The
Greenhouse Effect
To understand global warming we have to begin with a few ideas about radiation. The word "radiation" is loaded with negative connotations: we have to be careful about getting too many X-rays from medical examinations. Too much sun exposure (solar radiation) can damage the skin and cause cancer.
But any object emits some kind of radiation to a degree that depends in part on how hot it is. What we see when we observe an object is the light emitted (radiated) from the object that the retinas of our eyes process into an image (a picture) of the object. What we see, and what we call “light,” is called electromagnetic radiation.
Different colors correspond to different wavelengths of electromagnetic radiation, just as different keys on a piano produce sounds of different wavelengths.
Different colors correspond to different wavelengths of electromagnetic radiation, just as different keys on a piano produce sounds of different wavelengths.
What we feel as heat from
the Sun is actually solar radiation. Some is visible, but some is in the infrared or ultraviolet range of wavelengths, which cannot be detected by the human eye.
When solar radiation hits the outer atmosphere of the earth, some portion of this energy is reflected back into space by the earth's atmosphere. Another portion is dispersed and scattered by the molecules in the atmosphere and this reduces how much gets through to the surface.
But a large portion of solar radiation passes through the earth's atmosphere to reach the earth's surface. The radiation reaching the earth's surface is largely absorbed resulting in surface warming.
When solar radiation hits the outer atmosphere of the earth, some portion of this energy is reflected back into space by the earth's atmosphere. Another portion is dispersed and scattered by the molecules in the atmosphere and this reduces how much gets through to the surface.
But a large portion of solar radiation passes through the earth's atmosphere to reach the earth's surface. The radiation reaching the earth's surface is largely absorbed resulting in surface warming.
The roof of your car absorbs solar energy and gets hot to a degree that depends on the material it is made of, and upon its color. Since it is hot, it radiates energy.
This re-radiated solar energy is now in longer (infrared) wavelengths because as hot as the roof might be, it is a lot cooler than the sun!
As this radiation leaves the surfaces of the earth (whether the roof of your car or the vast surface of the oceans), it once again interacts with the atmosphere.
Some of this re-radiated energy escapes to space, but much of this re-radiated energy is absorbed by molecules in the earth's atmosphere and re-radiated back to the earth's surface.
This re-radiated solar energy is now in longer (infrared) wavelengths because as hot as the roof might be, it is a lot cooler than the sun!
As this radiation leaves the surfaces of the earth (whether the roof of your car or the vast surface of the oceans), it once again interacts with the atmosphere.
Some of this re-radiated energy escapes to space, but much of this re-radiated energy is absorbed by molecules in the earth's atmosphere and re-radiated back to the earth's surface.
On a smaller scale, this is similar to the warming that occurs inside your car parked in the sun. In the case of the car, the window glass passes solar radiation into the car easily (because glass is very transparent to sunlight), but the re-radiated energy does not pass out easily through the glass and so is “trapped” (absorbed) in the car.
The glass windows behave toward the car (in controlling how much the interior of the car heats up) in the same way that the atmosphere behaves toward the earth, controlling how much the earth heats up.
In the
atmosphere, the molecules responsible for retarding the escape of the re-radiated energy (which is called “radiation trapping”) are called greenhouse gases, and the major ones are water (H2O), nitrous oxide (N2O),
methane (CH4), and carbon dioxide (CO2).
These greenhouse gases act like the glass in a greenhouse, letting sunlight in but trapping re-radiated energy inside.
These greenhouse gases act like the glass in a greenhouse, letting sunlight in but trapping re-radiated energy inside.
Without these
gases most life on earth would not be possible, because the surface temperature of
the earth would likely be about 60°F colder.
So the greenhouse gases actually make the Earth hospitable to life.
But too much greenhouse gas will cause the Earth's temperature to rise to a degree that has harmful consequences. We have no problem living in places where the weather changes by 60°F from one season to another. Why would life be impossible if the earth’s surface were 60°F colder?
Suppose the earth’s surface were only 20°F colder, or 20°F warmer; would life be possible? Scientific evidence leads to predictions that if the earth’s surface temperature were to rise globally by only a few degrees, over a sustained period of time, the impact on life would be immense and in some places catastrophic!
So the greenhouse gases actually make the Earth hospitable to life.
But too much greenhouse gas will cause the Earth's temperature to rise to a degree that has harmful consequences. We have no problem living in places where the weather changes by 60°F from one season to another. Why would life be impossible if the earth’s surface were 60°F colder?
Suppose the earth’s surface were only 20°F colder, or 20°F warmer; would life be possible? Scientific evidence leads to predictions that if the earth’s surface temperature were to rise globally by only a few degrees, over a sustained period of time, the impact on life would be immense and in some places catastrophic!
In essence,
greenhouse gases act like an insulator or blanket above the earth, keeping the
heat in.
Increasing the concentration of these gases in the atmosphere increases the degree to which the atmosphere blocks the escape of infrared radiation.
Over several hundred thousand years, up to the present time, the level of greenhouse gases in the atmosphere has made life on earth sustainable.
But too great a concentration of any of the greenhouse gases can have dramatic effects on climate and significant repercussions upon the world around us.
Increasing the concentration of these gases in the atmosphere increases the degree to which the atmosphere blocks the escape of infrared radiation.
Over several hundred thousand years, up to the present time, the level of greenhouse gases in the atmosphere has made life on earth sustainable.
But too great a concentration of any of the greenhouse gases can have dramatic effects on climate and significant repercussions upon the world around us.
The
Energy Budget
Put a pot
of water on the stovetop and turn on the heat. The water gets hot, of course,
because of the heat input through the bottom of the pot.
But there are heat outputs (losses) from the pot that retard the rate at which the water heats up. For example there is radiation of heat away from the hot sides of the pot.
After a while, steam begins to rise from the surface of the water, and the escape of the steam also carries heat away. Put a lid on the pot and retard the heat loss due to steaming.
The rate of warming of the water in the pot is a consequence of the “heat budget” for the process—the imbalance between heat input and output.
The earth also has a heat budget.
But there are heat outputs (losses) from the pot that retard the rate at which the water heats up. For example there is radiation of heat away from the hot sides of the pot.
After a while, steam begins to rise from the surface of the water, and the escape of the steam also carries heat away. Put a lid on the pot and retard the heat loss due to steaming.
The rate of warming of the water in the pot is a consequence of the “heat budget” for the process—the imbalance between heat input and output.
The earth also has a heat budget.
Solar
radiation is the external source of the earth’s heat. Some of that input is reflected by clouds, some is absorbed by gases in the
atmosphere, some is absorbed by the earth’s surface and some is reflected back
to the atmosphere by the surface.
Different regions of the earth, and day-to-day weather variations have a strong effect on these absorptions and reflections. Ocean, sand, snow, clouds, all contribute in different ways to the earth’s energy budget.
Different regions of the earth, and day-to-day weather variations have a strong effect on these absorptions and reflections. Ocean, sand, snow, clouds, all contribute in different ways to the earth’s energy budget.
In order to
study climate change it is necessary to make measurements that “average out”
regional and seasonal changes.
Although the statistical procedures used to create these averages leave room for critique of the statistical methodologies, as well as of the methods of measuring temperatures of the land and the sea, the worldwide science community has produced widely accepted methods of making these measurements.
This is the basis for exploring the question of whether the earth’s temperature is changing in a global sense, or in a particular region, and whether this is due to man-made changes in the environment that must be reversed.
Although the statistical procedures used to create these averages leave room for critique of the statistical methodologies, as well as of the methods of measuring temperatures of the land and the sea, the worldwide science community has produced widely accepted methods of making these measurements.
This is the basis for exploring the question of whether the earth’s temperature is changing in a global sense, or in a particular region, and whether this is due to man-made changes in the environment that must be reversed.
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