University at Buffalo scientists working with ice cores have solved a mystery surrounding sunspots and their effect on climate that has puzzled scientists since they began studying the phenomenon.
The research, published in a paper in the May 15 issue of Geophysical Research Letters, provides striking evidence that sunspots -- blemishes on the sun's surface indicating strong solar activity -- do influence global climate change, but that explosive volcanic eruptions on Earth can completely reverse those influences.
It is the first time that volcanic eruptions have been identified as the atmospheric event responsible for the sudden and baffling reversals that scientists have seen in correlations between sunspots and climate.
"Knowing the mechanisms behind past climate changes is critical to our understanding of possible future changes in climate, such as global warming, and for assessing which of these changes are due to human activities and which arise naturally," explained co-author Michael Stolz, doctoral candidate in the Department of Physics in UB's College of Arts and Sciences.
According to the UB researchers, their work reveals two different mechanisms by which climate is affected by cosmic rays, charged particles that stream toward Earth and which are strongly influenced by solar activity.
"For a long time people have tried to find out whether, for example, periods of maximum sunspots will influence the climate to behave in a certain way," said Michael Ram, Ph.D., professor of physics at UB and co-author on the paper.
"Whenever scientists thought they had discovered something, say, they were seeing a positive correlation between temperature and sunspots, it would continue like that for several years and, all of a sudden, there would be a reversal and, instead, they would start to see a negative correlation," said Ram.
"There seemed to be no consistent relationship between what the sun was doing and what the climate was doing," he said.
To truly confirm any connection between sunspots and climate, a consistent correlation would have to be observed over a long period, covering many solar cycles, Ram explained.
That's what he and his graduate students and co-authors have done with their study of ice cores, long cylinders of ancient ice from Greenland that serve as a frozen archive in that they record climate details from thousands of years ago.
"This is the beauty of working with ice cores," said Ram. "They go back 100,000 years, so we can study how dust concentrations vary along the ice core, reflecting past atmospheric dust concentrations."
Plain old dust, Ram added, holds the key in these experiments because it reflects how dry conditions were in a particular year.
"Dust is a very sensitive parameter of climate," he explained.
Drawing on climate data derived from ice cores obtained through the Greenland Ice Sheet Project 2, (GISP2), the scientists used laser-light scattering techniques to determine the level of dust in the atmosphere for roughly the past 300 years, which is how far back sunspot data have been recorded.
The scientists started out with the assumption that a low level of cosmic rays on Earth resulting from high sunspot activity would lead to less cloud cover and less rain, with resulting high dust levels.
"This was true for the first three or four solar cycles we studied, from about 1930 to 1962, but then the correlation reversed itself, demonstrating that the mechanism couldn't be what we thought," said Ram.
It turned out that during those 32 years of positive sun/dust correlation, there was relatively little explosive volcanic activity worldwide. The researchers found that the same conditions existed between 1860 and 1882. Each of these relatively "quiet" periods came to an end with increased volcanic activity.
For example, in 1883, the Indonesian volcano Krakatau erupted in one of the deadliest volcanic disasters, killing 36,000 people. At exactly the same time, the data started to exhibit low dust concentration whenever there was high sunspot activity, a correlation that violated the scientists' original assumptions.
"By carefully studying the timing of other volcanic eruptions, we found that they coincided with all of the correlation reversals between sunspots and climate," said Ram.
A chart in the paper shows how six major volcanic eruptions between 1800 and 1962 occurred during precisely the same years when there were reversals in the correlation between sunspot activity and climate.
That revelation provided a further insight into how sunspots affect climate.
"All energy comes from the sun, but the change in visible radiation from the sun during any one solar cycle is less than one half of a percent," explained Stolz. "Scientists have said it's impossible that so small a change could influence any signal in the climate. But here we have evidence to show that it's not just radiation energy from the sun that is affecting climate, it's the solar-modulated cosmic rays that have a strong influence because of their impact on cloud cover."
With fewer clouds, and therefore less rain, the scientists reasoned, maximum sunspots should cause levels of atmospheric dust to rise.
"That is true sometimes," said John Donarummo, Jr., UB doctoral candidate in the UB Department of Geology and a co-author on the paper.
But, the researchers discovered, during periods of high volcanic activity, high sunspot activity also results in high levels of atmospheric dust.
According to Donarummo, it long has been known that volcanoes add more dust and more sulfates to the atmosphere.
The UB team discovered that these additional sulfates cause cosmic rays to have a more pronounced effect on Earth by spurring the formation of small droplets in the atmosphere that, in turn, cause the formation of a type of cloud that does not produce rain.
"During these times of high volcanic activity, the sunspot/climate correlation reverses and dust levels rise, even in the absence of high sunspots," explained Stolz.
The work was funded in part by the National Science Foundation.- By Ellen Goldbaum
[Contact: Ellen Goldbaum]