ESCI B.S3.O1.A3 Greenhouse Effect

As people become more concerned about global warming, we often hear the term "Greenhouse Effect," but what exactly does that mean?

A greenhouse used for growing plants works by capturing the heat that is absorb by the plants and objects inside it. Glass or plastic is used to allow the visible light through to the plants, but once that light is converted to heat (infrared radiation) the glass or plastic will act like an insulating blanket and keep the heat from escaping quickly. Not all of the heat is captured, but by capturing some of it, the temperature inside the greenhouse will rise to a much higher level than the outside air. In fact, greenhouse have to be equipped with vents and even cooling systems to prevent them from getting too hot for the plants inside. Our planet isn't equipped with vents and cooling systems. That is why people are concerned about global warming!

Take a look at this illustration of a greenhouse below:

Our planet's atmosphere works in much the same way that the glass in a greenhouse does. The atmosphere lets the light from the sun through without much interference, but once that energy is radiated back toward space as heat there are certain gases in the atmosphere that that trap that heat and keep it here on Earth. Please understand that the greenhouse effect isn't bad. In fact, without the greenhouse effect our planet would be much colder, around -40^oC (-40^oF), and probably couldn't support very much life. The greenhouse effect provides us with a comfortable living environment just like a greenhouse can allow gardeners to grow tropical plants in areas that would normally be much too cold. However, human activities are contributing to an increase of greenhouse gasses in the atmosphere and that is like closing the vents on a greenhouse and letting it get hotter. If we aren't careful, we might end up with an environment that is too hot!

 Take a look at this diagram to see how the greenhouse effect works in our atmosphere:

According to J. T. Kiehl and Kevin E. Trenberth of the National Center for Atmospheric Research Links to an external site., the major contributors to the greenhouse effect in order of importance are:

Notice that atmospheric nitrogen (N₂) and atmospheric oxygen (O₂) are not on this list. Nitrogen and oxygen make up more than 98% of our planet's atmosphere, but these gasses are not greatly affected by infrared radiation and so are not contributors to the greenhouse effect. The planet Venus has an atmosphere that is almost entirely carbon dioxide. That is one of the reasons it is so hot: over 462^oC (864^oF)!

As you can see from the table, water vapor has an influence on the greenhouse effect that is nearly two and a half times greater than carbon dioxide. And since warm air can hold more water vapor and also causes greater evaporation from the oceans, one of the biggest threats facing our climate is the danger of creating a positive feedback loop where warmer air leads to greater humidity which contributes to higher air temperatures.

Your Assignment: Let's check your understanding. Please answer the following questions.

1. Compare a greenhouse like the one pictured above to the parts of Earth that play a similar role in the greenhouse effect. Include the following parts and describe the role each plays:
        the plants
        the glass structure
        a closed vent
2. Describe a way that continuing uncontrolled carbon dioxide emissions could lead to a positive feedback loop that could cause rapid global warming.
3. Venus is only one fourth closer to the sun than the Earth is and yet its average surface temperature is 31 times hotter than Earth's surface. How can out of control global warming help explain this?
4. What would our planet be like if we didn't have any greenhouse effect?