Although we are currently concerned about global warming caused by mankind's enhancement of the natural greenhouse effect through greenhouse gas pollution, the Earth's climate has fluctuated many times in the past in response to natural mechanisms of climate change. Such changes in global climate have also occurred over much longer time scales, from hundreds and thousands of years to millions and hundreds of millions of years. In fact the current global warming trend which began at the end of the 19th century spans only a tiny fraction of the Earth's climatic history. Taking a longer perspective on climate variability can help us gain a better understanding of the global climate and of the evolution of the climate today and in the future.
The overall state of the global climate is largely determined by balance between energy the Earth receives from the Sun and the heat which the Earth releases back to space, called the global energy balance. Many causes of climate change involve processes that alter the global energy balance. The enhanced greenhouse effect is a potential cause of climate change because the extra quantities of greenhouse gases put into the air by mankind trap more heat in the atmosphere. This upsets the global energy balance and causes a rise in global surface temperature. Although most scientists agree that global warming is the result of the enhanced greenhouse effect, natural processes can also upset the global energy balance in this way, causing similar rises in global temperature. A change in the amount of energy received by the Sun for example has often been proposed as the cause of global warming. The challenge facing scientists is to establish beyond doubt a link between global warming and man-made greenhouse gas pollution.