When the Sun lost its heat

Locked away in fossils is evidence of a sudden solar cooling. Kate Ravilious meets the experts who say it could explain a 3,000-year-old mass migration - and today's global warming

Just under 3,000 years ago, a group of horse-riding nomads, known as the Scythians, started to venture east and west across the Russian steppes. At about the same time, African farmers began to explore their continent, and Dutch farmers abandoned their land and moved east. All over the world people became restless and started to move - but why? Archaeologists have never found a clear answer, but now one scientist thinks the explanation may lie on the surface of the Sun.

Just under 3,000 years ago, a group of horse-riding nomads, known as the Scythians, started to venture east and west across the Russian steppes. At about the same time, African farmers began to explore their continent, and Dutch farmers abandoned their land and moved east. All over the world people became restless and started to move - but why? Archaeologists have never found a clear answer, but now one scientist thinks the explanation may lie on the surface of the Sun.

Bas van Geel, a biologist from the University of Amsterdam, believes that the Earth's climate took a dramatic turn about 2,800 years ago, due to a quiet period in the Sun's activity, making the tropics drier and the mid-latitudes colder and wetter. Previously damp areas, like parts of the Netherlands, became flooded and uninhabitable, while very dry, desert-like areas, such as southern Siberia, became viable places to live. Meanwhile, in the tropics, land dried out and created savannahs where lush forests had grown before. "People living where the changes were most dramatic were forced to move," he explains.

Until now, climate scientists haven't taken too much notice of the changes in the activity of the Sun, believing them to be small fry compared with the effects of greenhouse gases and wobbles in the Earth's orbit. But now a growing number of scientists are convinced that fluctuations in the activity on the Sun's surface (such as flares, sunspots and gas boiling off) may be amplified, causing significant changes to the Earth's climate. Van Geel has gathered evidence that supports the idea that such solar activity is an important influence on our climate, and he has also shown how people are affected when the Sun decides to have a snooze.

Over the past 10 years, van Geel and his colleagues have been studying fossil plants in peats and muds from all over the world. They have been measuring carbon 14, the heaviest isotope of carbon, which is used to date things. Carbon 14 is created in the atmosphere when high-energy cosmic rays smash into nitrogen atoms. Carbon 14 atoms then team up with oxygen and become radioactive carbon dioxide, which is then absorbed by all living things. Once the plant or animal dies it stops interchanging its carbon with the atmosphere and, over time, the carbon 14 decays. Because scientists know approximately how quickly carbon 14 decays they can work out how old an object is. But this isn't the whole story.

The level of carbon 14 in the atmosphere varies according to how many cosmic rays are bombarding the Earth. When the Sun is very active, cosmic rays are deflected by the strong solar wind. This means that as well as indicating how old something is, carbon 14 can give scientists an idea of how intense the cosmic ray flux was. And this is just what van Geel has been using carbon 14 for. By measuring the detailed variations of the isotope of carbon at different levels in peat deposits, he can estimate the ups and downs in the intensity of the cosmic rays hitting the Earth at the particular time that the peat was formed from dead plant matter in wetlands.

"I use the carbon 14 as an indicator of solar activity because an increase in it means an increase in the cosmic ray flux and, therefore, a decrease in solar activity," he explains.

He has shown that, about 2,800 years ago, there was an abrupt, worldwide, increase in carbon 14 levels, which occurred at the same time as climate change. He believes the increase in carbon 14 means that solar activity suddenly declined. But how can little blips on the Sun's surface have such a drastic effect on the Earth's climate?

Proponents of the solar activity theory have come up with two possible mechanisms that might be transmitting the effects of fluctuations in activity on the Sun's surface.

The first is that changes in solar activity alter the level of cosmic rays hitting the Earth, which influences cloud formation. Clouds affect climate by altering the amount of sunlight reflected back into space, and by varying the level of rainfall.

Alternatively, changes in solar activity affect the amount of ultra-violet radiation leaving the Sun, which may have an impact on the amount of ozone created in the higher levels of the atmosphere. Ozone influences how much solar energy is absorbed by the atmosphere, and, indirectly, affects atmospheric circulation and associated weather.

Teaming up with archaeologists has enabled van Geel to back up his theory by showing that many people were migrating at this time. Along with Dutch specialists, he has found that farming communities in west Friesland suffered increasing rainfall about 2,800 years ago. They resorted to building homes on artificial mounds, but eventually they were washed out of their farms and had to move to drier places. Meanwhile, work in Cameroon has shown that there was an arid crisis that started at about the same time. This dry patch caused some of the forest to die and savannahs to open up. These openings in the forest made it easier for people to move. Archaeological remains show that farming communities began to migrate inland.

Most recently he has worked with Russian archaeologists to show that, also about 2,800 years ago, the Scythian people took advantage of a wetter climate to explore east and west across the steppe landscapes that lie north of Mongolia. Prior to this, the land had been hostile semi-desert, but the extra moisture turned it into green, grassy steppes, enabling these nomadic tribes to travel towards both China and south-east Europe.

Without a doubt there was a change in climate about 2,800 years ago, and it seems that this encouraged, or even forced, many groups of people to move. But was this a one-off change, or has solar activity played havoc with the climate at other times, too?

"Carbon 14 records show a major decrease in solar activity roughly every 2,300 years," says van Geel. "The most recent time this happened was during the 'little ice age', which peaked around 1650." At this time frost fairs were held on the Thames, harvests were poor all over Europe and glaciers marched down mountains.

Taking a look at the Sun right now reveals that we are in a period of high activity, with many sunspots, solar flares and an increasing magnetic field of the corona (the Sun's outer atmosphere). Van Geel and other proponents of the solar activity theory believe this high solar activity could be behind the global warming we have experienced over the last 50 years. "My impression is that there is an over-estimation of the greenhouse effect," says van Geel. It is controversial, but if he is right, then there is little we can do to control the Earth's climate. Instead, we can make the most of the sunshine and, perhaps, start preparing for the next chill in western Europe - due to peak about AD3950.

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