Natural Processes
Climate change has always occurred as a result of natural processes such as plate tectonics, volcanic activity, interactions between land, oceans and the atmosphere, as well as variations in sunlight intensity. However, it is believed that human activity has played a large part in recent changes in climate, particularly since the industrial revolution. Because of this the Earth has been subjected to increasing temperatures, melting of ice-caps and glaciers, rising sea levels, and unpredictable weather patterns.
Human Activity
Changes in climate resulting from human activity is not just a recent phenomenon. For thousands of years we have been cutting down trees both to use the wood for fuel and construction, and to develop land for agriculture. Fewer trees to absorb carbon dioxide will result in more CO2 building up in the atmosphere. There can even be a more direct effect, as is believed to have occurred in Greece and other Mediterranean countries over the last 2000 years. Widespread deforestation led to fewer trees absorbing the incoming sunlight, as well as greater soil erosion due to exposure. The modern climate is now significantly hotter and drier.
Greenhouse Gases
In more recent years, rising levels of greenhouse gases in the atmosphere, such as nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) in particular, have been identified by scientists as the primary cause of global warming. Although CO2 levels have varied hugely over the last 600 million years, the current atmospheric concentration of CO2 was recently calculated at 379ppm (parts per million), compared to pre-industrial levels of just 280ppm.
Carbon Dioxide
Most of our emissions, CO2 in particular, come from the burning of fossil fuels such as coal, gas and oil. Fossil fuels are used primarily for transport, heating and electricity generation. Every time we turn on the radio or television, light a fire, or drive to the shops, we are contributing to the increase of CO2 in our atmosphere. Most of what we purchase will also lead to CO2 emission in some way, either as a result of its manufacture and packaging, or transport of the item, or both. This includes the purchase of tropical fruits and vegetables, or fruit and vegetables bought out of season, which may have been transported large distances to reach your kitchen.
Finally, other factors including livestock and cement manufacture play an important role in emissions of CO2 and other human-induced greenhouse gases such as nitrous oxide and methane.