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climate science
Solar Effects On Climate
A survey of the current scientific literature leaves little doubt that the sun's radiant output impacts the Earth's climate on both decadal and centennial time scales. However, it is only one component of many affecting terrestrial climate. (for reviews, see Nesme-Ribes, 1994 and Valdes, 2000). The debate continues to focus not on whether the sun affects climate but to what extent.
The sun exerts forcing of Earth's climate as a result of heat transfer from solar irradiance striking the Earth's surface. Determining the magnitude of this effect is an area of active research. One recent paper correlates the sun's 11 year sunspot cycle to climate cycles on Earth during the last 9,000 years (Perry and Hsu, 2000). The sun also exerts forcing of the Earth's climate through variations in UV irradiance and through precipitating chemical reactions which produce radicals that contribute to the destruction of stratospheric ozone (O3 is both a shield against UV radiation, an important air pollutant, and a powerful GHG), thereby affecting both the composition and structure of the atmosphere. Both affect weather and climate ("Solar Influences on Global Climate," NAS, 1994; Haigh, 1996).
Although few question the importance of the sun on climate in Earth's past, the preponderance of the evidence shows that solar forcing has been significantly smaller than anthropogenic forcing for the last 30 years and will continue to be so into the future (Beer et al., 2000). Willson (1997), for example, found an increase in total solar irradiance of 0.036% between 1986 and 1996 in results of the Active Cavity Radiometer Irradiance Monitor experiment. Global circulation models indicate this would increase global temperature by ~0.03-0.09°C. If this is a trend sustained over the next century, Willson estimated that climate warming of approx. 0.4°C could result. By comparison, the Intergovernmental Panel on Climate Change (IPCC) in its latest assessment report (TAR, 2001) predicts that human activities affecting the climate system could raise global average temperature by 2.4-5.8°C if no action were taken to reduce GHG emissions.
The recent newspaper articles report on an as yet unpublished study of solar influence on climate by Danish scientists Svensmark and Marsh, who compared satellite measurements of galactic cosmic rays and low cloud cover. Since their data and reasoning have not yet been presented in a peer-reviewed journal, it is premature to judge the validity of their arguments. In our view, it is also inappropriate to dismiss the role of greenhouse gases in climate change, particularly in light of the vast body of work that points to the importance of these gases for global warming.
The Policy Implications
As the international negotiations of the climate treaty continue and the IPCC's Third Assessment Report (TAR) is made public, the Svensmark/Marsh study and others may be invoked to justify questioning the effect of GHGs on our atmosphere. It is important to remember that current scientific evidence shows that anthropogenic influences are a significant factor in causing modern global warming and that these influences will continue to grow in the next century unless action is taken to restrain fossil fuel burning and land-use changes.
The Messages
In discussing this complex issue with the media, the following messages may be helpful:
* Changes in the sun's output influence the Earth's climate, but the sun is only one of several components affecting terrestrial climate. Current scientific understanding suggests that the sun's influence is significantly less than the human-related climate influences, such as emissions of greenhouse gases (GHG) and sulfate aerosols, land use changes, and ozone depletion.
* The larger human influence on climate will continue to grow into the future. Over the next century, the IPCC predicts in its latest assessment (TAR, 2001) that human activities affecting the climate system could raise global average temperature by 2.5-10.4 degrees Fahrenheit (2.4-5.8 °C). If emissions of long-lived CO2 emissions continue while we reduce the emissions of relatively short-lived, cooling aerosols, the resulting warming could in fact be higher. By comparison, if current solar trends continue, warming of only approx. 0.4°C (0.6°F) would result.
* Humans have a responsibility to reduce their pollution of the atmosphere. Policies must be implemented in the short term to restrain fossil fuel burning and land-use changes. In order to achieve this, the international climate negotiations must focus on the human influence on climate.
References
1. Beer J, Mende W, and Stellmacher R (2000). The role of the sun in climate forcing, Quaternary Science Reviews, 19, 403-415.
2. Haigh J D (1996) The impact of solar variability on climate. Science, 272, 981-984.
3. IPCC (1994) Climate Change 1994: Radiative Forcing of Climate and an Evaluation of the IPCC IS92 Emissions Scenarios. Cambridge University Press.
4. IPCC (1955) Climate Change 1995: The Science of Climate Change. Cambridge University Press.
5. Nesme-Ribes E (1994) The Solar Engine and Its Influence on Terrestrial Atmosphere and Climate. Springer Verlag.
6. National Research Council (1994) Solar Influences on Global Change. National Academy Press.
7. Perry C and Hsu K (2000) Geophysical, archaeological, and historical evidence support a solar-output model for climate change. Proc. of the National Academy of Sciences, 10.1073.
8. Valdes P (2000) Warm climate forcing mechanisms, in Warm Climates in Earth History, Huber B, MacLeod K, and Wing S eds. Cambridge University Press.
9. Willson R C (1997) Total solar radiance trend during solar cycles 21 and 22. Science, 277, 1963-1965.
November, 2000
This report may not be reprinted or posted to electronic networks without permission and acknowledgement.
Acknowledgements
This Update was prepared by UCS consultant Peggy Shannon, reviewed by Stephen Schneider and Marcia Baker, and reviewed and updated by Susanne Moser. |
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© Union of Concerned Scientists
Page Last Revised: 08/11/05 |
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