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Attribution of recent climate change is the effort to scientifically demonstrate which mechanisms are responsible for observed changes in the Earth's climate. The endeavor 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. The latest Intergovernmental Panel on Climate Change (IPCC) report 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." .

The report defines "very likely" as a greater than 90% probability and represents the consensus of the scientific community.

Attribution of 20th century climate change

Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. This has led to increases in mean global temperature or global warming. Other human effects are relevant—for example, sulphate aerosols are believed to lead to cooling—and natural factors also contribute. According to the historical temperature record of the last century, the Earth's near-surface air temperature has risen around 0.74 ± 0.18 °Celsius (1.3 ± 0.32 °Fahrenheit).

An important 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". The 2007 fourth assessment report (AR4) report strengthened this and noted, "Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns".

Over the past five decades there has been a global warming of approximately 0.65 °C (1.17 °F) 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.
• The plateau in warming from the 1940s to 1960s can be attributed largely to sulphate aerosol cooling.

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 GCMs can reproduce observed temperature trends (including early twentieth 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 twentieth century temperature change inexplicable.

Subsequent to the TAR

Following the publication of the Third Assessment Report (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; and the 2006 US CCSP report .
• 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, observed conditions that "could be an indication of a climate regime shift to pre-1976 conditions." Bratcher and Giese conclude: "The results presented here do not preclude the possibility that anthropogenic sources of greenhouse gases have contributed to global warming. However the results do indicate that the human forced portion of global warming may be less than previously described."
• The IPCC Fourth Assessment Report states that human actions are "very likely" the cause of most global warming, meaning a 90% or greater probability that more than 50% of observed warming is attributable to human actions.
• Svensmark at Danish National Space Center published a report in 2006 titled "Experimental evidence for the role of ions in particle nucleation under atmospheric conditions." A press release described 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.

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 IPCC's Fourth Assessment Report dated 2007, the statement is made 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." where 'very likely' is quantified as more than 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.
• 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).

Findings that complicate attribution to CO₂

Warming sometimes leads CO₂ increases

The ice core record shows that on some occasions temperature starts rising hundreds of years before CO₂ increases. This confirms that factors other than increased CO₂ concentrations have triggered warming and cooling processes at certain times in geological history. (See Orbital forcing, for instance.) It also provides evidence that the relationship between CO₂ and climate can go in both directions: changes in CO₂ concentrations affect climate, while changes in climate can affect CO₂ concentrations. One proposed mechanism for this effect is increased release of sequestered CO₂ from oceans as circulation patterns shift in response to climate change.

A more speculative and polemical inference sometimes drawn is that the causal relationship between temperature rises and global CO₂ concentrations might be only one-way, that historical increases in CO₂ have been nothing more than the product of independently rising temperatures. However, a strictly "one-way" view of the relationship between CO₂ and temperature would contradict basic results in physics, specifically that the absorption and emission of infrared radiation by CO₂ increases as its atmospheric concentration increases. Such first principles as well as empirical observation would suggest that positive feedbacks from CO₂ concentrations amplify warming initially caused by other factors:

Close analysis of the relationship between the two curves shows that, within the uncertainties of matching their timescales, the temperature led by a few centuries. This is expected, since it was changes in the Earth’s orbital parameters (including the shape of its orbit around the Sun, and the tilt of Earth’s axis) that caused the small initial temperature rise. This then raised atmospheric CO2 levels, in part by outgassing from the oceans, causing the temperature to rise further. By amplifying each other’s response, this “positive feedback” can turn a small initial perturbation into a large climate change. There is therefore no surprise that the temperature and CO2 rose in parallel, with the temperature initially in advance. In the current case, the situation is different, because human actions are raising the CO2 level, and we are starting to observe the temperature response.

The ice core data says relatively little about the pattern of modern warming (that is, warming since about 1960). Recent CO₂ levels greatly exceed the range witnessed in the ice core data. Isotope analysis of atmospheric CO₂ changes implicates human activity as the driver, unlike during prior interglacial periods. As noted above, models that give a significant amount of weight to increased CO₂ levels when attempting to explain recent temperature rises match the observed data far better than those that do not. It is from this (and other observations) that the IPCC concluded that humans (because of CO₂ emissions) were 90% likely to be the cause of the recently observed warming.

Warming elsewhere in the solar system

In the last ten years at least localized warming (or rather proxies for warming) has been observed on Mars , Pluto , and Neptune's largest moon Triton . It has been asserted, though not in any scientific papers, that this points to a solar explanation for the recent warming on Earth. This is however hotly disputed by astronomers and climate scientists. Jay Pasachoff, an astronomy professor at Williams College, said that Pluto's global warming was:

likely not connected with that of the Earth. The major way they could be connected is if the warming was caused by a large increase in sunlight. But the solar constant--the amount of sunlight received each second--is carefully monitored by spacecraft, and we know the sun's output is much too steady to be changing the temperature of Pluto.

In the absence of an increase in the amount of sunlight, it is believed that other factors, such as orbital changes, must be at work (in the case of Triton, for example, researchers "believe that Triton's warming trend could be driven by seasonal changes in the absorption of solar energy by its polar ice caps.").

For more information on the climate of the best understood of the planets and moons mentioned in this argument, see: Climate of Mars. For more information on the effects of solar variation on the Earth, see: Solar variation

Footnotes

1. Indermühle, Andreas (2000). "Atmospheric CO2 concentration from 60 to 20 kyr BP from the Taylor Dome ice core, Antarctica" (PDF). Geophysical Research Letters. 27. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
2. Fischer, Hubertus (1999). "Ice core records of atmospheric CO2 around the last three glacial terminations". Science. 283. doi:10.1126/science.283.5408.1712. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
3. Skinner, L.C. (2006). "Glacial – interglacial atmospheric CO2 change: a simple "hypsometric effect" on deep-ocean carbon sequestration?" (PDF). Climate of the Past Discussions. 2.
4. Paillard, Didier (2000). "Glacial Cycles: Toward a New Paradigm" (PDF). Reviews of Geophysics. 39:3.
5. Schimel, D. (1996). "CO2 and the carbon cycle, pp.76-86 in Climate Change 1995: The Science of Climate Change. J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg and K. Maskell (eds), Cambridge University Press 1996". {{cite journal}}: Cite has empty unknown parameter: |1= (help); Cite journal requires |journal= (help)

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
• US EPA climate change and global warming website

• Global warming