Attribution of recent climate change

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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 gases into the atmosphere that theory and climate models 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.

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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" [1].

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

  • 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 changes in greenhouse gases and aerosols, reproduce the observed global 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 [2]

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.”[3][4][5]

Global climate models (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.

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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:

  • 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 [6] [7] 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 [8].
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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 [9].

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

  • 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.

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Scientific literature and opinion

Some examples of published and informal support for the consensus view:

  • A recent paper (Estimation of natural and anthropogenic contributions to twentieth century temperature change, Tett SFB et al., JGR 2002), says "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." [11]
  • In 1996, in a paper in Nature entitled "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 doubtful view of global warming accept that recent climate change is mostly anthropogenic. John Christy said: "...he supports the AGU declaration, and is convinced that human activities are the major cause of the global warming that has been measured..."

Willie Soon and Richard Lindzen say that there is insufficient proof for anthropogenic attribution. For more information, see:

    • "Modeling climatic effects of anthropogenic carbon dioxide emissions: unknowns and uncertainties", Soon W et al., 2001, Climate Research 18(3).
    • "Climate hypersensitivity to solar forcing?", Soon W et al., 2000, Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences 18(5).
    • "Environmental effects of increased atmospheric carbon dioxide", Soon W et al., 1999, Climate Research 13(2).
    • "Reconciling observations of global temperature change", Lindzen RS, Giannitsis C, 2002, Geophysical Research Letters 29(12).
    • "Can increasing carbon dioxide cause climate change?", Lindzen RS, 1997, PNAS 94(16).
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See also

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References

  • Le Quéré, How much of the recent CO2 increase is due to human activities?, 2005 [12]
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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
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