Efforts to scientifically ascertain and attribute mechanisms responsible for recent global warming and related climate changes on Earth have found that the main driver is elevated levels of greenhouse gases produced by human activities, with natural forces adding variability. The likely range of human-induced surface-level air warming by 2010–2019 compared to levels in 1850–1900 is 0.8 °C to 1.3 °C, with a best estimate of 1.07 °C. This is close to the observed overall warming during that time of 0.9 °C to 1.2 °C, while temperature changes during that time were likely only ±0.1 °C due to natural forcings and ±0.2 °C due to variability in the climate.
According to the Intergovernmental Panel on Climate Change (IPCC) in 2021, it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." 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: increasing atmospheric concentrations of greenhouse gases, for a warming effect global changes to land surface, such as deforestation, for a warming effect 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, changes in solar activity, and volcanic activity.
Multiple lines of evidence support attribution of recent climate change to human activities: A physical understanding of the climate system: greenhouse gas concentrations have increased and their warming properties are well-established. Historical estimates of past climate changes suggest that the recent changes in global surface temperature are unusual. Computer-based climate models are unable to replicate the observed warming unless human greenhouse gas emissions are included. Natural forces alone (such as solar and volcanic activity) cannot explain the observed warming.
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. ()
Background
Factors affecting Earth's climate can be broken down into forcings, feedbacks and internal variations. A forcing is something that is imposed externally on the climate system. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output. Human activities can also impose forcings, for example, through changing the composition of Earth's atmosphere.
Radiative forcing is a measure of how various factors alter the energy balance of planet Earth. A positive radiative forcing will lead towards a warming of the surface and, over time, the climate system. Between the start of the Industrial Revolution in 1750, and the year 2005, the increase in the atmospheric concentration of carbon dioxide (chemical formula: ) led to a positive radiative forcing, averaged over the Earth's surface area, of about 1.66 watts per square metre (abbreviated W m−2).
Climate feedbacks can either amplify or dampen the response of the climate to a given forcing. There are many feedback mechanisms in the climate system that can either amplify (a positive feedback) or diminish (a negative feedback) the effects of a change in climate forcing.
The climate system will vary in response to changes in forcings. The climate system will show internal variability both in the presence and absence of forcings imposed on it, (see images opposite). This internal variability is a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean (see also the later section on Internal climate variability and global warming). An example of internal variability is the El Niño–Southern Oscillation.