This question has been debated enormously, because climate change can be “due to natural variability or as a result of human activity” (IPCC 2007) and because the climate system is very complex. However, it is scientifically supported that our climate is changing, largely due to the observed increases in human produced greenhouse gases.
Greenhouse gases (including carbon dioxide, methane gas, oxides of nitrogen, and halocarbons) absorb heat from the sun in the atmosphere and reduce the amount of heat escaping into space. This extra heat has been found to be the primary cause of observed changes in the climate system over the 20th century.
Greenhouse gases are produced by human activities, including:
burning fossil fuels, such as coal, oil or gas
using energy generated by burning fossil fuels
some aspects of farming, e.g. raising cattle and sheep, using fertilisers and growing some crops
clearing land, including logging
the breakdown of food and plant wastes and sewerage
some industrial processes, such as making cement and aluminum.
These changes include increases in global average air and ocean temperature, widespread melting of snow and ice and rising global sea levels. The extra heat in the climate system has other impacts, such as affecting atmospheric and ocean circulation, which influences rainfall and wind patterns. Ocean acidification is also another negative impact of increased carbon dioxide. Around a quarter of human-produced carbon dioxide is absorbed by the oceans. As the carbon dioxide dissolves in sea water it forms a weak carbonic acid, making the ocean more acidic. There are early indications that some marine organisms are already being affected by ocean acidification.
Furthermore, since climate patterns play a fundamental role in shaping natural ecosystems, and the human economies and cultures that depend on them, changes in the climate will affect such systems.Therefore where and how people, plants and animals live will be affected. Other impacts include human health, forestry, water resources and availability, energy use and transportation
Dept. of Ecology, Washington State, USA.
Dept. Climate Change and Energy Efficiency, Australia.