By Dr Paal Brekke from the European Space Agency
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| Satellites now monitor solar activity constantly. |
Natural processes
involving changes in the Sun could have at least as powerful an
effect on global temperature as increased emissions of carbon
dioxide (CO2).
Climate scientists
have already looked at changes related to Sun spot activity -
a cycle of approximately 11 years - and long-term changes in the
Sun's brightness, which has a cycle that lasts for centuries.
They have
discounted the effect of both on the temperature increase over
the last century because they either happen over too short a timescale,
or they are too weak.
But so far
they have omitted to take two other factors into account:
* Changes
in the amount of ultraviolet radiation from the Sun affect the
ozone layer. This is a very important part of the atmosphere where
lots of chemical reactions take place that govern the way the
rest of the atmosphere works;
* The Sun's
magnetic field and solar wind - mainly in the form of electrons
and protons coming out of the Sun - protects the entire Solar
System by acting as a sort of shield from cosmic rays (very energetic
particles and radiation from outer space).
This shield
does not stop all the cosmic rays from getting though, and its
effectiveness varies with the long-term changes in the activity
of the Sun, which can rise and fall on a timescale of centuries.
Cloud cover
One of the
effects that cosmic rays have is to influence how cloudy the Earth
is.
So if the
Sun undergoes long-term changes in activity - which it does -
the amount of cosmic rays reaching the Earth will also vary over
the same timescale, and so will the planet's overall cloudiness.
The amount
of cloud affects the amount of radiation from the Sun reaching
the planet surface, which in turn affects the global temperature.
Data collected
from satellites show that the amount of low clouds over the Earth
closely follows the amount of cosmic rays reaching the Earth.
The resulting
warming due to this effect over the last century could be comparable
to the amount of warming people think has been due to the greenhouse
effect.
Add to that
the other effects due to the Sun, and greenhouse gases become
less than 50% responsible for rising global temperatures.
Little
effect
The other
side of this coin is that reducing greenhouse emissions will have
much less effect in halting rising temperatures than some people
think, and it might have hardly any effect at all.
The energy
emitted from the Sun drives the climate system, and natural changes
in its behaviour can have a far greater effect than human behaviour.
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| Reducing carbon emissions could make no difference
to the climate . |
Thus, some
people may ask: "So why bother worrying about greenhouse
gases, and adding billions to the costs of industry to force them
to cut emissions, when it could well be a pointless exercise?"
If the Sun
is indeed the main contributor to the recent climate change, the
money may be better spent providing clean air in big cities and
clean drinking water to the Third World.
The author
is a solar physicist serving as the European Space Agency's deputy
project scientist for the Esa-Nasa Solar and Heliospheric Observatory
(Soho)
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