Nicola Scafetta is an atmospheric scientist with Duke University’s highly regarded Department of Physics, and in a recent article in the Journal of Atmospheric and Solar-Terrestrial Physics, he provides an excellent introduction for us stating “Estimating the solar contribution to global mean air surface temperature change is fundamental for evaluating the anthropogenic contribution to climate change. This is regarded as one of the most important issues of our time. While some theoretical climate model studies indicate that the solar variability has little effect on climate (these studies estimate that less than 10% of the global warming observed since 1900 is due to the sun), several empirical studies suggest that large climatic variations are well synchronized with solar variations and, therefore, climate is quite sensitive to solar changes.”

No doubt about it – a considerable debate rages in the climate community about the role of the sun in controlling the temperature of the earth. There are many leading climate scientists who believe that the sun’s impact is negligible while others believe the sun’s control on earth’s temperature is substantial. Both groups publish regularly, and there is no end of empirical and theoretical evidence to support both camps. If you think the debate is over in the world of climate change, look at the literature on solar control of climate, and you will immediately conclude the debate is as lively as ever.

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The American Geophysical Union (AGU) recently released its new and improved “position statement” on global warming. Andy Revkin of the New York Times featured the AGU’s release on this DotEarth blog site and asked AGU members to chime in on their opinions of the statement that was developed by the AGU’s ruling Council. While there were definitely members who expressed dismay at the position statement, a majority of commentors gave it their hearty endorsement. Apparently, most of the endorsers have not given a very in depth consideration of all that is contained in the AGU’s statement, for otherwise, (we would hope anyway) that they would have been a bit more reserved.

For instance, the AGU’s position statement includes the following sentence: “If this 2 degrees Celsius warming [above 19th century levels] is to be avoided, then our net annual emissions of carbon dioxide must be reduced by more than 50 percent within this century.” This is akin to stating “If pigs had wings, they could fly.” Sure, you could endorse the statement, but to do so would seem a bit foolish. First off, pigs don’t have wings, and it would take nothing short of a miracle for them to acquire them, and secondly, even if they had wings, it is not guaranteed that they could fly. The most pig-shaped bird we can think of—the penguin which is large and rotund and flopping around on its belly a lot of the time—has wings, but can’t fly. Thus even if the impossibility of pigs sporting wings was overcome, it wouldn’t insure a successful flight.

The same is true of the AGU’s statement about a 50% CO2 reduction this century and its impacts on global temperature. First off, it will take nothing short of a miracle for the 50% reduction to take place, and secondly, it probably wouldn’t stop the temperature from rising 2ºC above “natural” levels. Endorse it if you want, but it doesn’t reflect well on your scientific reasoning skills.

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Climate scientists have known for many years that the energy output from the Sun varies and believe it or not, when the Sun is putting out more energy, the Earth heats up and when the Sun cools down, so does the Earth. What appears to be so simple is actually much more complex as the Sun can vary its output differentially in the various portions of the electromagnetic spectrum. For example, the Sun can actually increase its production of gamma rays while decreasing the level of infrared emission, and these patterns of energy output can reveal themselves quite differently in terms of response of Earth’s climate.

Nonetheless, over the past century, the correlation between solar irradiance (in Watts per square meter) and the planetary temperature anomaly (in °C) is highly statistically significant; the Pearson product-moment correlation coefficient is 0.51 and is beyond the 0.99 level of statistical confidence (see below). The line on the plot basically represents a simple statistical linear “model” that predicts the planetary temperature anomaly given a specific output level of the Sun. For every year we could examine the difference between the observed temperature anomaly and the predicted anomaly given solar output – the difference (observed – predicted) is called the residual which should have a mean of zero over the study period. The residuals should fluctuate from year to year, and they should behave in a random fashion.

Scatterplot of annual solar irradiance values (in Watts per square meter) and the annual global near-surface temperature anomalies (in °C) over the period 1910-2003 (from Balling and Sen Roy, 2005)

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Just when you were starting to believe that variations in the amount of energy coming from the sun weren’t responsible for much of the observed surface warming during the past 20 years, comes along a paper in Geophysical Research Letters from two researchers at Duke University, Nicola Scafetta and Bruce West, that concludes otherwise:

We estimate that the sun contributed as much as 45–50% of the 1900–2000 global warming, and 25–35% of the 1980–2000 global warming. These results, while confirming that anthropogenic-added climate forcing might have progressively played a dominant role in climate change during the last century, also suggest that the solar impact on climate change during the same period is significantly stronger than what some theoretical models have predicted.

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That appears to be what is happening, judging from three papers in the May 6 issue of Science.

These three papers argue that the amount of incoming solar radiation reaching the surface of the earth has increased dramatically in the last two decades. While the values vary from paper to paper, in toto the new studies suggest that the increase in solar radiation absorbed at the earth’s surface had almost 10 times as much warming power during that time as the concurrent increases in carbon dioxide, the main global warming gas. Therefore, the warming observed over the past 20 years must have little to do with changes in greenhouse gases.

Before you kill the greenhouse effect, please note that we think this is a lot of hooey. But if you accept these results, that’s where you have to go.
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