The Greenhouse Effect
The Greenhouse Effect:
Global Warming and Climate Change
You have probably noticed that when you leave your car under direct sunlight
on a sunny day, the interior of the car gets much warmer than the air
outside, and you may have wondered why the car acts like a heat trap. This
is because glass at thicknesses encountered in practice transmits over 90
percent of radiation in the visible range and is practically opaque (nontransparent)
to radiation in the longer wavelength infrared regions. Therefore,
glass allows the solar radiation to enter freely but blocks the infrared radiation
emitted by the interior surfaces. This causes a rise in the interior temperature
as a result of the thermal energy buildup in the car. This heating
effect is known as the
greenhouses.
The greenhouse effect is also experienced on a larger scale on earth. The
surface of the earth, which warms up during the day as a result of the
absorption of solar energy, cools down at night by radiating part of its
energy into deep space as infrared radiation. Carbon dioxide (CO
vapor, and trace amounts of some other gases such as methane and nitrogen
oxides act like a blanket and keep the earth warm at night by blocking the
heat radiated from the earth (Fig. 2–66). Therefore, they are called “greenhouse
gases,” with CO
taken out of this list since it comes down as rain or snow as part of the
water cycle and human activities in producing water (such as the burning of
fossil fuels) do not make much difference on its concentration in the atmosphere
(which is mostly due to evaporation from rivers, lakes, oceans, etc.).
CO
CO
The greenhouse effect makes life on earth possible by keeping the earth
warm (about 30°C warmer). However, excessive amounts of these gases disturb
the delicate balance by trapping too much energy, which causes the
average temperature of the earth to rise and the climate at some localities to
change. These undesirable consequences of the greenhouse effect are
referred to as
greenhouse effect, since it is utilized primarily in2), water2 being the primary component. Water vapor is usually2 is different, however, in that people’s activities do make a difference in2 concentration in the atmosphere.global warming or global climate change.The global climate change is due to the excessive use of fossil fuels such
as coal, petroleum products, and natural gas in electric power generation,
transportation, buildings, and manufacturing, and it has been a concern in
recent decades. In 1995, a total of 6.5 billion tons of carbon was released to
the atmosphere as CO
is about 360 ppm (or 0.36 percent). This is 20 percent higher than the level
a century ago, and it is projected to increase to over 700 ppm by the year
2100. Under normal conditions, vegetation consumes CO
2 and releases O2during the photosynthesis process, and thus keeps the CO
the atmosphere in check. A mature, growing tree consumes about 12 kg of
CO
deforestation and the huge increase in the CO
decades disturbed this balance.
In a 1995 report, the world’s leading climate scientists concluded that the
earth has already warmed about 0.5°C during the last century, and they estimate
that the earth’s temperature will rise another 2°C by the year 2100. A
rise of this magnitude is feared to cause severe changes in weather patterns
with storms and heavy rains and flooding at some parts and drought in others,
major floods due to the melting of ice at the poles, loss of wetlands and
coastal areas due to rising sea levels, variations in water supply, changes in
the ecosystem due to the inability of some animal and plant species to
adjust to the changes, increases in epidemic diseases due to the warmer
temperatures, and adverse side effects on human health and socioeconomic
conditions in some areas.
The seriousness of these threats has moved the United Nations to establish
a committee on climate change. A world summit in 1992 in Rio de
Janeiro, Brazil, attracted world attention to the problem. The agreement prepared
by the committee in 1992 to control greenhouse gas emissions was
signed by 162 nations. In the 1997 meeting in Kyoto (Japan), the world’s
industrialized countries adopted the Kyoto protocol and committed to
reduce their CO
the 1990 levels by 2008 to 2012. This can be done by increasing conservation
efforts and improving conversion efficiencies, while meeting new
energy demands by the use of renewable energy (such as hydroelectric,
solar, wind, and geothermal energy) rather than by fossil fuels.
The United States is the largest contributor of greenhouse gases, with over
5 tons of carbon emissions per person per year. A major source of greenhouse
gas emissions is transportation. Each liter of gasoline burned by a
vehicle produces about 2.5 kg of CO
produces about 20 lbm of CO
about 12,000 miles a year, and it consumes about 600 gallons of gasoline.
Therefore, a car emits about 12,000 lbm of CO
year, which is about four times the weight of a typical car (Fig. 2–67). This
and other emissions can be reduced significantly by buying an energyefficient
car that burns less fuel over the same distance, and by driving sensibly.
Saving fuel also saves money and the environment. For example,
choosing a vehicle that gets 30 rather than 20 miles per gallon will prevent
2 tons of CO
reducing the fuel cost by $400 per year (under average driving conditions of
12,000 miles a year and at a fuel cost of $2.00/gal).
It is clear from these discussions that considerable amounts of pollutants
are emitted as the chemical energy in fossil fuels is converted to thermal,
mechanical, or electrical energy via combustion, and thus power plants,
motor vehicles, and even stoves take the blame for air pollution. In contrast,
no pollution is emitted as electricity is converted to thermal, chemical, or
mechanical energy, and thus electric cars are often touted as “zero emission”
vehicles and their widespread use is seen by some as the ultimate
solution to the air pollution problem. It should be remembered, however,
that the electricity used by the electric cars is generated somewhere else
mostly by burning fuel and thus emitting pollution. Therefore, each time an
electric car consumes 1 kWh of electricity, it bears the responsibility for the
pollutions emitted as 1 kWh of electricity (plus the conversion and transmission
losses) is generated elsewhere. The electric cars can be claimed to be
zero emission vehicles only when the electricity they consume is generated
by emission-free renewable resources such as hydroelectric, solar, wind, and
geothermal energy (Fig. 2–68). Therefore, the use of renewable energy
should be encouraged worldwide, with incentives, as necessary, to make the
earth a better place to live in. The advancements in thermodynamics have
contributed greatly in recent decades to improve conversion efficiencies (in
some cases doubling them) and thus to reduce pollution. As individuals, we
can also help by practicing energy conservation measures and by making
energy efficiency a high priority in our purchases.
2 concentration in2 a year and exhales enough oxygen to support a family of four. However,2 production in recent2 and other greenhouse gas emissions by 5 percent below2 (or, each gallon of gasoline burned2). An average car in the United States is driven2 to the atmosphere a2 from being released to the atmosphere every year while2. The current concentration of CO2 in the atmosphere
