The amount of sunlight
reaching the Earth's surface is increasing, two new studies
in Science magazine suggest.Using
different methods, they find that solar radiation at the
surface has risen for at least the last decade.
Previous work had found
the opposite trend, leading to a popular theory known
as "global dimming". But
the latest Swiss and US research indicates the dimming
in the past has now been reversed, possibly because of
reduced atmospheric pollution.
Hard-won data
The idea of global dimming holds
that tiny particles - aerosols - in the atmosphere are
reflecting sunlight back into space, and the effect is
to cool the Earth's surface.
The aerosols - a large proportion
of which come from human activities - are therefore acting
against any human-induced greenhouse effect. And only
when societies clean up the production of aerosols will
the true extent of global warming become apparent.
That is the theory, but global dimming
has been hard to study definitively.
As with many other issues
relevant to climate science, the answers researchers seek
are not easily obtained, because previous generations did
not build the instruments and set up the experiments that
present-day investigators now suddenly need.
With the growing realisation
that climate change may be a major hazard for the planet
and for human society, the gaps which are left in this
area are quickly being plugged.
Martin Wild's team, from
the Swiss Federal Institute of Technology in Zurich -
one of the groups reporting in this week's edition of
the journal Science - has taken advantage of recent developments.
"We needed a system
which allowed high-quality measurements of solar radiation,"
he told BBC News.
"Since 1990, many
stations giving high-quality data have been built - and
overall, they show an increase in radiation reaching the
Earth's surface."
Looking down
Between about 1960 and
1990, however, Dr Wild believes that global dimming did
occur, as found in records from an earlier generation
of monitoring stations, and from experiments measuring
the rate of evaporation of water - an indirect measure
of the Sun's energy at the Earth's surface.
The second paper in the
magazine uses data gathered by satellites observing the
Earth, rather than monitoring stations on its surface.
"We have satellite
observations that are global, which can look at the global
picture starting from 1983 until 2001," principal
investigator Dr Rachel Pinker, from the University of
Maryland, US, told BBC News.
"We have found in many
places that indeed we do agree with observations that have
been made at the ground, that there was a decrease up to
the '90s and then there was an increase.
"When we analyse the
entire record for the 20 years, we don't see a dimming;
we see a slight increase in the amount of radiation when
averaged over global scales."
So are other scientists
convinced - have the lights unequivocally gone out on
the global dimming hypothesis?
"These studies do
add important information to what we knew previously,"
commented Dr Eleanor Highwood, a climatologist at the
UK's Reading University, "and there is some consistency
between the papers."
The really crucial question,
she said, was why things changed around 1990 - why light
which had slowly been fading started to regain its strength.
Effective
legislation
The most obvious theory
of all - that the Sun has simply increased its output
- was swiftly demolished by Dr Wild.
"The size of the changes
we are seeing are just too big to be explained that way,"
he said.
So whatever the explanation
is, it must lie in the atmosphere - and Dr Wild believes
that regional differences in his data may give a clue.
"In India, we see no upturn in
radiation, nor in China," he explained. "But over
most of the rest of the world, notably Europe, we do.
"Since 1990, the atmosphere
has become much cleaner with the introduction in many
places of clean air legislation. "The
other thing that happened in 1990 was the breakdown of
the Soviet Union and its bloc of countries, which led
to less output from their highly polluting industries."
So it could simply be that over
many areas of land, the atmosphere is cleaner - more sunlight
is penetrating to the surface; whereas Asia is still pumping
out aerosol particles and, as a consequence, no brightening
is observed there.
Heated debate
If global dimming was masking the
true scale of the human-induced greenhouse effect, and
that dimming has now been superseded by global brightening,
what does it all mean for the climate?
"We need to understand the
basis of this," observed Dr Highwood, "and see
what types of aerosol particles might be involved.
If it's due to changes in soot-type
aerosols, which absorb radiation, it might not affect things
very much; but if it's changes in particles which scatter
sunlight, such as sulphate aerosols, then we might not have
the computer models of climate change right. "But there is a third possibility
here, which is changes in cloud cover. Until we can sort
this out, we're going to struggle."
Once again, then, a conclusion arrived
at by decades of painstaking observation raises a host
of further questions, which can only be answered by yet
more decades of painstaking observation and the expensive
instruments needed to make them.
One thing which shows absolutely
no sign at all of dimming is the heat of scientific debate
between climate change scientists.
