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In 1991, there was a study published in Science magazine showing a stunning correlation between sun spot length and the rising temperature trends. (Friis-Christensen, E. Science (1991), 254, 5032, 698 - 700). This study gained popularity in arguments against global warming stating that it showed any warming trends in the 20th century could be easily explained by an increase in solar irradiation.
For years this has been the lone study that successfully made the correlation, but it was legitimate at the time and was published in a well-respected journal...
Here's what has happened since 1991:
In 1999, Knud Lassen (a co-author of the original paper) published "Solar forcing of the Northern Hemisphere land air temperature: new data" (Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62, Issue 13, p. 1207-1213.) in which he concluded "About 10 years ago, FCL91 found that a good fit existed between the solar cycle length curve and the cycle mean temperatures. Today we conclude that addition of data for the 1990s has changed that picture."
The second graph on that image shows deviation from the expected temperature based on sun spot lifetimes. If irradiation were to fully explain the warming trend, then these 2 lines should run coincident, but from 1970 on they clearly do not.
Since then, there have been a number of papers that bring to light the mistakes made in the original study including one by Peter Laut in 2003 (Journal of Atmospheric and Solar-Terrestrial Physics, Volume 65, Issue 7, May 2003, Pages 801-812) in particular the mishandling of the most recent data in the plot in which the data points are not included in the filtered averaging that the authors performed.
To bring things full circle, as part of an interview last year Friis-Christensen (the first author of the original study) was quoted as saying "that any correlation between sunspots and global warming that (he) may have identified in the 1991 study has since broken down. There is a clear 'divergence' between the sunspots and global temperatures after 1986, which shows that the present warming period cannot be explained by solar activity alone." (link)
There were 2 studies in 2009 on the effects of solar irradiation. The first (A D Erlykin et al 2009 Environ. Res. Lett. 4) concluded that the maximum impact that could be attributed since 1956 was 14%. The second (Benestad, R. Journal of Geophysical Research, 114, D14) found that 7 +/- 1% of warming in the 20th century was attributable to solar effects.
For years this has been the lone study that successfully made the correlation, but it was legitimate at the time and was published in a well-respected journal...
Here's what has happened since 1991:
In 1999, Knud Lassen (a co-author of the original paper) published "Solar forcing of the Northern Hemisphere land air temperature: new data" (Journal of Atmospheric and Solar-Terrestrial Physics, Volume 62, Issue 13, p. 1207-1213.) in which he concluded "About 10 years ago, FCL91 found that a good fit existed between the solar cycle length curve and the cycle mean temperatures. Today we conclude that addition of data for the 1990s has changed that picture."
The second graph on that image shows deviation from the expected temperature based on sun spot lifetimes. If irradiation were to fully explain the warming trend, then these 2 lines should run coincident, but from 1970 on they clearly do not.
Since then, there have been a number of papers that bring to light the mistakes made in the original study including one by Peter Laut in 2003 (Journal of Atmospheric and Solar-Terrestrial Physics, Volume 65, Issue 7, May 2003, Pages 801-812) in particular the mishandling of the most recent data in the plot in which the data points are not included in the filtered averaging that the authors performed.
To bring things full circle, as part of an interview last year Friis-Christensen (the first author of the original study) was quoted as saying "that any correlation between sunspots and global warming that (he) may have identified in the 1991 study has since broken down. There is a clear 'divergence' between the sunspots and global temperatures after 1986, which shows that the present warming period cannot be explained by solar activity alone." (link)
There were 2 studies in 2009 on the effects of solar irradiation. The first (A D Erlykin et al 2009 Environ. Res. Lett. 4) concluded that the maximum impact that could be attributed since 1956 was 14%. The second (Benestad, R. Journal of Geophysical Research, 114, D14) found that 7 +/- 1% of warming in the 20th century was attributable to solar effects.