13 January 2005
Horizon producer David Sington on why predictions about the Earth's climate will need to be re-examined.
This is a film that demands action. It reveals that we may have grossly underestimated the speed at which our
climate is changing. At its heart is a deadly new phenomenon. One that until very recently scientists refused to believe even
existed. But it may already have led to the starvation of millions. Tonight Horizon examines for the first time the power
of what scientists are calling Global Dimming.
Runtime 49 Minutes, click play to start
We are all seeing rather less of the Sun. Scientists looking at five decades of sunlight measurements have reached the disturbing
conclusion that the amount of solar energy reaching the Earth's surface has been gradually falling. Paradoxically, the decline
in sunlight may mean that global warming is a far greater threat to society than previously thought.
The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records
from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. "There was a staggering
22% drop in the sunlight, and that really amazed me," he says.
Intrigued, he searched out records from all around the world, and found the same story almost everywhere he looked, with
sunlight falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British
Isles. Although the effect varied greatly from place to place, overall the decline amounted to 1-2% globally per decade between
the 1950s and the 1990s.
Gerry called the phenomenon global dimming, but his research, published in 2001, met with a sceptical response from other
scientists. It was only recently, when his conclusions were confirmed by Australian scientists using a completely different
method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.
Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires,
produces not only invisible carbon dioxide (the principal greenhouse gas responsible for global warming) but also tiny airborne
particles of soot, ash, sulphur compounds and other pollutants.
This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also
changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain
a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would
otherwise be, again reflecting the Sun's rays back into space.
Scientists are now worried that dimming, by shielding the oceans from the full power of the Sun, may be disrupting the
pattern of the world's rainfall. There are suggestions that dimming was behind the droughts in sub-Saharan Africa which claimed
hundreds of thousands of lives in the 1970s and 1980s. There are disturbing hints the same thing may be happening today in
Asia, home to half the world's population. "My main concern is global dimming is also having a detrimental impact on the Asian
monsoon," says Prof Veerhabhadran Ramanathan, one of the world's leading climate scientists. "We are talking about billions
But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power
of the greenhouse effect. They know how much extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide
(CO2) we have placed there. What has been surprising is that this extra energy has so far resulted in a temperature rise of
This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide
than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of 6°C. But it now appears the
warming from greenhouse gases has been offset by a strong cooling effect from dimming - in effect two of our pollutants have
been cancelling each other out. This means that the climate may in fact be more sensitive to the greenhouse effect than thought.
If so, then this is bad news, according to Dr Peter Cox, one of the world's leading climate modellers. As things stand,
CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution
is at last being brought under control. "We're going to be in a situation, unless we act, where the cooling pollutant is dropping
off while the warming pollutant is going up. That means we'll get reduced cooling and increased heating at the same time and
that's a problem for us," says Cox.
Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards. That means a temperature
rise of 10°C by 2100 could be on the cards, giving the UK a climate like that of North Africa, and rendering many parts of
the world uninhabitable. That is unless we act urgently to curb our emissions of greenhouse gases.