The record temperatures in Britain last weekend made global warming feel all too real. Atmospheric carbon dioxide from burning fossil fuels usually gets the blame, and, consequently, alternative, "clean" energy sources such as wind farms, solar panels and fuel cells are the new vogue. But is carbon dioxide really the culprit? Or might the global-warming theory be based on hot air? A small but significant number of scientists are now suggesting that carbon dioxide may not be such a major player, and that in fact the ups and downs in the Earth's climate could be linked to the exploding of far- away stars.
Such a controversial theory requires some explaining. When a massive star reaches its supernova state, it explodes and releases high-energy particles called cosmic rays. Some of these cosmic rays enter the Earth's atmosphere. Two specialists in this field - Nir Shaviv from the Hebrew University of Jerusalem and Jan Veizer from the University of Ottawa in Canada - claim to have found evidence that vari-ations in cosmic rays could be influencing Earth's climate.
Our solar system rotates around the Milky Way, passing in and out of the four spiral arms of stars belonging to the galaxy. When the solar system is inside a spiral arm it receives a high cosmic ray flux (CRF) because it is surrounded by many exploding stars. "The spiral arms move slower than the stars and this creates a stellar 'traffic jam'," explains Shaviv. "Almost all star formation takes place in these stellar traffic jams and so massive stars, which live short lives, get stuck in the jam and die on the spiral arms too."
What Shaviv and Veizer have noticed is that cold periods in the Earth's climate tend to occur at the same time as high CRF relating to the Earth passing through a spiral arm of the Milky Way. "Approximately every 150 million years the Earth has entered a spiral arm of the Milky Way, and there has been a corresponding cold period with more ice at the poles and many ice ages," Shaviv says.
The scientists used a clever technique involving meteorites to measure the changes in the CRF hitting the Earth over the last 545 million years. When a meteorite forms and breaks off from its parent asteroid, its fresh surface is exposed to cosmic rays. These blast into meteorites and break down some of the meteorite atoms, creating lighter elements. The greater the CRF, the greater the accumulation of light elements in the meteorite. Hence, by using pre-existing light element data, measured from meteorites of different ages, Shaviv and Veizer were able to calculate how the CRF has changed over time.
Comparing their CRF record with a corresponding seawater temperature record (computed from chemical changes in fossil shells and corals) revealed a startling correlation between CRF and temperature changes on Earth. Periods of low CRF (when the Earth was outside the spiral arms of the Milky Way) corresponded to warm "greenhouse" times, while periods of high CRF (when the Earth was passing through a spiral arm) corresponded to cooler "icehouse" times.
As yet it is far from clear how cosmic rays can influence the Earth's climate, but one possibility is that cosmic rays affect the cloud cover. When cosmic rays hit the Earth's atmosphere they knock electrons off the atoms that they bump into, creating charged particles. Some recent experiments and computer models suggest that cloud droplets form more easily on these charged particles. "A greater CRF could encourage more cloud formation, especially over the ocean where it is normally difficult for clouds to form," Shaviv says. More clouds (especially low-level, white ones) can help to reduce the global temperature by reflecting solar radiation back into space.
The change in CRF from the Earth passing into a spiral arm of the Milky Way every 150 million years or so helps to explain the long- term ups and …