overall albedo is an average of 30%. James Lovelock, who
proposed the Gaia hypothesis, even invented an imaginary place
called Daisyworld, inhabited by black and white daisies. When the
temperature in Daisyworld rises, the white daisies multiply,
reflecting more heat. However, when the temperature drops, the
black daisies proliferate. More recently, Peruvian scientist Eduardo
Gold has suggested that we “paint” the brown parts of the
glaciers white to stop the rocks and soil, which absorb more heat,
from accelerating the melting phenomenon.
Since the 19th-century Industrial Revolution, human activities have
contributed to a substantial increase in CO2, CH4 and N2O in the atmosphere. They have also released synthetic gases that do not exist
in nature, such as chlorofluorocarbons (CFCs). These GHGs, which are
banned today because of their negative impact on the stratospheric
ozone, have a global warming potential up to 10,720 times higher
than that of CO2 and a life expectancy of 45 to 1,700 years. It is
important to note that each new GHG that is created and released
into the atmosphere, and there are many, captures a specific spectrum of infrared radiation (or heat) reflected by the Earth’s surface.
By multiplying the number of GHGs, we eliminate as many pathways that allow the atmosphere to evacuate excess heat. The proliferation of new gases is at least as worrisome as the accumulation
of any one gas, for example, carbon dioxide.
Based on educational material from the Des idées dans l’airž! (DIDA) kit available from the Centrale des syndicats du Québec (CSQ) http://eav.csq.qc.net/dida/