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. Climate: The Vanishing Solar Factor

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by Dan Whipple
Boulder CO (UPI) Jul 26, 2004
It should be no mystery to anyone who has ever been outdoors that the sun influences the climate. But recent scientific evidence indicates solar forcing of climate change actually may be less than previously thought.

Conventional wisdom follows the line pursued in a recent article in the London Daily Telegraph, headlined, Hotter-burning sun warming the planet. The story quoted Sami Solanki, director of the Max Planck Institute for Solar System Research in Gottingen, Germany.

The sun has been at its strongest over the past 60 years and may now be affecting global temperatures, Solanki told the newspaper. This offers a possible explanation for global warming that needs to be weighed when proceeding with expensive efforts to cut emissions of greenhouse emissions, the Telegraph story said.

Solanki himself only partially subscribes to this conclusion, however.

In the 2002 Harold Jeffreys Lecture to the Royal Astronomical Society in London, Solanki said: After 1980, however, the Earth's temperature exhibits a remarkably steep rise, while the sun's irradiance displays at the most a weak secular trend. Hence the sun cannot be the dominant source of this latest temperature increase, with man-made greenhouse gases being the likely dominant alternative.

This line of reasoning found more support in a 2003 paper that analyzed beryllium-10 concentrations in ice cores.

As found by Solanki and Ilya G. Usoskin of the Sodankyla Geophysical Observatory at the University of Oulu in Finland: The reconstruction shows reliably that the period of high solar activity during the last 60 years is unique throughout the past 1,150 years. The current high level of solar activity may also have an impact on the terrestrial climate, the authors conclude.

We note a general similarity between our long-term sunspot number reconstruction and different reconstructions of temperature, the pair wrote, although they did not estimate how much of the observed increase in global temperature might be accounted for by solar activity.

Beryllium-10 -- or 10Be, as it is commonly known -- is an isotope of the metal produced by the effects of cosmic radiation in the atmosphere.

In the absence of any cosmic ray flux, this production rate should be constant, Caspar Ammann, a paleoclimatologist at the National Center for Atmospheric Research in Boulder, told United Press International. The place to find 10Be is in ice cores.

When the sun is very active, producing a lot of sunspots -- which it does in roughly 11 years cycles -- it forms a kind of windshield across the Earth, reducing the number of cosmic rays reaching the surface. Fewer cosmic rays means less 10Be in the ice cores. It functions as a tidy proxy for solar activity.

The Usoskin and Solanki paper shows a neat graph with a sharp spike in a Greenland ice core that seems to indicate stronger solar activity over the last 60 years than any time in the last 1,200 years or so.

The devil, as they say, mucks around in the details, however. In the first place, the Solanki-Usoskin paper relies on a single ice core record from Greenland and one from the Antarctic.

The reconstruction based on 10Be from Southern Greenland is in my opinion very problematic, Raimund Muscheler, a geologist at Lund University in Sweden and an expert on cosmic ray-generated isotopes -- also known as cosmogenic radionucleides -- told UPI.

(Beryllium 10) is not only influenced by changes in solar activity and geomagnetic field changes, he continued, but it can also be influenced by local effect due to changes in atmospheric transport and deposition.

The Greenland record, for example, does not always agree with 10Be from Antarctica, as well as with certain tree ring records, Muscheler explained. He added that Usoskin and Solanki did not use the last 100 years of the South Pole record.

In other words, the beryllium proxy is not clean and foolproof. There may already be a signal from a warming climate included.

My conclusion about past solar activity based on radionucleide records would be the following: Solar activity was relatively high during the last 50 years, but there were similar periods during the last 1,000 years, Muscheler said.

Ammann added: If you would take averages of all the ice cores, you would not get this increase in (solar activity) in the last 50 years, but it would stay relatively flat. It is only one core that shows the rise. This is not the common feature of all of them.

A second fundamental problem is new research is showing that many long-held assumptions about solar forcing of the climate might be incorrect. Following this part of the story requires going back to the Maunder Minimum, a period of virtually no sunspot activity from 1645 to 1715, when Earth was undergoing a little ice age.

The sun has an 11-year cycle, in which it goes from zero sunspots (and its lowest energy) to about 150 sunspots, on average, at its highest energy. During the Maunder Minimum, this cycle essentially disappeared for 70 years, and the sun just floated out there at its minimum energy output.

People say that during the Maunder Minimum, the sun must have been very unusual, because we had a cold climate around the world, Ammann said. They suggested that we have a lower energy coming from the sun.

A change in the lowered energy of the sun at the Earth's surface is very small and very hard to measure. The sun's energy is expressed as watts per meter squared, and the solar constant, as it is called, is 1,367 watts per meter squared. The variation has been measured at about 0.1 percent -- one thousandth -- or 1.36 watts per meter squared.

In order for the sun to force the climate to the little ice age observed during the Maunder Minimum, the change in the solar constant had to be about twice what has been observed during modern, zero-sunspot periods. In other words, the zero of the Maunder Minimum has to be lower than a modern zero at the bottom of the 11-year solar cycle. Climate models that include solar forcing do, in fact, include this larger influence, making them more sensitive to solar forcing.

In the 1970s, astronomers observed about 10 sun-like stars, six of which had 11-year cycles like the sun's, and four of which did not. Based on this small sample, they concluded the assumption about the low-energy Maunder Minimum was reasonable.

Since then, however, astronomers have surveyed many more sun-like stars, and recent work indicates the energy output at the low end of the solar cycle does not warrant this assumption.

In another line of research that supports this conclusion, Peter Foukal of Heliophysics Inc., compared sunspot cycles and ultraviolet irradiance. He found a factor (three times to five times) lower than expected to produce a significant global warming contribution based on present-day climate model sensitivities.

For the 20th Century, Ammann said, there should be a pretty big change based on the earlier assumption. But they raise significant doubts on the existence of any low-frequency additional change. We have no physical basis to expect that the zeros (of the Maunder Minimum) are any different than the zeros that we measure today.

Ammann has conducted numerous experiments on his own climate models and found they are, if anything, oversensitive to solar forcing, based on what now known about the observed physical processes.

It is making a relatively small forcing even smaller, he said.

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