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Attribution of recent climate change
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, which 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. According to the IPCC, the current best answer is, roughly, most of the warming observed over the last 50 years is attributable to human activities.
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" .
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 (GHG for this article). 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 GHG forced change than from natural change .
However, there is room for disagreement, both about the magnitude of the observed temperature changes and the certainty of the attribution.
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 "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". 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 or 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 that surveyed  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." 
- 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, and is convinced that human activities are the major cause of the global warming that has been measured..."
Some examples of scientists who have spoken or published against the consensus viewpoint:
- Astrophysicist Sallie Baliunas said in a speech delivered at Hillsdale College on February 5, 2002, "...about 80 percent of the carbon dioxide from human activities was added to the air after 1940. Thus increased carbon dioxide in the air cannot account for the pre-1940 warming trend. That trend had to be largely natural. Then, as the air's carbon dioxide content increased most rapidly, temperatures dropped for nearly 40 years. And it seems that human effects amount at most to about 0.1 degree Celsius per decade -- the maximum increase in warming seen after the 1970s." ().
- Scientists Willie Soon and Richard Lindzen say that there is insufficient proof of the recent attribution as yet - though they have not written papers showing this.
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