The Green house effect is the build-up of carbon dioxide. This gas is used by plants, but we are cutting them down so fast they can not use up the amount being released into the air. In addition, there are more people and more advances that are creating the gas (factories, cars). Once the gas begins to exceed the "normal" level, it can cause a rise in temperature for the planet. this causes the ice sheets over the north and south poles to melt, resulting in flooding. Also, a change of even a few degrees in the oceans can cause coral reefs (living creatures) to die off. This has been seen recently in the Great Barrier Reef off the coast of Australia. An interesting, but somewhat technical, talk about Global Warming is linked below. Do some more reading, then see what is being done to help.