[Paleopsych] Independent: When the Sun lost its heat

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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

    29 September 2004

    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

    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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