Electromagnetic radiation can be absorbed by materials. These materials may then emit the absorbed energy in other ways, such as through the release of heat (infrared radiation). This section is devoted to three important wavelengths of radiation - microwave, infrared, and ultraviolet.

Microwave Radiation:

Most of us have microwave ovens in our kitchens, but how many of us actually know how it works? It all comes down to absorption of electromagnetic radiation by none other than water!

Follow this link to see how a microwave oven works.

Infrared (IR) radiation:

Our bodies have nerve endings that are sensitive to IR radiation - in other words, we can sense heat or cold.

Heat energy is a major component of chemical reactions, especially those dealing with combustion. Some reactions require heat (known as an endothermic reaction), and others release heat (exothermic reactions).

When an object such as a bar of iron is placed in a fire, or set out in the hot sunlight, it will absorb infrared radiation. This causes its temperature to rise; eventually it will cool (emit IR radiation) when removed from the source of heat.

Heat energy can also be created by the force of friction. When a meteor enters the Earth's atmosphere, it will vaporize (we're talking about vaporizing metallic rock here!) due to the amount of friction it encounters with the air. Have you ever experienced "wind burn" on a blustery day?

Some gases in the atmosphere are able to absorb infrared radiation. Those that are good at it are called greenhouse gases. The most common greenhouse gases are carbon dioxide, water vapor, and methane.

As solar radiation (recall that it is composed of a wide range of wavelengths, including UV) strikes the surface of the Earth, the ground, water, pavement, etc. will heat up. At night, this heat is emitted in the form of IR radiation. Greenhouse gases will re-absorb this radiation, not allowing some of it to escape into space. Over time, this causes a slight warming effect on the atmosphere.

external image Greenhouse_Effect.png

Here is a chart depicting some gases and their absorption spectra.

external image Atmospheric_Transmission.png

Water vapor is the most potent greenhouse gas in Earth's atmosphere, and the most abundant. However, water vapor has a unique property that none of these other gases has - it can stick together. As humidity (water vapor in the air) increases, it has a tendency to condense into the liquid phase, creating clouds, fog, rain, or ice particles. This phase change allows the amount of water in the air to be regulated by nature, so no runaway greenhouse effect can occur because of water vapor.

The other gases are not so lucky. Carbon dioxide will only 'stick together' at or below -78 degrees C under normal pressure conditions, and methane does so at -182 degrees C. These temperatures are not attainable under natural conditions on the Earth's surface. Therefore, if the atmospheric concentration of these gases increases, they will end up absorbing more IR radiation, and will cause the Earth's atmosphere to warm (over extended periods of time). This is called the "greenhouse effect" or "global warming" effect.

Here is a chart of atmospheric greenhouse gas levels measured over the past few decades.
external image Major_greenhouse_gas_trends.png

And here is a look at carbon dioxide levels over the past few centuries and thousands of years.
external image Carbon_Dioxide_400kyr_Rev.png

The amount of carbon dioxide has varied greatly over the course of Earth's history. Its concentration is tied in with the carbon cycle and with volcano activity. To understand its role in today's climate, one must first look at the climate of the past. Here is a plot showing the global temperature changes that have occurred over the past 450,000 years.
external image Ice_Age_Temperature_Rev.png
In the above chart, the blue and green (top two) plots are from Antarctic ice core samples; the bottom is taken from sea floor sediment data. Note the correlation; this indicates that the local climate conditions in Antarctica can be safely compared to global conditions.

Also note the following:
  • Global climate has fluctuated between short, warm periods and longer-lived cool periods.
  • The cool periods are considered ice ages. The Earth came out of its last ice age about 12,000 years ago (right side of plot).
  • Compare the previous plot of carbon dioxide concentration to the temperatures in the plot above. There is correlation, but also lag times between changes in each variable in some cases.

Here is another look at the past 18,000 years:

Note the correlation between changes in carbon dioxide (red) and temperature anomaly.

Now examine the current warm period we are experiencing (called the Holocene era):
external image Holocene_Temperature_Variations_Rev.png
Note the black curve (average of all measurement systems - the colored lines) and how it rises dramatically from the end of the last ice age. From there, it generally decreases in a cooling trend. The rise and fall of temperature anomaly between 2,000 years ago and 2004 correspond to the Medieval Warm Period and the subsequent Little Ice Age.

Next, examine the temperature anomaly trend over the past 1,000 years:
external image 1000_Year_Temperature_Comparison.png

All of the colored lines are different measurement techniques. The line that garners the most attention is the black one beginning around the 1850s. Zooming in on the last 200 years reveals:

external image Instrumental_Temperature_Record.png

And finally, the past 25 years:
external image Short_Instrumental_Temperature_Record.png
The oscillations in temperature anomaly can be attributed to a volcanic eruption (1991 Mount Pinatubo) and the El Nino/La Nina ocean current events. In general, however, the atmosphere is warming, rapidly so in comparison to the past, and in light of the fact that the Earth seems to have been in a cooling trend ever since it entered the Holocene.

Anthropogenic Carbon Dioxide

The term "anthropogenic" means human-caused or human-induced. Therefore anthropogenic global warming (AGW for short) means human-caused global warming. The most apparent case for AGW is the dramatic increase in atmospheric carbon dioxide that began in the Industrial Era and has continued to grow rapidly, even accelerate, in modern times. See the first chart on greenhouse gases above, and the following chart below. These changes in carbon dioxide levels are mostly to blame on fossil fuel consumption.

external image Carbon_History_and_Flux_Rev.png
external image Global_Carbon_Emission_by_Type.png

Climate Change

Earth's climate system is extremely complex, and has many potential 'game changers' called forcings. Changes in solar output, orbital variations, planetary axial tilt, plate tectonics, ocean currents, atmospheric composition, volcanic activity, vegetation, etc. all play a role in our planet's climate.

Some evidence pointing towards recent climate changes include melting polar ice caps and retreating glaciers, habitat and migration pattern changes for birds and fish, extensive droughts in Australia, excessive rains in India, rising sea levels, etc., all of which have been observed to some extent.

Because carbon dioxide is a greenhouse gas, and human activity has caused such a dramatic change in its atmospheric concentration, there is concern that AGW will cause climate changes. The extent, severity, and duration of these changes is difficult to predict at best. The question becomes one of principle: do we act now to prevent or minimize climate change despite the enormous cost it will incur to society? Or do we play the wait-and-see card, and take the risk that climate changes will or will not be severe?

UV Radiation

Go to the ozone page.

Here is an Ozone depletionlink.