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 best answer, as expressed by the IPCC and the NAS, is that the scientific view can be summed up roughly as 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 (http://www.grida.no/climate/ipcc_tar/wg1/); the NAS report and an overview of the report may be found here (http://www4.nationalacademies.org/onpi/webextra.nsf/web/climate?OpenDocument); 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 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] (http://www.grida.no/climate/ipcc_tar/wg1/007.htm).
Over the past 5 decades there has been a warming of approximately 0.3°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] (http://www.grida.no/climate/ipcc_tar/wg1/460.htm).
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] (http://www4.nationalacademies.org/onpi/webextra.nsf/web/climate?OpenDocument)[4] (http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html)[5] (http://www.eldis.org/static/DOC8822.htm)
Global climate models 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 (http://www.sciencemag.org/cgi/content/full/301/5641/1886)"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. Note that since GCMs can reproduce observed 20C temperature trends (including early 20C 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 20C temperature change inexplicable.
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 (http://www.grida.no/climate/ipcc_tar/wg1/439.htm) or the IPCC TAR, which discusses The Meaning of Detection and Attribution (http://www.grida.no/climate/ipcc_tar/wg1/443.htm#1211), Quantitative Comparison of Observed and Modelled Climate Change (http://www.grida.no/climate/ipcc_tar/wg1/456.htm), Pattern Correlation Methods (http://www.grida.no/climate/ipcc_tar/wg1/458.htm) and Optimal Fingerprint Methods (http://www.grida.no/climate/ipcc_tar/wg1/460.htm).
- An essay in Science that surveyed [6] (http://www.sciencemag.org/cgi/content/full/306/5702/1686) of abstracts related to climate change and concluded that most accepted the consensus 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 "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." [7] (http://groups-beta.google.com/group/sci.environment/msg/aa8521b67cd216ac?dmode=source)
- 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.". Note that this earlier work only addressed the most recent period, and that estimates of natural variability are important 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 (http://www.agu.org/sci_soc/policy/climate_change_position.html), 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).