Climate changes have occurred in the past naturally, over decades to millenia for various reasons. Interannual variations are also an important ingredient of climate and can arise through, for example, interactions between the atmosphere and the oceans, as is the case with El Niño. This article discusses weather and climate variations in the context of the Earth system as a whole, and provides a basis for understanding the reasons why climate may vary, and how that may be manifested in terms of weather. The main focus is on the atmosphere as the most variable component of the Earth system; it is afterall where we live and makes up the air we breathe. But the atmosphere interacts with the oceans, the land surface and its vegetation and the other components of the climate system, so those too are important, even from this perspective. Their role in climate is also addressed here.
Like the oceans, the atmosphere is a global commons (Sorros, 1997). It is globally connected and air that is over one nation can easily lie over another on the next continent a day later. Recent attempts by manned balloons to circumnavigate the globe have dramatically shown how the winds can carry the balloon half way around the world in a week or less and that air currents often take the balloon in unwanted directions. So the atmosphere belongs to no one nation, rather all nations may use it for their own purposes (such as discharging pollution and effluent into it) and thus it is also subject to abuse and the phenomenon known as the "tragedy of the commons" (Hardin 1968) in which the best interests of an individual or individual nation may conflict with the health of the commons itself. Human influences on climate change, often referred to as "global warming", are therefore also discussed.
The Earth System includes many other important processes and phenomena that can not be dealt with here. Even in the atmosphere alone, environmental problems include ozone depletion and the "ozone hole", acid rain, air quality and pollution, and a few decades ago, radioactivity and atomic bomb test debris. Other problems exist in the oceans and on land, such as biodiversity, deforestation, desertification, exploitation of water resources and fisheries, and so on, and many of these environmental problems may be exacerbated by climate change, so that it is the intersection of these that make for major challenges in the years ahead.