Man vs. Milky Way revisited

Published August 1, 2000

The chess-board is the world, the pieces are the phenomena of the universe, the rules of the game are what we call the laws of Nature. The player on the other side is hidden from us. We know that his play is always fair, just, and patient. But also we know, to our cost, that he never overlooks a mistake, or makes the smallest allowance for ignorance.
—T. H. Huxley, A Liberal Education

When you see a cumulus or stratus cloud, do you think of the Milky Way Galaxy? You should, in light of some new cloud observations of interest to all climate and environment watchers.

Clouds can be anything from fog at the surface of the Earth to altocumulus in the middle troposphere or noctilucent clouds in the mesosphere at 90 kilometers (km) above the Earth. One thing all clouds have in common is water, in the form of droplets, ice crystals, or both. The type of cloud, which affects the climate’s energy budget, is determined by the size and amount of its water and ice content.

High clouds (above 8 km altitude) tend to warm the Earth’s surface by absorbing infrared radiation and re-emitting it downward. Low clouds (below 3 to 4 km) tend to cool the surface, for two reasons: They reflect some incoming sunlight, and they emit infrared radiation to higher altitudes where it can escape to space.

Measurements show that, averaged globally, clouds cool by reflecting sunlight and warm by trapping infrared radiation; the cooling is the greater of the two. As changes in cloud area and type occur, the balance of energy in the climate system tips, which may cause climate to change.

Researchers with the International Satellite Cloud Climatology Project (ISCCP) work hard to gather cloud observations from meteorological centers scattered over the world. They group clouds by altitude: low, high, and middle.

The ISCCP’s latest release documents changing cloud coverage from 1983 to 1994. Although high and middle clouds vary in coverage from one year to the next, they do not show the stunning pattern seen for low clouds, which show an 11-year cycle. Astute readers will recognize that 11 is also the magic number for sunspot cycles.

What a coincidence! To emphasize it, we added to the charts the change in flux of high-energy particles from the Galaxy–the cosmic rays. Recall that cosmic rays are ions traveling near the speed of light.

Created continuously by the blast waves of supernovae, cosmic rays result when a massive star explodes with a power so great that it brightens by a factor of 100 million and briefly outshines the combined light of the 100 billion stars in the Galaxy. Supernovae occur once per century or so in the Galaxy, energizing nearby gas into fast ions to create a steady rain of cosmic rays hitting the solar system.

Those cosmic rays are so speedy that they reach deep into the Earth’s atmosphere before disintegrating. They hit the air’s atoms and molecules, spraying neutrons that we have measured since the 1950s. One record, made in Climax, Colorado, at an altitude of 3,400 meters, is shown overlooking the cloud record.

By no coincidence, those neutron counts show an 11-year cycle. The reason is that the sun’s magnetic field, which varies on the same cycle, modulates the flux of incoming cosmic ions to the Earth. When the sun’s magnetism rises, cosmic rays are deflected, so the amount of neutrons produced in the air decreases. Conversely, when the sun’s magnetism weakens, the neutron counts rise because the incoming cosmic ray flux increases.

Given that low cloud coverage shows a surprisingly strong correlation with the neutron counts, cosmic rays are implicated in cloud formation on the Earth.

A similar effect may occur in the methane cloud layer of Neptune. Like the cosmic ray-sunspot link, Neptune’s albedo varies with the 11-year cycle of solar magnetism, suggesting that the flux of cosmic rays actually changes the properties of the methane cloud layer.

On Earth, the changes in low cloud coverage may also be linked to global temperature change. NASA’s satellite record of the lower troposphere temperature reveals an 11-year variation that strikes a familiar note. The temperature is lowest when the cosmic ray flux is highest–and the coverage by low clouds most extensive.

The observed correlations seem to hint at a physical process: cosmic ions, modulated by the sun’s 11-year magnetic cycle, may affect low terrestrial clouds, which may in turn alter the lower tropospheric temperature.

Still, to rule out coincidence, researchers must find the underlying physics of those correlations. But this much is already clear: The strengths of those observed correlations contrast with the poor agreement of the predicted warming trend for the air’s increased carbon dioxide content and tropospheric temperature.

Sallie Baliunas, Ph.D., and Willie Soon, Ph.D., are colleagues at the Harvard-Smithsonian Center for Astrophysics. Their contributions to Environment & Climate News are made possible by the George C. Marshall Institute, Washington, DC, where Baliunas is senior scientist and Soon is a visiting fellow.


Lockwood, G. W., et al.,1991. The brightness, albedo, and temporal variability of Neptune. The Astrophysical Journal, 368, 287-297.

Moses, J. I., M. Allen, and Y. L. Yung, 1989. Neptune’s visual albedo variations over a solar cycle: A pre-Voyager look at ion-induced nucleation and cloud formation in Neptune’s troposphere. Geophysical Research Letters, 16, 1489-1492.

Soon, W., et al., 2000. Variations of solar coronal hole area and terrestrial lower tropospheric air temperature from 1979 to mid-1998: Astronomical forcings of change in Earth’s climate? New Astronomy, 4, 563-579.

Svensmark, H., and E. Friis-Christensen, 1997. Variation of cosmic ray flux and global cloud coverage–A missing link in the solar-climate relationship. Journal of Atmospheric Solar-terrestrial Physics, 59, 1225-1232.