Climate change is a long–term shift in weather conditions identified by changes in temperature, precipitation, winds, and other indicators. Climate change can involve both changes in average conditions and changes in variability. Over the last 100 years, the global mean temperature has increased by 0.74oC and atmospheric CO concentration has 2 increased from 280 ppm in 1750 to 400 ppm in 2013. Causes of Climate Change The earth′s climate is naturally variable on all time scales. However, its long–term state and average temperature are regulated by the balance between incoming and outgoing energy, which determines the Earth′s energy balance. Any factor that causes a sustained change to the amount of incoming energy or the amount of outgoing energy can lead to climate change. As these factors are external to the climate system, they are referred to as ′climate forcers′ invoking the idea that they force or push the climate towards a new longterm state â€“ either warmer or cooler depend on the cause of change. Different factors operate on different time scales, and not all of those factors that have been responsible for changes in earth′s climate in the distant past are relevant to contemporary climate change. Factors that cause climate change can be divided into two categories – those related to natural processes and those related to human activity. In addition to natural causes of climate change, changes internal to the climate system, such as variations in ocean currents or atmospheric circulation, can also influence the climate for short periods of time. This natural internal climate variability is superimposed on the long–term forced climate change. Natural Causes The Earth′s climate can be affected by natural factors that are external to the climate system, such as changes in volcanic activity, solar output, and the Earth′s orbit around the Sun. Of these, the two factors relevant on timescales of contemporary climate change are changes in volcanic activity and changes in solar radiation. In terms of the Earth′s energy balance, these factors primarily influence the amount of incoming energy. Volcanic eruptions are episodic and have relatively short–term effects on climate. Changes in solar irradiance have contributed to climate trends over the past century but since the Industrial Revolution, the effect of additions of greenhouse gases to the atmosphere has been over 50 times that of changes in the Sun′s output.