Global warming is an average increase in the temperature of the atmosphere near the Earth's surface and in the troposphere, which can contribute to changes in global climate patterns. Global warming can occur from a variety of causes, both natural and human induced. In common usage, "global warming" often refers to the warming that can occur as a result of increased emissions of greenhouse gases from human activities.
a speech on global warming =  Global Warming occurs as a result of increased concentration of greenhouse gases in the atmosphere. The main greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide and ozone. These gases do now allow all the radiations from the Earth to escape back info space. This is known as the Greenhouse Effect. This trapped radiation or heat helps to keep the Earth at a constant temperature. Greenhouse effect is thus very important to main the temperature of the Earth at a habitable level.Over the past few 100 years, human have been burning trees and fissile fuels, thereby releasing millions of tones of carbon dioxide into the atmosphere. The increased level of carbon dioxide in the atmosphere has caused the temperature of the Earth to rise. This, in turn, causes more water to evaporate into the atmosphere. Water vapor is also very good at absorbing heat. This cause the Earth’s atmosphere to become even warmer. Scientist has called this effect ‘Global Warming’. They have warned that Global Warming could shift rainfall patterns, raise the sea level and alter the ecological balance.