Attribution of recent climate change
Scientific studies have investigated the causes of climate change. They have found that the main cause and driver of recent climate change is elevated levels of greenhouse gases produced by human activities. Natural forces add climate variability as well. Based on many scientific studies, it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times.": 3 Studies on attribution have focused on changes observed during the period of instrumental temperature record, particularly in the last 50 years. This is the period when human activity has grown fastest and observations of the atmosphere above the surface have become available. Some of the main human activities that contribute to global warming are: (a) increasing atmospheric concentrations of greenhouse gases (mainly carbon dioxide), for a warming effect; (b) global changes to land surface, such as deforestation, for a warming effect; and (c) increasing atmospheric concentrations of aerosols, mainly for a cooling effect.
In addition to human activities, some natural mechanisms can also cause climate change, including for example, climate oscillations (for example El Niño–Southern Oscillation (ENSO)), changes in solar activity, and volcanic activity.
The IPCC's attribution of recent global warming to human activities reflects the view of the scientific community, and is also supported by 196 other scientific organizations worldwide.
Four main lines of evidence support attribution of recent climate change to human activities: Firstly, a physical understanding of the climate system: greenhouse gas concentrations have increased and their warming properties are well-established. Secondly, there are historical estimates of past climate changes suggest that the recent changes in global surface temperature are unusual. Thirdly, computer-based climate models are unable to replicate the observed warming unless human greenhouse gas emissions are included. And finally, natural forces alone (such as solar and volcanic activity) cannot explain the observed warming.