In the second of his looks into the negative effects of aviation, Bob Maddox claims we should also be aware of Global Dimming

WHAT do you say to a scientist who asks you to believe that, while the Earth is indisputably getting warmer, it is at the same time receiving significantly less sunlight than it did 40 years ago? Since sunlight is the only source of heat for our planet, it does not take the likes of a Vulcan Science Officer to arrive at the conclusion: “illogical.”

Yet this, indisputably appears to be what is happening, as climate scientists are beginning to wake up to the existence of a previously unsuspected phenomenon we now know as Global Dimming.

“Illogical” was the response that greeted Dr Gerald Stanhill in 1989, when he reported Israel had lost 22 per cent of its sunlight since 1960. His findings, utterly inconsistent with the reality of rising temperatures, were ignored. After all, a 22 per cent reduction in sunlight is the stuff of Ice Ages, not of Global Warming. As one leading climatologist put it at the time: “That is bull$hit! If that were true, we would all be freezing!”

But then, a similar trend was noticed in the Alps by a clever young lady named Beate Liepert, who measured a whopping 28 per cent loss of sunlight since 1960. Not consistent with the Alpine glaciers melting all around her, but real nonetheless.

By the mid 1990s, others were reporting significant drops in sunlight around the world: Antarctica had 9 per cent less, Germany 20, the British Isles 16 and the USA 10. Russia was a whopping 30 per cent dimmer than in 1960.

The world it seemed, was getting darker. But why? Since scientists knew there was nothing wrong with the Sun, the cause had to be right here on Earth and there was one obvious suspect: atmospheric pollution.

Man and aviation

It had long been suspected that sooty particles and other aerosols from the burning of fossil fuels might have an effect on climate, but there was simply no way to measure it – until tragedy took a hand.

September 12, 2001. The skies over the US have been swept clear of aircraft as a shocked nation wakes to the realization an airliner full of people can be used as a ballistic missile as deadly as any possessed by the military.

To climate scientist David Travis, 9/11 offered an opportunity. Travis had been studying the effects condensation trails, or contrails, left behind by high-flying jet aircraft might be having on surface temperatures.

Over the busy airways of the USA and Western Europe, contrails can obscure much of the sky, resembling a web of high cirrus clouds.

Then for three days with the skies clear of contrails, Travis collected data from over 5,000 weather stations and what he found astonished him: the temperature range – the difference between the hottest part of the day and the coldest part of the night – had jumped by one degree centigrade.

From the perspective of climate change, this was a huge number and the first real indication the effect aerosol pollutants were having on our climate was much bigger than previously suspected.

Suddenly, scientists began to wake up to the idea that bronchitis, asthma and dirty buildings were not the only consequences of our sooty way of generating power. In the theatre of Climate Change, the villain of the piece had long been our old friend Global Warming.

No one had noticed its shadowy dim sister waiting quietly in the wings.

All this was about to change with the work of two Aussies and a professor from the Maldives.

Pan’s people

In the 1990s, Graham Farquhar and Michael Roderick of the Australian National University were puzzling over an apparently illogical set of results: the rate at which water evaporated all around the world had declined over the last 30 years despite the warmer climate.

Farquhar and Roderick were measuring something called the Pan Evaporation Rate. What’s that? Well, as Farquhar puts it with commendable Aussie directness: “It’s called the Pan Evaporation Rate because it’s the evaporation rate from a pan. But there’s an apparent paradox here – the evaporation rate is going down, but the temperature is going up.”

Surely, higher temperatures should evaporate water faster, like turning up the heat on a stove? Not so, says Roderick: “It turns out that the dominant force in evaporation is the energy of sunlight itself – photons hitting the surface of the water and tearing away water molecules, not the air temperature.”

So, in a world of less sunlight, even with higher temperatures, we can expect less evaporation. Global Dimming had a real effect – and if it reduced the evaporation rate from a pan, then what of the world’s oceans?

Anyone who paid attention to Geography at school will remember the oceans are the primary link in the Hydrological Cycle – that great turning wheel by which sea water is evaporated and returned to the land as rain, forming the basis of all terrestrial life. Does Global Dimming affect this?

Yes, according to climatologist Professor Veerabhadran Ramanathan (keep trying…you’ll get it eventually). To test this, Prof Ramanathan took measurements from the Maldives, where the northern most islands lie in a stream of dirty air from India, while those to the south bask under clean air direct from Antarctica.

And what he found should disturb us all, for it suggests Global Dimming is a mass killer.

Ramanathan found clouds forming in India’s dirty airstream were composed of smaller droplets than those formed in the clean air of Antarctica, an effect of the billions of microscopic particles of pollution.

Many smaller particles reflect more sunlight than fewer large ones, turning the clouds into giant mirrors and reflecting more sunlight back into space – the same effect Travis had noted with contrails.

Beneath these mirrors evaporation from the ocean surface falls, endangering the established global rainfall patterns upon which millions depend.

Thirst and hunger

The region of sub-Saharan Africa known as the Sahel is one of the most fragile on Earth. Few who experienced it will forget Michael Buerk’s iconic television report from the Plain of Korum as the 1984 Ethiopian famine raged across the Sahel, shocking the world into action and giving rise to Band Aid and its legacy. At the time, overgrazing and poor land-management techniques were blamed. It was all the fault of the people themselves.

But was it?

Atmospheric physicist Dr Leon Rotstayn blames Global Dimming, with dirty air from the USA and Europe streaming out across the Atlantic.

With less sunlight reaching the ocean surface, evaporation rates dropped and the life-giving summer monsoon rains repeatedly failed to reach the Sahel, leading to a record 28-year drought.

And if Global Dimming was responsible for the Ethiopian famine, could it strike again and where?

Prof Ramanathan points to the giant pollution plumes streaming from India and China out across the Indian and Pacific Oceans. If these interfere with the seasonal monsoons of Asia, the results could be truly catastrophic, involving roughly 3.6 billion people – nearly half the world’s population.

But there may be light on the horizon. A 2007 NASA satellite survey reports Global Dimming began to reverse around 1990. Clean Air legislation in the USA and Europe began to take effect by removing evermore dirty aerosol pollutants from our skies. So, will the future really be brighter?

Yes – and here may lie a terrible catch. For it appears Global Dimming may well have been protecting us from something far worse.
For decades we appear to have been unwittingly living in a world where the cooling effects of our sooty pollutants have been damping down the warming effects of our gasses. Now by cutting Global Dimming without tackling carbon dioxide (CO2) emissions, we may also be taking the brakes off Global Warming.

David Travis’ measurements after 9/11, gave us a brief glimpse into a world without Global Dimming. With governments doing far more to reduce aerosol pollutants than CO2 emissions, we may all be in for a much hotter ride than expected.

By failing to notice the damping effect of Global Dimming over the past 40 years, we may have seriously underestimated the magnitude and speed of Global Warming.

Unless governments act quickly and decisively to reduce both types of pollutants simultaneously, the future may hotter than we thought.

As Prof Ramanathan puts it: “There really is no choice here. We have to cut down all air pollution, if not eliminate it altogether.”

The alternative, after all, is surely illogical?

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  1. The study found that global warming since 1985 has been caused neither by an increase in solar radiation nor by a decrease in the flux of galactic cosmic rays. Some researchers had also suggested that the latter might influence global warming because the rays trigger cloud formation.

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