Global warming by definition is an increase in the average temperature of the earth's surface. Since the late 1800's, the average temperature has increased about 0.5 to 1.5 Fahrenheit degrees (0.3 to 0.8 Celsius degree). Scientists disagree about the causes of this warming trend. Some say that the warming trend is a natural occurrence; they say that natural process could cause the rise in temperature. Changes in the path of revolution of the Earth around the sun could cause global warming. Orbital variation changes how sunlight is distributed on the surface of the planet. But, evidence suggests that human activities have caused this unusual change in climate. .
The greenhouse effect is the major cause. Energy from the sun heats the earth's surface and in turn heat energy is radiated back into space. Greenhouse gasses trap some of this outgoing energy to retain heat and heat the planet. Without this natural process, the global temperature would be much lower than it is now. The problem is that these valuable and necessary greenhouse gases build up in the atmosphere and cause more and more heat to be trapped. This causes a long term warming of the planet. When excess amounts of carbon dioxide, methane, and nitrous oxide (the greenhouse gasses) are released into the atmosphere, the greenhouse effect is increased. These gasses trap more and more heat and raise the global temperature. .
Emissions of carbon dioxide or CO2 are increasing the greenhouse effect. Natural conditions can release large amounts of CO2 and cause these fluctuations. Forrest fires and volcanic activity produce large amounts of CO2. Global warming is caused primarily by human activities most importantly - the burning of fossil fuels. When coal gas and oil are burned, they release CO2 in the atmosphere. This coincides with industrial development. Deforestation and the clearing of land reduce the amount of carbon dioxide that trees and other vegetation remove from the atmosphere in the process of photosynthesis.