March 14, 2000
In a paper that appears in the February 28 issue of the journal New Astronomy, climatologist Eric Posmentier of Long Island University's Brooklyn Campus, solar physicists Willie Soon and Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics and physicist Pius Okeke of the University of Nigeria chart temperature anomalies seen in the Earth's lower troposphere (i.e., the region of atmosphere in which we live) using Microwave Sounding Unit (MSU) radiometers aboard weather satellites.
The scientists compared the Earth's temperature with the size of coronal holes reported on the Sun during a two-decade period, starting in January 1979 and ending April 1998. Results show a clear drop in terrestrial atmospheric temperature after the Sun's magnetic field activity is most intense. At this point, there is a dropping off of magnetic activity and an enlargement of the coronal holes. "This is the first time anyone has combined these modern, reliable data sets to link solar activity and climate, and to cite several alternative mechanisms that might explain this link," Posmentier explained.
Coronal holes are, literally, gaps in the Sun's outer atmosphere through which the stream of hot, supersonic particles known as the solar wind pours out into space to engulf the entire planetary system. At Earth, this hot bath of charged particles produces the aurorae (i.e., the aurora borealis), interferes with electrical and radio transmissions, and may threaten passengers aboard high-flying airliners or astronauts aboard unshielded spacecraft. The solar wind has also been long suspected as a possible indirect contributor to terrestrial climate change.
Posmentier and colleagues think that the connection between the solar wind and climate may be more direct, suggesting that the charged particles hitting the Earth's atmosphere may affect the properties of terrestrial water clouds, particularly the percentage of those clouds covering the Earth. In turn, significant changes in the cloud cover influence the temperature of the lower troposphere, with temperatures falling with increased cloud cover. Another possibility is that the charged particles change ozone chemistry in the upper atmosphere, in turn affecting the dynamics of the climate.
The scientists note, however, that the charged particles hitting the Earth could come from either the Sun, or from galactic cosmic rays that are modulated by the solar wind. Or, from a combination of both sources. Regardless, the percentage of the Sun's surface covered by coronal holes seems to be a fairly accurate indicator of temperature in the Earth's troposphere over months or years.
The correlation comes with some caveats. As Posmentier and colleagues note, other major climate factors are also at work concurrently, thus complicating attempts to correlate Sun-Earth phenomena. Most notable in the past two decades have been the warming effects of the 1997-98 El Nino and the general cooling that followed the eruption of Mount Pinatubo in 1991.
According to Posmentier, their results do not rule out the possible climate influence of man-made fossil fuels, which have caused the atmosphere's CO2 levels to rise. "During some parts of the last century, as the amount of CO2 increased, the temperature increased," he explained. "I don't dispute that, and I'm not saying that CO2 can't have significant effects in the future.
"What I am saying is the data do not unambiguously support the contention that CO2 increases are the dominant cause of climate variability," he added. "There are other reasons for climate variations that are significant. In fact, we've found that the strongest correlation is the one between the area of the Sun's surface covered with holes and the globally averaged temperature of the Earth."
Support for this research came from the Mount Wilson Institute and the Electric Power Research Institute, with additional funding from the Massachusetts Space Grant Consortium, the Smithsonian Institution, the Richard C. Lounsbery Foundation, and NASA.
Full-text version of the New Astronomy paper.
Goddard Space Flight Center
Greenbelt, Maryland 20771
April 8, 1999
For decades, scientists have tried to understand the link between winds and temperature and the sun and its cycles. There were tell-tale signs of a connection. For instance, the Little Ice Age recorded in Europe between 1550 and 1700 happened during a time of very low solar activity. But how the sun and climate were linked continued to elude researchers.
According to Drew Shindell, a climate researcher from NASA's Goddard Institute for Space Studies in New York, NY, and lead author of the new study, a key piece of the puzzle was missing. Previous studies neglected to take into account the effects of increased solar activity on the ozone layer or the complex chemistry of the upper atmosphere where most of the high-energy radiation, including ultra-violet radiation (the kind responsible for creating the ozone layer) gets absorbed.
"When we added the upper atmosphere's chemistry into our climate model, we found that during a solar maximum major climate changes occur in North America." The changes, according to Shindell, are caused by stronger westerly winds. Changes also occur in wind speeds and directions all over the Earth's surface.
"Solar variability changes the distribution of energy," said Shindell. "Over an 11-year solar cycle, the total amount of energy has not changed very much. But where the energy goes changes as wind speeds and directions change." During the sun's 11-year cycle, from a solar maximum to a solar minimum, the energy released by the sun changes by only about a tenth of a percent.
When the solar cycle is at a maximum, it puts out a larger percentage of high-energy radiation, which increases the amount of ozone in the upper atmosphere. The increased ozone warms the upper atmosphere and the warm air affects winds all the way from the stratosphere (that region of the atmosphere that extends from about 6 to 30 miles high) to the Earth's surface. "The change in wind strength and direction creates different climate patterns around the globe," said Shindell.
According to Shindell, the new study also confirms that changing levels of energy from the sun are not a major cause of global warming.
Many scientists have argued that the radiation change in a solar cycle -- an increase of two to three tenths of a percent over the 20th century -- are not strong enough to account for the observed surface temperature increases. The GISS model agrees that the solar increases do not have the ability to cause large global temperature increases, leading Shindell to conclude that greenhouse gasses are indeed playing the dominant role.
The general circulation model used in the study included solar radiation data from NASA's Upper Atmospheric Research Satellite, launched in 1991. With data from UARS, which was used to calculate ozone changes, scientists have good measurements of how much radiation the sun puts out, increasing the accuracy of the new model.
National Center for Atmospheric Research (NCAR)
FEBRUARY 13, 1998
Harry van Loon, a scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and Karin Labitzke of the Free University of Berlin (FUB) had previously found that a 10- to 12-year oscillation in the stratosphere of the Northern Hemisphere corresponded to four 11-year solar cycles, beginning in 1958. Now, with the help of a vast data reanalysis conducted by NCAR and the National Centers for Environmental Prediction, the two researchers have revealed a mirror image of the solar-stratosphere correlation in the Southern Hemisphere, spanning three solar cycles from 1968 to 1996. NCAR's primary sponsor is the National Science Foundation.
Solar activity cycles from one minimum to the next about every 11 years. During the intervening maximum, explosive activity on the sun intensifies, radiative output increases, and more sunspots are visible on the solar surface. As the measure of this cycle, van Loon and Labitzke used the flux in the 10.7-centimeter radio waveband, an objectively observed quantity highly correlated with the 11-year cycle. They compared these radio data with FUB's daily analyses of the stratosphere. The results show a strong correlation between the solar cycle and the 10- to 12-year oscillation of the lower stratosphere's mean temperatures and constant pressure heights above sea level.
"The emergence of a correlation in the Southern Hemisphere similar to that in the Northern Hemisphere has increased our confidence that the solar-stratospheric relationship is more than a statistical coincidence," says van Loon.
For many years scientists have tried to find an earthly link to the sun's 11-year cycle. Previous attempts have turned up humorous correspondences to the number of Republicans in the House of Representatives and the length of women's skirts. Until van Loon and Labitzke's research on the stratosphere, even serious scientific stabs at the problem eventually proved false. A solid link takes on added significance now as scientists search for a clear sun-earth connection for computer models used to predict climate change.
"The role of the sun in climate change is still an unsolved problem," says van Loon. "Any relationship between changes in solar output and what happens here on earth is important for understanding long-term climate."
The sun's output has varied about 0.1% over one solar cycle during the past several decades. Over centuries, however, larger variations may occur. For example, an extended quiet period on the sun may have chilled the earth during the Little Ice Age between the mid-1550s and mid-1800s. During the long, severe winters and short, wet summers of that period, alpine glaciers advanced down river canyons, Dutch canals froze over, and farming became difficult farther north.
Van Loon and Labitzke found that the highest correlations of the stratosphere's pressure heights with the solar cycle are concentrated in two well-defined latitude zones, which move from lower latitudes in winter to higher latitudes in summer, thus tracking the sun's interseasonal journey.
The annual mean temperatures of the lower stratosphere are well correlated with the solar cycle in the summer of either hemisphere, but only weakly correlated in winter. That is, during the summer months in either hemisphere, the average temperatures of the lower stratosphere rise and fall with the waxing and waning of the sun's energy output over its 11-year cycle.
NCAR is managed by the University Corporation for Atmospheric Research (UCAR). UCAR is a consortium of more than 60 universities offering Ph.D.s in the atmospheric or related sciences.
National Center for Atmospheric Research (NCAR)
Zhenya Gallon
FEBRUARY 13, 1998
Fox presented the findings of his research (with NCAR's Oran White, Juan Fontenla of GTE, and Eugene Avrett of the Smithsonian Astrophysical Observatory) at the annual meeting of the American Association for the Advancement of Science, in Philadelphia on February 14.
Previous methods relied on a single parameter estimate of the sun's activity (the flux in the 10.7-centimeter radio waveband) to represent radiative changes in the sun. Those methods are based on correlations between the sun's activity and radiative variations. "However," explains Fox, "the physical connections that the correlations imply have not been established; these relations are purely statistical. So the use of the parametric representation has no physical basis -- it's an indirect relationship."
The NCAR team's breakthrough is the ability to represent more accurately the physical processes of the entire surface of the sun. They do this by taking a detailed look at how individual features, like sunspots, change the amounts of radiation in several different wavelengths. This allows them to estimate all of the radiation reaching the earth over several different wavelengths as the physical state of the sun continually changes.
The team uses observations from the precision solar photometric telescope (PSPT), combined with computer modeling that incorporates prior observations and solar physics theory. The PSPT is operated by NCAR's High Altitude Observatory at its Mauna Loa Solar Observatory in Hawaii. The PSPT project is part of the National Science Foundation's Radiative Input from the Sun to the Earth (SunRISE) program.
"Tree rings and ice cores tell us a little about variations in the sun over centuries. But we have only the 11-year sunspot record for the last 300 years to link directly to today's sun, and we don't have direct measurements of the radiation before the 1970s," Fox explains. New information from the team will help scientists understand the effects of solar variability on the chemistry of the upper atmosphere, including the ozone layer, the shield protecting the earth from too much ultraviolet radiation. The new techniques will also provide better information to scientists modeling the earth's climate by providing more accurate data about the sun's infrared radiation. In addition, the green and yellow bands in the visible light spectrum are now believed to affect ocean circulation. The team expects to make the first accurate estimates of solar radiation in those bands, adding important information on the link between the oceans and the atmosphere.
NCAR is managed by the University Corporation for Atmospheric Research (UCAR). UCAR is a consortium of more than 60 universities offering Ph.D.s in the atmospheric or related sciences.
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