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Attribution of recent climate change is the problem of discovering what mechanisms are responsible for 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 gases into the atmosphere. Theory and climate models imply that this should lead to increases in mean global temperature — colloquially known as global warming. Other human effects are relevant—for example, sulphate aerosols are believed to lead to cooling—and natural factors also act.

Temperatures have risen over the last century (somewhere between 0.4 and 0.8 °C) and, while the exact 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 U.S. 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" .

Over the past five decades there has been a global warming of approximately 0.4°C at the Earth's surface (see historical temperature record). This warming might have been caused by internal variability of the climate system, by external forcing, by an increase in concentration of "greenhouse" gases, or by any combination of these factors. Current studies indicate that the increase in greenhouse gases, most notably CO₂, has been most influential, on the grounds that:

• estimates of internal variability from climate models, and reconstructions of past temperatures, indicate that the warming is unlikely to be entirely natural.
• climate models forced by natural factors AND increased greenhouse gases and aerosols reproduce the observed global temperature changes; those forced by natural factors alone do not.
• "fingerprint" methods indicate that the pattern of change is closer to that expected from greenhouse gas-forced change than from natural change.

In 2001 the U.S. 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.”

Another candidate mechanism for climate change is solar forcing. Most global climate model (GCM) studies indicate that the direct effects of solar variation would be too small to significantly affect climate. Much of the solar research centers around possible mechanisms to amplify the effect, possibly through increasing solar activity reducing cosmic ray flux and, speculatively, modifying cloud cover ; however there is no agreement on whether this is correct within the scientific community. 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:

• A review of detection and attribution studies by the International Ad Hoc Detection and Attribution Group found that The evidence indicates that natural drivers such as solar variability and volcanic activity are at most partially responsible for the large-scale temperature changes observed over the past century, and that a large fraction of the warming over the last 50 yr can be attributed to greenhouse gas increases. Thus, the recent research supports and strengthens the IPCC Third Assessment Report conclusion that “most of the global warming over the past 50 years is likely due to the increase in greenhouse gases.”
• Multiple independent reconstructions of the temperature record of the past 1000 years confirm that the late 20th century is probably the warmest period in that time
• Two papers in Science in August 2005 resolve the problem, evident at the time of the TAR, of tropospheric temperature trends. The UAH version of the record contained errors, and there is evidence of spurious cooling trends in the radiosonde record, particularly in the tropics. See satellite temperature measurements for details.
• Barnett et al. "Penetration of Human-Induced Warming into the World's Oceans" (Science, Vol 309, Issue 5732, 284-287, 8 July 2005), say that the observed warming of the oceans cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences . However, ocean temperatures have cooled significantly since 2003.
• Bratcher and Giese, "Tropical Pacific decadal variability and global warming" published in 2002, points to oceanic events (with a four year lag in temperatures) causing climate shifts - a 1972 event caused a climate shift resulting in warmer temperatures from 1976 until 2002. An event in 2002 caused Giese to predict a climate shift to pre-1976 conditions by 2006.. Giese's prediction came true when 2006 temperatures were cooler than 2005 even though 2006 was an El Nino year. Giese concludes the results of his study "indicate that the human forced portion of global warming may be less than previously described."
• Svensmark at Danish National Space Center published a report in 2006 of his team's ability to demonstrate low-level clouds (which cool the Earth) are formed when cosmic rays come into the low atmosphere. Previous research has demonstrated a reduction in cosmic rays between 1920 and 1980 when measurements were stopped. Cosmic rays have not been considered as a climate forcing in any computer models. This indicates that mankind's contribution to global warming may be less than the IPCC has indicated.

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 .

Detection of a signal requires demonstrating that an observed change is statistically significantly different from that which can be explained by natural internal variability.

Attribution requires demonstrating that a signal is:

• unlikely to be due entirely to internal variability;
• consistent with the estimated responses to the given combination of anthropogenic and natural forcing
• not consistent with alternative, physically plausible explanations of recent climate change that exclude important elements of the given combination of forcings.

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:

• The attribution of climate change is discussed extensively, with references to peer-reviewed research, in chapter 12 of the IPCC TAR, which discusses The Meaning of Detection and Attribution, Quantitative Comparison of Observed and Modelled Climate Change, Pattern Correlation Methods and Optimal Fingerprint Methods.
• An essay in Science surveyed 928 abstracts related to climate change, and concluded that most journal reports accepted the consensus. This is discussed further in scientific opinion on climate change.
• A recent paper (Estimation of natural and anthropogenic contributions to twentieth century temperature change, Tett SFB et al., JGR 2002), says that "Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming."
• In 1996, in a paper in Nature titled "A search for human influences on the thermal structure of the atmosphere", Benjamin D. Santer et al. wrote: "The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability." This earlier work only addressed the most recent period. Estimates of natural variability matter for assessing the significance of the trend.
• Even some scientists noted for their somewhat skeptical view of global warming accept that recent climate change is mostly anthropogenic. John Christy has said that he supports the American Geophysical Union (AGU) declaration, and is convinced that human activities are the major cause of the global warming that has been measured.

Some scientists do disagree with the consensus: see list of scientists opposing global warming consensus. For example Willie Soon and Richard Lindzen ("Can increasing carbon dioxide cause climate change?", Lindzen RS, 1997, PNAS 94(16)) say that there is insufficient proof for anthropogenic attribution. Generally this position requires new physical mechanisms to explain the observed warming; for example "Climate hypersensitivity to solar forcing?", Soon W et al., 2000, Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences 18(5).

References

• Le Quéré, How much of the recent CO₂ increase is due to human activities?, 2005

External links

• RealClimate - Blog on current climate change issues by active climatologists
• "The Climate of Man", The New Yorker (2005): Part 1, Part 2, Part 3
• Anthropogenic or Solar? by Shaviv
• Newest reports on US EPA website

• Global warming