The world heats as it absorbs the sun's energy or when atmospheric gases prevent heat emitted by the earth from escaping into space (the greenhouse effect). A multitude of natural and human-made causes can have an impact on the Earth's climate system. These include changes in the amount of solar energy that reaches the surface of the planet, due to variations in the angle of the earth's orbit around the sun or because of clouds or other atmospheric effects; fluctuations in the number of volcanoes active at any given time; changes in the amount or type of land use; and even events as large as asteroid impacts or major volcanic eruptions.
In general, there are two ways in which global warming may affect the climate: by changing the average temperature of the planet or by changing the distribution of rainfall across the globe. It is also possible for global warming to have a mixed effect - i.e., having both positive and negative consequences for different regions of the world. Let's take a look at these issues in more detail.
Average Temperature: Global warming will cause the average temperature of the planet to rise. If you warmed up a cup of coffee using a microwave, you would be heating it up faster than if you used the oven. This is because radiation gets absorbed by objects that are closer to the source.
The temperature of the Earth is determined by a balance between incoming solar energy and energy reflected back into space. Heat that would otherwise be lost to space is absorbed by carbon dioxide. Some of this energy is re-emitted back to Earth, causing the planet to heat up even more. The remaining fraction becomes available for use by living things.
Solar radiation passes through the atmosphere and land surfaces and is either reflected back into space or loses its energy as long-wave radiation. Long-wave radiation is heat that travels in straight lines away from the source. It can be emitted into space or it can travel toward the surface of the Earth and be re-radiated into space at night when the surface is cold. The total amount of long-wave radiation that reaches Earth's surface is about 600 watts per square meter.
At any given time, some parts of Earth are exposed to more sunlight than others. For example, during the day, the sun is never far from the center of the Earth because we are surrounded by air which has very high optical density (refractive index). So even though the sun is relatively close to the horizon at sunset and sunrise, a lot of its light still makes it all the way across Earth to illuminate another part of the planet. At night, however, everything on Earth is dark except for a small region around the moon and stars. This area with no darkness or opacity is called the sky.
When carbon dioxide (CO2) and other air pollutants accumulate in the atmosphere, they absorb sunlight and solar energy that has bounced off the earth's surface. This reduces the amount of solar energy reaching the earth-air interface and causes temperatures to drop. CO2 emissions from human activities increase the concentration of this gas in the atmosphere and so cause further cooling. However many other factors influence Earth's climate, including variations in the intensity of the sun's rays, changes in the position of the earth with respect to the moon, and eruptions of large volcanoes, that make it difficult to distinguish effects of CO2 alone.
The average temperature of the planet over time is called "global warming", but there are local effects of this heating due to different factors affecting heat gain or loss at different locations. For example, an area of land covered in vegetation will tend to absorb more radiation than one covered in ice or snow, so vegetation can have a significant effect on the local temperature. Also, oceans absorb much of the radiation that would otherwise reach the earth's surface, so deep ocean currents can affect the temperature of places far away from any continent.
Heat is also lost the atmosphere, which can lead to thick clouds forming and blocking out much of the sun's radiation before it reaches the ground.