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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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