- 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.
-
- ©2004 Independent Digital (UK) Ltd. All rights
reserved
-
- http://news.independent.co.uk/world/
- science_technology/story.jsp?story=566685
|
