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Attribution of recent climate change attempts to discover what mechanisms are responsible for the observed changes in climate. The endeavour centers on the observed changes over the last century and in particular over the last 50 years, when observations are best and human influence greatest. Over the past 150 years human activities have released increasing quantities of Greenhouse Gas es into the Atmosphere that theory and Climate Model s say should lead to increases in temperature - colloquially known as Global Warming . Other human effects are relevant—for example, sulphate Aerosol are believed to lead to cooling—and natural factors also act. Temperatures Have Risen In The Last Century (somewhere between 0.4 and 0.8 °C) and the proportion of this warming that is due to human influence is still open to question. The current scientific consensus, as expressed in 2001 by the Intergovernmental Panel On Climate Change (IPCC) and the National Academy Of Sciences (NAS), and recently confirmed by a joint statement of the G8 academies of science, is that ''most of the warming observed over the last 50 years is attributable to human activities''. A summary of IPCC climate research may be found in the IPCC assessment reports ; the NAS report and an overview of the report may be found here ; the degree of consensus is discussed at Scientific Opinion On Climate Change . ATTRIBUTION OF 20TH CENTURY CLIMATE CHANGE The most fiercely-contested question in current climate change research is over attribution of climate change to either natural/internal or human factors over the period of the instrumental record - from about 1860, and especially over the last 50 years. In the 1995 second assessment report (SAR) the IPCC made the widely quoted statement that "The balance of evidence suggests a discernible human influence on global climate”. The phrase "balance of evidence" was used deliberately to suggest the (English) common-law standard of proof required in civil as opposed to criminal courts: not as high as "beyond reasonable doubt". In 2001 the third assessment report (TAR) upgraded this by saying "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities" {Link without Title} . Over the past 5 decades there has been a warming of approximately 0.4°C at the Earth's surface (see Historical Temperature Record ). This warming might have been caused by internal variability, or by external forcing, or by "greenhouse" gases. Current studies indicate the latter is most likely, on the grounds that
In 2001 the US National Academy Of Sciences released a report supporting the IPCC's conclusions regarding the causes of recent climate change. It stated: ''“Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes are also a reflection of natural variability.”'' {Link without Title} {Link without Title} {Link without Title} Global Climate Model s (GCM) do not incorporate the indirect Solar Forcing through modulation of cosmic ray flux (increased solar activity reduces cosmic ray flux and is speculated to modify cloud cover). This is because there is no known mechanism for this effect; climate models cannot incorporate unknown mechanisms. One possible mechanism for the cosmic ray flux to influence climate is via Particle Formation by Ion Nucleation in the Upper Troposphere and Lower Stratosphere ''"These findings indicate that, at typical upper troposphere and lower stratosphere conditions, particles are formed by this nucleation process and grow to measurable sizes with sufficient sun exposure and low preexisting aerosol surface area. Ion-induced nucleation is thus a globally important source of aerosol particles, potentially affecting cloud formation and radiative transfer. ... Atmospheric aerosols affect climate directly by altering the radiative balance of the Earth (1) and indirectly by acting as cloud condensation nuclei (CCN) (2), which in turn change the number and size of cloud droplets and the cloud albedo"''; however there is no agreement within the community for the correctness of this. Since GCM ''can'' reproduce observed temperature trends (including early 20th century changes, where solar forcing is non-negligible) there is no obvious need for a high sensitivity to solar forcing. Indeed, a significantly higher sensitivity to solar forcing would make early 20th century temperature change inexplicable. Subsequent to the TAR Following the publication of the TAR in 2001 "detection and attribution" of climate change has remained an active area of research. Some important results include:
DETECTION AND ATTRIBUTION Detection and attribution of climate signals, as well as its common-sense meaning, has a more precise definition within the climate change literature, as expressed by the IPCC {Link without Title} . ''Detection'' of a signal requires demonstrating that an observed change is statistically significantly different than can be explained by natural internal variability. ''Attribution'' is to demonstrate that a signal is
Detection does not imply attribution, and is easier than attribution. Unequivocal attribution would require controlled experiments with multiple copies of the climate system, which is not possible. Attribution, as described above, can therefore only be done within some margin of error. For example, in the TAR , the statement is made that ''most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations'' where "likely" is quantified as 66-90% certain. SCIENTIFIC LITERATURE AND OPINION Some examples of published and informal support for the consensus view:
Willie Soon and Richard Lindzen say that there is insufficient proof for anthropogenic attribution. For more information, see:
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