Attribution of recent climate change

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Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for 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 troposphere have become available. The dominant mechanisms to which recent climate change has been attributed all result from human activity. They are:[1]

Attribution of recent change to anthropogenic forcing is based on the following facts:

  • The observed change is not consistent with natural variability.
  • Known natural forcings would, if anything, be negative over this period.
  • Known anthropogenic forcings are consistent with the observed response.
  • The pattern of the observed change is consistent with the anthropogenic forcing.

Recent reports from the Intergovernmental Panel on Climate Change (IPCC) 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] It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing (i.e., it is inconsistent with being the result of internal variability), and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling.[3]
  • "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 defines "very likely," "extremely likely," and "virtually certain" as indicating probabilities greater than 90%, 95%, and 99%, respectively.[1]

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