Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for relatively recent changes observed in the Earth's climate. The effort has focused on changes observed during the period of instrumental temperature record, when records are most reliable; particularly on the last 50 years, when human activity has grown fastest and observations of the upper atmosphere have become available. The dominant mechanisms to which recent climate change has been attributed all result from human activity. They are:[1]
- increasing atmospheric concentrations of greenhouse gases
- global changes to land surface, such as deforestation
- increasing atmospheric concentrations of aerosols.
Recent reports from the Intergovernmental Panel on Climate Change (IPCC) report have concluded that:
- "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."[2]
- "From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols, and land surface changes, it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750."[1]
- "It is virtually certain that anthropogenic aerosols produce a net negative radiative forcing (cooling influence) with a greater magnitude in the Northern Hemisphere than in the Southern Hemisphere.[1]
The panel, which represents consensus in the scientific community, defines "very likely," "extremely likely," and "virtually certain" as indicating probabilities greater than 90%, 95%, and 99%, respectively.[1]