快猫短视频

OUR WORLD is ruled by the Sun. Its life-giving energy warms the Earth, fuels
our crops and drives the weather. This much we can take for granted. But now
signs are emerging that the Sun has a more subtle influence, too, which arises
from regular changes in its activity. Variations in the amount of energy
streaming our way seem to influence the climate in myriad ways.

But there is a puzzle here. During these fluctuations, which typically vary
over an 11-year cycle, the energy emitted changes by a tiny 0.1 per cent. From
cycle to cycle, the differences are even smaller. Yet evidence is mounting that
these minuscule variations lead to measurable changes in climate around the
world. 鈥淚t is not clear how such small changes can be responsible for the
fluctuations in climate,鈥� says Joanna Haigh, an atmospheric physicist at
Imperial College, London.

Something must be amplifying the small variation in energy coming from the
Sun, but what? And perhaps more importantly, by how much? Global average
temperatures have risen by around 0.6 掳C over the past century, and until
recently human activity has been fingered as the main culprit. But now a few
climatologists have begun to suggest that the Sun鈥檚 influence could be much
greater鈥攂ig enough to swamp that of greenhouse gases and let human
polluters off the hook. This led the British journalist Nigel Calder to conclude
in his book The Manic Sun, published last year, that there is 鈥渓ittle
scope for any contribution [to the recent warming] from greenhouse warming鈥�. Or,
as The Observer newspaper put it in April, humans are 鈥渘ot to blame鈥�
for global warming. So is this hype, mischief-making or the climate breakthrough
of the decade?

快猫短视频s have been looking for a link between climate and changes in the
Sun for more than 200 years. The latest claim has its origin in a paper
published in 1991 by Eigil Friis-Christensen and Knud Lassen of the Danish
Meteorological Institute in which they compared solar activity and global
temperatures over the past 250 years. The researchers uncovered a striking
correlation over the past century between rising temperatures and the length of
the solar cycle. This has been steadily decreasing and is now several months
shorter than it was at the end of the 19th century. Astrophysicists agree that
shorter solar cycles imply more solar activity, which in turn means that more
solar radiation reaches the Earth. The conclusion seemed inescapable: changes in
the Sun were causing warming on Earth.

However, there were some concerns about a graph Friis-Christensen and Lassen
used, which compared temperature with the length of the solar cycle. Most
scientists do not use the length of the solar cycle as an indicator of the Sun鈥檚
activity. A plot in the same paper comparing temperature with the number of
sunspots was far less successful, especially for the period since 1940. While
the solar cycle length has followed the temperature鈥攄ropping in
mid-century and then rising again as temperatures have hit new records鈥攖he
trend in sunspot numbers has been downward since 1960.

Hotting up

Nevertheless, Friis-Christensen and Lassen鈥檚 work caused a flurry of concern
among greenhouse scientists in the run-up to the Earth Summit of 1992, when the
world鈥檚 governments called for action to cut emissions of greenhouse gases. That
year Tom Wigley, then of the Climatic Research Unit at the University of East
Anglia, and his colleague Mick Kelly tried to work out how much of the world鈥檚
warming was due to a more active Sun, and how much to greenhouse effects. Using
computer models, the researchers looked at what happened when they gave various
weightings to the effects of solar activity and greenhouse gases. They found
that they obtained the best match with global temperatures recorded over the
past 200 years when 100 per cent of the change was attributed to the Sun, and
none to the greenhouse gases. But the match was almost as good for the same
period if they ignored solar changes altogether and just considered the
greenhouse effects. For both these results they had to make assumptions about
the strength of these effects which few scientists would consider reasonable. If
they kept to more realistic levels of amplification, they found that greenhouse
gases explained most of the recent warming.

Undeterred, Friis-Christensen returned to the fray last summer, along with
Henrik Svensmark, also from the Danish Meteorological Institute. This time, they
had a mechanism to explain why tiny changes in the Sun could have big effects on
Earth. It was all down to an intriguing correlation they spotted between cloud
cover in the tropics and the number of small but very energetic particles called
cosmic rays reaching the Earth from deep space.

To explain this correlation, the two researchers revived an idea proposed 20
years ago, that cosmic rays may encourage clouds to form in the Earth鈥檚
atmosphere. More cloud might reflect more of the Sun鈥檚 energy back into space,
leading to a cooler world. The link between cosmic rays and solar activity is
the solar wind, the charged particles streaming from the Sun. The solar wind is
known to push cosmic rays aside, preventing them from bombarding the Earth. So
the more active the Sun is, the stronger the solar wind, leading to fewer cosmic
rays and a drop in the cloud cover. Result: global warming. Or so the reasoning
goes.

The Danish team鈥檚 new graph certainly suggests a link between solar activity,
cosmic rays and cloud cover. Satellite data show changes of 3 to 4 per cent in
average cloud cover over the tropics from 1980 to 1995, in step with monthly
figures for the intensity of cosmic rays arriving at the Earth
(see Diagram).

Intriguing as this correlation is, it is far from proof that solar activity
and cloud cover are connected. Friis-Christensen and Lassen used data for cosmic
rays covering only 15 years, which is not even one-and-a-half solar cycles, and
their figures for cloud cover spanned only seven years. Such a short time period
makes it hard to rule out the possibility that the correlation is simply a
matter of chance, and that something else caused the cloud changes.

For instance, climate scientist Keith Shine from the University of Reading,
points out that the cloud data begin shortly after one major volcanic eruption,
from El Chich贸n in Mexico, and end just before another, Pinatubo in the
Philippines. Both eruptions poured large amounts of debris into the upper
atmosphere and had a worldwide impact on climate. To add to the confusion, the
peak period of cloud cover, in 1987, also coincided with a strong El Ni帽o
event. This redistribution of warm water in the tropical Pacific Ocean is the
largest natural perturbation of the tropical climate. Researchers from Caltech
are about to publish a paper showing that the data on cloud cover in the 1980s
correlate as well with El Ni帽o as they do with cosmic rays.

Clouding the issue

In last year鈥檚 paper, Friis-Christensen also attempted to extend his data for
cloud cover back to 1975, and forwards to 1995. He found that the correlation
with cosmic rays remained, while the correlation with the other sources like El
Ni帽o decreased drastically. However, critics point out that the extra
data are from three separate satellite sources and relate only to oceans in the
southern hemisphere. Friis-Christensen himself admits that 鈥渁 detailed
comparison of absolute levels is difficult鈥�.

There are other uncertainties with the cosmic ray theory, not least the fact
that no one knows exactly why these particles from space should affect the
Earth鈥檚 cloud cover at all. There are several competing ideas. Friis-Christensen
and Lassen suggest that cosmic rays create an electrical charge in tiny
particles in the atmosphere that act as nuclei for the formation of cloud
droplets. The charge would make the particles much more effective at seeding
clouds. Brian Tinsley, a British researcher now working at the University of
Texas in Dallas, has an alternative idea. He believes that electric fields
caused by cosmic rays could encourage water droplets to freeze. These would then
act as nuclei for the formation of cloud droplets.

Whether either of these mechanisms would create enough cloud to affect the
climate remains to be seen. 鈥淲e don鈥檛 know whether the effect is large enough,鈥�
says Friis-Christensen. Even assuming that cosmic rays do produce clouds, it is
not certain whether they would actually cool the world. Clouds at low altitude
do tend to reflect more sunlight, and so have a cooling effect. But
higher-altitude clouds generally have the opposite effect, trapping heat and
warming the atmosphere.

Jasper Kirkby and Frank Close of CERN, the European Laboratory for Particle
Physics in Switzerland, hope to perform experiments that will answer at least
some of these questions. Their idea is to re-create the conditions of the upper
atmosphere in a cloud chamber鈥攁 piece of kit common in particle physics
labs. They would fill the chamber with mixtures of gases and cloud-forming
nuclei characteristic of different heights in the atmosphere and then blast them
with CERN particle beams, which stand in for cosmic rays. By seeing whether
clouds form, and if so what kind and by what mechanism, they hope to provide
data for climatologists to plug into their global climate models. Ultimately
Kirkby believes the experiment could tell us how important a contribution cosmic
rays have made to global warming in the past century.

Testing time

It is still early days. 鈥淭his is not even an official proposal to CERN yet.
It is just an idea,鈥� says Kirkby. Friis-Christensen believes their experiment
could work. But Giles Harrison, a leading expert on atmospheric electricity from
the University of Reading, doubts whether cloud chamber experiments will ever
clinch the argument. 鈥淚t would be hard to generate microphysical properties
genuinely typical of the atmospheric conditions before a cloud forms.鈥� One
problem, he says, is the massive variability of electrical conditions in the
atmosphere, even in a clear sky. Harrison plans his own experiments using
weather balloons.

While Kirkby agrees that measurements in the real atmosphere are important,
he points to the greater control that is possible with cloud chamber
experiments. 鈥淲e hope to be able to switch things on and off to see what
happens. Do clouds form and disappear?鈥� he says. And he insists that the
experiment will be able to model the atmosphere effectively. 鈥淚t is just air and
water vapour and aerosols.鈥�

Some greenhouse scientists are angered by all this interest in solar forces.
They know that industry lobbyists are keen to promote ideas of this kind, as a
way of blocking action to limit emissions of known greenhouse gases. And while
Shine says the solar work is interesting and worth pursuing he, too, despairs at
commentators who leap from conjecture to conclusion without waiting for research
to confirm or refute them. And he insists that whatever the effect on global
climate of changes in the Sun鈥檚 activity, it does not undermine the role of
greenhouse gases. The greenhouse effect is 鈥渉ard, repeatable science鈥�, he says.
鈥淲e know that human activity has increased the concentrations of these molecules
and has enhanced the greenhouse effect.鈥�

Most solar scientists side with Shine. Even Friis-Christensen does not insist
that his proposed cosmic ray effect is the only force for global climate change.
鈥淚 cannot claim that our ideas disprove the greenhouse theory,鈥� he says. 鈥淏oth
mechanisms may work together. Greenhouse gases could have played a significant
role.鈥� Kirkby agrees: 鈥淭here is certainly a greenhouse effect,鈥� he says. 鈥淭he
question is whether it is responsible for all the 0.6 掳C warming in the past
century, or two-thirds or a fifth鈥攐r what?鈥�

Tinsley thinks a slow increase in solar activity was largely to blame for the
0.3 掳C rise in global temperatures between 1890 and 1970. But he says the
further 0.3 掳C warming since then must have been caused by something else.
鈥淭here are good theoretical reasons for expecting greenhouse gas effects to
become obvious in the early 21st century,鈥� he warns.

Wigley, now at the National Center for Atmospheric Research in Boulder,
Colorado, has also been refining his estimates of the relative contributions of
the two effects. His latest computer model suggests that the solar influence on
global warming between 1890 and 1997 was 20 to 40 per cent. He says his models
can re-create the rapid warming between 1910 and 1940, the levelling off until
the end of the 1960s, and the warming since then, but only by including solar
effects.

It seems a new consensus could be emerging. The influence of the Sun and of
greenhouse gases marching hand in hand appears to account for the past century鈥檚
slow rise in global temperature. But greenhouse gases could well continue to
dominate鈥攗nless the Sun has more surprises in store.