Globale Verdunkelung

Erst kürzlich haben Klimaforscher festgestellt, dass die Sonnenhitze auf der Erde seit Jahrzehnten dramatisch abnimmt. In den USA hat sie zum Beispiel um bis zu zehn Prozent, in Teilen Großbritanniens sogar um 16 Prozent abgenommen. Das Phänomen wird inzwischen "Global Dimming" - globale Verdunkelung - genannt. Diese Entwicklung ist nicht aufgefallen, weil die globale Erwärmung den Verlust an Sonnenhitze kompensiert hat. Dennoch hatte sie wahrscheinlich bereits katastrophale Folgen. Denn die Intensität der Sonneneinstrahlung beeinflusst den Wechsel von Regen- und Trockenzeit in vielen Regionen der Erde. Neue Forschungen belegen, dass zum Beispiel Dürrekatastrophen der 80er Jahre des 20. Jahrhunderts, die Tausenden Menschen in Äthiopien das Leben kosteten, in direktem Zusammenhang mit diesem Phänomen stehen. Es gibt bereits weitere Alarmzeichen: In Indien setzt der Monsunregen von Jahr zu Jahr später ein. Was passiert, wenn er irgendwann ganz ausbleibt?
Die globale Verdunkelung hat letztlich die gleiche Ursache wie der Treibhauseffekt. Es ist ein Nebenprodukt der Verbrennung fossiler Energien: Winzige Partikel aus Ruß, Asche und Schwefeldioxid in der Atmosphäre führen zu vermehrter Tröpfchenbildung in den Wolken. Deshalb wird die Sonnenwärme stärker reflektiert. Die Reduzierung der Luftverschmutzung durch Abgas- und Rußfilter könnte das Problem also beseitigen.

Doch die Sache hat einen Haken: Ohne "Global Dimming" würde das ganze Ausmaß des Treibhauseffekts erst voll wirksam werden. Ein Temperaturanstieg von zwei Grad Celsius innerhalb von 20 Jahren brächte Grönlands Gletscher zum Schmelzen. Viele Inseln und manche Städte würden überflutet. Wenn nichts unternommen wird, könnte am Ende des Jahrhunderts in Europa ein Klima wie in Nordafrika herrschen.

Dieser Film behauptet nicht, dass es so kommen wird. Aber er warnt vor den Folgen eines Phänomens, das erst vor wenigen Jahren entdeckt wurde und noch heute von vielen Wissenschaftlern und Politikern ignoriert wird. Die deutsche Erstausstrahlung der BBC Exklusiv-Dokumentation "Schwarze Sonne - Die Folgen der Globalen Verdunkelung" zeigt auf, wie wir einer Klimakatastrophe entgehen können, sofern auch der politische Wille zum Handeln da ist.

Original

Global Dimming

Horizon producer David Sington on why predictions about the Earth's climate will need to be re-examined.

Questions and answers about global dimming

Programme transcript

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 of people."

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 just 0.6°C.

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.