TCS Daily

The Sun, Cosmic Rays and Our Environment

By Sallie Baliunas - August 16, 2004 12:00 AM

The capability of new instruments to detect fine amounts of matter created high in the Earth's atmosphere by cosmic rays leads to a strange notion -- that some local environmental change on Earth is linked with the fluctuations of cosmic rays over periods of years to millennia.

Cosmic rays are charged particles that travel through the disk of the Galaxy at speeds near that of light. Scientists have observed strong correlations between the fluctuation of these rays and changes in local environments. But an exact description of how cosmic rays might influence ecosystems is missing, leaving forecasts of future local ecosystem change with considerable uncertainty.

The Galaxy's flattened, spinning disk is its main visible feature. It houses not only the Earth, which orbits the Galaxy's central bulge every 250,000 years, but also massive stars. These stars end their lives in cataclysmic supernova detonations, which then vanish from normal view as neutron stars and perhaps pulsars, or even black holes. The massive stars are so rare among the Galaxy's 100 billion stars that a supernova occurs only about once or twice per century.

Energy released by a supernova blast is immediately injected into the material (mostly hydrogen gas) in the volume of space between stars in the Galaxy's disk. A supernova's debris remnant, sometimes shaped in a shell, generates lasting, powerful magnetic fields that twist and accelerate the charged particles to speeds close to the speed of light, thus making cosmic rays. Threaded throughout the disk of the Galaxy, past supernova remnants create a haze of cosmic rays to the solar system that has been relatively steady over many millennia.

Just a fraction of these cosmic rays strike atoms or molecules in the top of the Earth's atmosphere. When they do, they create rare isotopes, radiocarbon (carbon-14) and beryllium-10. Some beryllium-10 is washed out of the air by precipitation and deposited in annual snowfall layers of ice sheets in Greenland and Antarctica. Radiocarbon that joins with oxygen to form carbon dioxide in the air may then be incorporated in tree growth rings as a result of photosynthesis.

Scientists have extracted a record of changes in these rare isotopes over time from corings made into the environmental repositories, with European scientists in the lead in measuring the small amounts of beryllium-10. The records reflect the fact that cosmic ray flux delivered to the top of the earth's atmosphere varies as the Sun's magnetism does. Fewer cosmic rays reach the atmosphere when the Sun's magnetism is strong because the rays, as charged particles, are easily swept aside by magnetic fields. The most famous solar magnetic change is the 11-year sunspot cycle, but more widely-spaced swings appear over centuries and longer.

Records of cosmic ray byproducts from Antarctica and Greenland suggest a pattern of low cosmic ray flux (and high solar magnetism) approximately 800 to 900 years ago, followed by several periods of high cosmic ray flux (low solar magnetism) for the next few centuries, and a return to generally low cosmic ray fluxes (and high solar magnetism) beginning in the 19th Century.

Remarkably, the records of cosmic-ray byproducts sometimes show surprisingly good correlation with independent records of past change in local environments.

  • One example comes from cave formations in southern Arabia, where researchers at Heidelberg and Bern find that periods of smaller calcite growth rings, indicating reduced rainfall during northerly shifts in monsoon patterns, correspond to periods of high amounts of cosmic ray byproducts between 9,900 and 5,500 years ago, the period of their study.

  • Researchers in The Netherlands provide another. They found increases in moisture-thriving plants in bogs in eastern Netherlands from 4,500 to 2,500 B.C., generally during periods of greater concentrations of cosmic ray byproducts.

  • Seafloor sediments from the North Atlantic, signifying sharp cooling periods very roughly every 1,500 years over the last ten thousand years, largely coincide with periods of increased cosmic ray byproducts, according to Columbia University work.

  • And elevated populations of diatoms in Lake Arolik, Alaska, denote periods of greater lake productivity that tend to recur during periods of high cosmic ray flux, according to researchers at the University of Illinois and elsewhere.

Many similar correlations between local environmental indicators and cosmic ray byproduct concentrations have been found, largely owing to advances in measuring technology.

There have been many attempts to explain the good correlations between the dramatic changes in observed local environments and observed production of cosmic ray byproducts. But most amount to guesswork, rather than providing truly satisfactory answers.

One suggestion made has been that cosmic rays striking dust particles in the air might help to trigger clouds to form, perhaps thereby changing precipitation or temperature patterns or both. But how and which types of clouds might be formed, and their effects on climate or other environmental measures, remain too sketchy to conclude anything reliable and quantitative.

Another suggestion involves the varying Sun itself. While the sun's magnetism modulates the incident of cosmic ray flux, at the same time the sun's total energy output is changing, along with particular wavelengths of the sun's energy and fast moving-particles streaming out of the Sun's wind. Total energy output of the Sun has been observed by satellite instruments to vary so slightly over the last two decades, that the change seems too small to create the observed dramatic ecosystem changes. Yet, it may be that some aspects of ecosystems are very sensitive to tiny amounts of some or many of the changes in the solar output, or yet again that small solar changes are amplified by the oceans or clouds.

Currently, though, it is impossible to say with accuracy what causes the many newly discovered correlations between the cosmic ray byproducts found in terrestrial reservoirs and ecosystem changes. That leaves forecasting future ecosystem change with rather large gaps in reliability.


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