The term "global climate" is used to refer to the general state of the world's climate. Whilst different climate zones may be identified, with different types of weather in different parts of the world, climatologists sometimes like to study the general climate of the whole Earth, for example when investigating evidence for climate change.
Global climate is fundamentally influenced by the amount of energy the Earth receives from the Sun, but also by how that energy is stored and redistributed through the world's atmosphere and oceans. The positions of the world's continents and large mountain ranges also affect global climate. Greenhouse gases in the atmosphere trap a lot of heat which would otherwise escape to space, keeping the Earth much warmer than it would otherwise be. The general circulation of the atmosphere transports warm air from the equator where solar heating is greatest towards the higher latitudes, keeping the hot equatorial zone cooler and the cold polar regions warmer.
The simplest means of assessing the state of the global climate is to measure the global average temperature of the Earth's surface and atmosphere in contact with it. This is done by analysing thousands of records of temperature collected from stations all over the world, both on land and at sea. Today, the global average surface temperature is about 15C. This is 33C warmer than the average surface temperature of the moon, which is about the same distance from the Sun as the Earth. The moon, of course, does not have an atmosphere, and therefore no greenhouse gases to trap extra heat.
During the last Ice Age, global average surface temperature was 5C lower than today. At that time, carbon dioxide levels in the atmosphere were lower, heat redistribution by ocean circulation was weaker, and less energy was being received from the Sun as a consequence of variations in the Earth's orbit. During the Ice Age, which ended only 14,000 years ago, vast ice sheets covered much of the Northern Hemisphere.
Today, scientists are worried that mankind's pollution of the atmosphere with extra greenhouse gases will result in global warming. During the last 100 years, the global average surface temperature has risen by about 0.6C. It is projected to rise a further 2 to 3C by the end of the 21st century.