New study finds sun’s contribution to recent warming is ‘negligible’

Earth to deniers — global warming is caused by human emissions, not solar activity.

The Naval Research Laboratory and NASA report that, “if anything,” the sun contributed “a very slight overall cooling in the past 25 years.” D’oh! The study ($ub. req’d), “How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006,” finds:

Empirical models that combine natural and anthropogenic influences (at appropriate lags) capture 76% of the variance in the monthly global surface temperature record, suggesting that much of the variability arises from processes that can be identified and their impact on the global surface temperature quantified by direct linear association with the observations.

Natural influences produce as much as 0.2 K warming during major ENSO events, near 0.3 K cooling following large volcanic eruptions and 0.1 K warming near maxima of recent solar cycles. To properly quantify their amplitudes, the natural and anthropogenic changes must be accounted for simultaneously when analyzing the surface temperature anomalies, since neglecting the influence of one can overestimate the influence of another. For this reason, we suggest that estimated solar cycle changes of 0.2 K and Pinatubo cooling of 0.4 K are too large.

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None of the natural processes can account for the overall warming trend in global surface temperatures. In the 100 years from 1905 to 2005, the temperature trends produce by all three natural influences are at least an order of magnitude smaller than the observed surface temperature trend reported by IPCC [2007]. According to this analysis, solar forcing contributed negligible long-term warming in the past 25 years and 10% of the warming in the past 100 years …

Here are some excellent visual “reconstructions of the contributions to monthly mean global surface temperatures by individual natural and anthropogenic influences (at appropriate lags) are shown”:

The right hand ordinates give the native scales of each influence and the left hand ordinates give the corresponding temperature change determined from the multiple regression analysis. The gray lines are trends for the whole interval. The inset in Figure 2d shows the individual greenhouse gases, tropospheric aerosols and the land surface plus snow albedo components that combine to give the net anthropogenic forcing.

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What is significant about this analysis is that it is based on observations:

To distinguish between simultaneous natural and anthropogenic impacts on surface temperature, regionally as well as globally, we perform a robust multivariate analysis using the best available estimates of each together with the observed surface temperature record from 1889 to 2006. The results enable us to compare, for the first time from observations, the geographical distributions of responses to individual influences consistent with their global impacts.

Again, this is not really a big surprise to those who follow the scientific literature. A major 2007 study concluded [PDF]:

Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

Since the notion that changes in some spots or solar output has been the primary driver — or even a large component — of recent warming remains perhaps the biggest myth pushed by deniers, let’s look at some more analysis. The excellent debunking website, Skeptical Science, has a nice post on this. They explain that “The study most quoted by skeptics actually concluded the sun can’t be causing global warming.” Double D’oh!

And they list a bunch of other studies dismissing or minimizing the sun’s contribution:

• Ammann 2007 [PDF]: “Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century.”
• Foukal 2006 [PDF] concludes “The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years.”
• Usoskin 2005 [PDF] concludes “during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source.”
• Stott 2003 [PDF] increased climate model sensitivity to solar forcing and still found “most warming over the last 50 yr is likely to have been caused by increases in greenhouse gases.”
• Solanki 2003 [PDF] concludes “the Sun has contributed less than 30% of the global warming since 1970.”
• Lean 1999 concludes “it is unlikely that Sun-climate relationships can account for much of the warming since 1970.”
• Waple 1999 finds “little evidence to suggest that changes in irradiance are having a large impact on the current warming trend.”
• Frolich 1998 [PDF] concludes “solar radiative output trends contributed little of the 0.2°C increase in the global mean surface temperature in the past decade”

Only a pagan would continue to worship at the altar of the sun-explains-everything religion.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.