Greenhouse gases are gases that absorb energy emitted from the earth and radiate it back into the atmosphere. If there are too many greenhouse gases, the earth could become too warm. If greenhouse gases dramatically decrease, the earth may be too cool for human activities, such as farming, planting, and harvesting, to occur.
Why do I care? A certain amount of greenhouse gases is essential to life on earth. However, human activities are affecting the levels of these gases in the atmosphere, which are in turn affecting the climate we have adapted to.
Greenhouse gases are the gases that absorb long-wave energy and emit it back into our atmosphere. They are responsible for keeping the earth warm enough to live on. Most of these gases are present in the atmosphere naturally. However, anthropogenic (manmade) processes are dramatically increasing the concentration of these gases. This is one of the main reasons we think our earth is experiencing warming and climate changes.
Concentrations of greenhouse gases are commonly given in percentages as well as mixing ratios of gases to total air volume, such as ppt, ppb, and ppm. The percentages are the percentage of the atmosphere made up by these gases. Since the percentages are very small, most of the time concentrations are referred to in parts per trillion (ppt), parts per billion (ppb) or parts per million (ppm). This means that for every trillion, billion or million molecules, the number given is the amount of molecules that are composed of that gas per trillion air molecules, etc. For example, as of 2009, our atmosphere currently contains a CO2 concentration of 385ppm; for every million molecules, about 385 of them are carbon dioxide.
In the linked pages, you should also keep in mind that a fossil fuel is any hydrogen and carbon rich substance that was created by the decomposition of prehistoric plants and animals that can be burned to produce heat or energy. This includes coal, petroleum, and natural gas products.
The concentration of greenhouse gases can affect the temperature of the Earth's surface by trapping energy near the ground. If they increase, temperatures could also increase and that would affect productivity of crops and other vegetation. If there is plenty of water, an increase in temperatures could actually increase crop production and lengthen the growing season for many plants. The increase in carbon dioxide could also act like a fertilizer and increase plant growth by providing more of a basic building block for plant metabolism. However, the warming of the earth is expected to change the water balance in many regions. This could make the crops more succeptible to droughts and water stress and limit their ability to grow.