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Sunday, November 6, 2011

The Greenhouse Effect

The greenhouse effect is an increase in the temperature of a planet as heat energy from sunlight is trapped by the gaseous atmosphere. Excess carbon dioxide and water vapor increase this global warming effect.

Solar Energy (sunlight) is short-wavelength radiation which easily penetrates the Earth’s atmosphere and warms the Earth; only about one quarter of incoming sunlight is reflected by the atmosphere. The warmed Earth emits long-wavelength radiation (infrared waves or heat energy) back into space; these longer waves are mostly reflected back to Earth by the atmosphere.

The size of the atmosphere in the illustration above is greatly exaggerated in order to show the greenhouse effect. Most of the Earth’s atmosphere is within 10 miles (16 km) of the Earth’s surface).

What is the greenhouse effect?

1. What is a greenhouse?

Drawing of greenhouse, with glass walls and roof, plants growing inside. Sunlight coming in through roof, but bounces off inside of roof and cannot escape.
A greenhouse is made of glass. It traps the Sun’s energy inside and keeps the plants warm, even in winter.

A greenhouse is a house made of glass. It has glass walls and a glass roof. People grow tomatoes and flowers and other plants in them. A greenhouse stays warm inside, even during winter. Sunlight shines in and warms the plants and air inside. But the heat is trapped by the glass and can’t escape. So during the daylight hours, it gets warmer and warmer inside a greenhouse, and stays pretty warm at night too.

2. How is Earth like a greenhouse?

Drawing shows Earth, surrounded by atmosphere containing greenhouse gases, and Sun shining through. Labels say 'During the day, when the sun's energy reaches Earth's atmosphere, most of it goes right through. Some bounces off, back into space. At night, most of the Sun's energy escapes back into space. But some is trapped inside the atmosphere by the greenhouse gases, further warming Earth.
Greenhouse effect of Earth’s atmosphere keeps some of the Sun’s energy from escaping back into space at night.

Earth’s atmosphere does the same thing as the greenhouse. Gases in the atmosphere such as carbon dioxide do what the roof of a greenhouse does. During the day, the Sun shines through the atmosphere. Earth’s surface warms up in the sunlight. At night, Earth’s surface cools, releasing the heat back into the air. But some of the heat is trapped by the greenhouse gases in the atmosphere. That’s what keeps our Earth a warm and cozy 59 degrees Fahrenheit, on average.

3. Is it warm in here, or is it just me?

Three images: polar bear cub on snow, tropical rain forest, lakes surrounded by grassy hills.
Every place has its own climate, whether Arctic cold, tropical warm, or something in between. Averaging them all together makes global climate.

You might think 59 degrees Fahrenheit is pretty cold. Or, you might think that’s warm. It depends on what you are used to. That temperature would melt all the Arctic ice. Yes, it’s colder than 59 degrees in a lot of places, and hotter than 59 degrees in a lot of places, but 59 is the average of all of the places.

Drawing shows Earth, surrounded by atmosphere containing too much greenhouse gas, and Sun shining through. But some is trapped inside the atmosphere by the greenhouse gases. The atmosphere is colored yellow and orange to show that it is warming up too much.
If the atmosphere causes too much greenhouse effect, Earth just gets warmer and warmer

The point is, if the greenhouse effect is too strong, Earth gets warmer and warmer. This is what is happening now. Too much carbon dioxide and other greenhouse gases in the air are making the greenhouse effect stronger.

4. Why can’t we just open a window–or plant a tree?

You might wonder, why don’t we just plant more trees? After all, plants take in carbon dioxide and give off oxygen.

Well, that might help a little. But, instead of planting more forests, some people are cutting them down and burning them to make more farm land to feed the growing human population.

Photo of a burning forest.
A forest burns. (Photograph copyright Woods Hole Research Center).

Photo of white, sick-looking corals under water.
These coral are sick. They should be colorful, not bleached out.
The ocean also absorbs a lot, but not all, of the excess carbon dioxide in the air. Unfortunately, the increased carbon dioxide in the ocean changes the water, making it more like acid. Ocean creatures don’t like acidic water. The bleached out, unhealthy coral in this picture is just one example of what acidic water can do.

5. Don’t clouds keep Earth cooler?

Water in the atmosphere also acts as a greenhouse gas. The atmosphere contains a lot of water. This water can be in the form of a gas–water vapor–or in the form of a liquid–clouds. Clouds are water vapor that has cooled and condensed back into tiny droplets of liquid water.

Clouds as seen from space.
Earth’s clouds as seen from space.

Water in the clouds holds in some of the heat from Earth’s surface. But the bright white tops of clouds also reflect some of the sunlight back to space. So with clouds, some energy from the Sun never even reaches Earth’s surface.

Cloud effects on Earth's radiation. Diagram shows how clouds reflect some of the Sun's energy back to space.
Clouds prevent some of the Sun’s energy from ever reaching Earth’s surface.

6. Or do clouds make Earth warmer?

Here is a riddle:

As the ocean warms up, more water evaporates into the air. So does more water vapor then mean more warming? And does more warming mean more water vapor? And ‘round and ‘round we go?

Drawing shows how clouds have a greenhouse effect at night. Clouds trap some of the heat coming from Earth's surface, preventing it from escaping into space.
At night, clouds trap some of the heat from Earth’s surface. Thus, it does not escape back into space.

Or, since more water vapor means more clouds, will the fluffy white clouds reflect enough sunlight back into space to make up for the warming?

Drawing shows arrows that represent sun's energy reflecting off tops of clouds.
During the day, clouds reflect the Sun’s energy back to space, before it has a chance to heat Earth’s surface.

This cloud riddle has scientists scratching their heads and trying to figure it out. NASA is helping with satellites like Aqua and CloudSat, which study the Earth’s water cycle and clouds in 3-D.

Image taken by a satellite of a hurricane. Below is a cross-section of the storm clouds, colored to show how much water is contained in the clouds at different heights.
The top image is a hurricane, as seen by a satellite. Below is a cross-section of the storm clouds. This colorful image was made with data from the CloudSat satellite. It shows with different colors how much water is contained in the clouds at different heights.

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