Blame it on the supernova

Is this long hot spell a sign of climate change caused by greenhouse gases? Not necessarily. Scientists are looking into space for an explanation, and in particular at exploding stars that rain cosmic rays on to the Earth.

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 downs in the Earth's climate. One hundred million years ago the dinosaurs basked in tropical sunshine in England and alligators were roaming Alaska. They were experiencing a warm "greenhouse" period while the Earth travelled between spiral arms of the Milky Way. More recently we have suffered a colder, "icehouse" period as the Earth has travelled through the Sagittarius-Carina spiral arm. Ice sheets have fingered down from the North Pole as far south as Italy at times, and creatures such as the woolly mammoth have inhabited the British Isles.

Unfortunately, the cold patch isn't quite over yet. "In principle, we should be witnessing a warmer climate now because the Earth has left the Sagittarius-Carina arm, but in fact we also had to pass through the Orion arm, a temporary little spur sticking out of the Sagittarius-Carina arm," Shaviv explains. We are now leaving the Orion arm but it will be a few million years before we start really to experience a "greenhouse" climate again. This greenhouse period is due to last for 50 to 60 million years before we enter the next spiral arm, Perseus, and go back to "icehouse" conditions.

In the meantime, another short ice age is still a possibility. "On the time scale of tens to thousands of years, the appearance of ice ages is probably not due to CRF, but instead due to wobbles in the Earth's orbit around the Sun, known as Milankovitch cycles," Shaviv says. These wobbles change the amount of solar radiation reaching the Earth, and combined with a high CRF they can cause ice ages lasting for tens of thousands of years.

So where does carbon dioxide fit into all this? And do we really need to worry about man-made increases in atmospheric carbon dioxide? There is no doubt that carbon dioxide is a "greenhouse" gas and that it does have a warming effect, but it has always puzzled scientists that levels of atmospheric carbon dioxide often seem to lag centuries behind the temperature changes. In short, it is not clear whether the carbon dioxide is driving the climate or the climate is driving the changes in carbon dioxide.

Shaviv and Veizer believe that carbon dioxide does have a modulating effect on climate, but that it is not as extreme as many climatologists have predicted. "Clearly we are affecting climate, and emissions from fossil fuels are probably still responsible for about a third of the global warming," Shaviv says. "The other two-thirds can probably be explained by an increase in activity from our Sun."

When the Sun is active, it produces a strong solar wind. Shaviv and Veizer propose that on short timescales of days to millennia, the changes in solar activity can alter the CRF reaching the Earth, and this causes small changes in climate. "The cosmic rays from distant stars lose energy when they encounter the solar wind, just like salmon that get tired when they swim upstream," Shaviv explains. When the Sun is more active and the solar wind is stronger, fewer cosmic rays reach the Earth. During the last century, the solar wind has become stronger and we are experiencing warm temperatures on Earth. "Quite soon the solar wind is likely to decrease, meaning that more cosmic rays will reach the Earth and the temperature will decrease," Shaviv says.

Contrary to the global-warming scenario, Shaviv and Veizer are forecasting quite chilly weather for the next few hundreds to thousands of years. Wobbles in the Earth's orbit may give us more ice ages before we leave the Orion arm of the Milky Way. Glaciers will inch their way down from the poles and our descendants will probably want to move towards the equator.

Within our own lifetimes, we may also experience another cold snap owing to a decrease in the solar wind. In the late 17th century, the Thames used to freeze over during the winter and people held "frost fairs" on the ice. Within a few decades, or perhaps a century or two, we may well be holding frost fairs again, although it will take a lot more evidence from Shaviv and Veizer to convince the majority of climatologists, who are still convinced that rising carbon-dioxide levels are behind the global warming recorded over recent years.