Global warming refers to an increase in the average temperature of the earth's atmosphere. Many believe, based on substantial evidence, that global warming is occurring because of human activity. The burning of fossil fuels, a non-renewable source of energy, is leading to the intensification of greenhouse gases. When fossil fuels are burned, greenhouse gases such as carbon dioxide are released into the atmosphere. The fossil fuels we burn to run our cars, trucks, factories, planes and power plants add to the natural supply of greenhouse gases. The natural supplies of greenhouse gases keeps the planet inhabitable by preventing the earth from being a frozen "wasteland". "Greenhouse gases—which can stay in the atmosphere for at least fifty years—are building up beyond the Earth's capacity to remove the gases and, in effect, creating an extra-thick heat blanket around the Earth. " source Many scientists believe that such a build  up will lead to climate changes that will alter the world as we know it. There is speculation that global warming will lead to further melting of the glaciers, causing the sea level to rise. The rising of the seas could dramatically effect the coastal zones by causing severe flooding, thus putting large populations at risk. Some parts of the world could become a desert, as other deserts expand even further. Some people are skeptical of these possible scenarios for various reasons. If the hypothesis is true, it would require changes in the way we live. Companies that rely on the sale and consumption of fossil fuels would have a lot to lose if people were to reduce their use of these resources in order to slow global warming. Other skeptics might be those who simply don't want to change their way of living.