Documentary Review: BBC Global Dimming

Not only is global warming a real and present threat that may yet in conjunction with impending energy shortages doom industrial civilization, it may have even been dangerously underestimated. “What have you been smoking!?,” you might say to me. Get off the doom train and enjoy the Sun. Unfortunately, we might not have much of it during the next decades – at least metaphorically speaking. To see why, I recommend you watch this video on global dimming or read its transcript.

So here’s the plot-line. After 9/11, the US air fleet was grounded for three days in the name of national security. Though presumably a major inconvenience for travelers, it was a boon for climatologist David Travis who was studying the effects of contrails, or vapor trails, left behind by high-flying aircraft on the world’s climate.

He predicted that removing contrails would have a significant impact on global temperatures, but was shocked to discover that the daily temperature range – the difference between the hottest and coldest temperature measures in a day – shifted up by an unprecedented 1.1C during those three days!

The Beginning

This story begins with Gerald Stanhill, who was tasked to measuring solar radiation over Israel as part of its plans to develop an irrigation system in the 1950’s. Repeating these experiments in the 1980’s, he found that there had been a whopping 22% drop in solar radiation over Israel!

The results were dismissed by mainstream researchers, who could not believe that the Earth’s atmosphere was darkening because there had begun a clear warming trend from the 1970’s. But then Beate Liepert combed through meteorological records in Germany and discovered the same thing. Working independently, Stanhill and Liepert discovered that from the 1950’s to the early 1990’s, the level of solar energy reaching the Earth’s surface had dropped 9% in Antarctica, 10% in the USA, 16% in parts of the British Isles and almost 30% in Russia. They christened the phenomenon global dimming.

Global Dimming

Scientists still ignored these results, since they could not square global dimming with global warming.

In Australia, two researchers, Michael Roderick and Graham Farquhar, were investigating the factors influencing the so-called  “pan evaporation rate”. Basically this is a really boring set of repeated experiments in which you fill up a pan of water, leave it out in the Sun for constant intervals of time and plot a time series of how much water vanishes during those periods. Really sad. But very useful in agricultural science.

Their research indicated that the key things determining evaporation rates are wind levels, humidity and the brightness of sunlight, with the latter dominant (the photons kick the water molecules out of the pan into the atmosphere). Temperatures actually play only a relatively minor role. So they reasoned that the number of photons hitting the Earth’s surface was going down…but why?

Mike Roderick happened across a paper called “Evaporation Losing Its Strength” in the magazine Nature, which reported a global decline in pan evaporation rates across the US, Europe and Russia. Putting two and two together, they compared it with the decline in observed sunlight from Stanhill’s and Liepert’s work. The trends towards decline matched perfectly.

The global dimming theory now had a bright future.

Reflecting Away the Asian Monsoon?

During the mid-1990’s climate scientist Veerabhadran Ramanathan noticed a decline in sunlight over large parts of the Indian Ocean. He reasoned this was due to atmospheric pollution. Industrial civilization emits soot and sulfate particulates into the atmosphere, creating the hazes which shroud its major cities.

This effect is especially pronounced over the plains of northern India, where the fires from hundreds of millions of primitive peasant cook-stoves and the exhaust from the millions of rickshaws that ply its gridlocked cities play a major part in forming the “Asian Brown Cloud”, the dusky haze that envelops much of South Asia.

A multinational experiment was conducted to study this in more detail in the Maldives. In the north, air is polluted by aerosols from the Indian subcontinent; in the southernmost islands, it is cleared away by clean Antarctic winds. This fortunate conjunction lent itself well to comparative study.

When water vapor attaches itself onto naturally occurring particles, they eventually become too heavy to remain airborne and fall to the ground as rain. There are far more particles suspended in polluted air – ash, soot, sulfur dioxide, etc – than in normal air. By a factor of 10, to be precise. Thus the man-made particles provide ten times as many sites for water droplets to attach themselves to. Therefore, polluted clouds contain many more water droplets than naturally occurring clouds – each one far smaller than it would be naturally.

Many small water droplets reflect more sunlight than a few larger ones, so polluted clouds reflect far more light back into space, preventing the Sun’s heat from getting through. This is the mechanism by which global dimming works – not only are the particles themselves reflecting more sunlight, but most importantly they form brighter clouds over polluted areas.

(I think this is also a feedback mechanism. During Ice Ages, you have a lot of dust-laden winds which would reflect back sunlight, dim the Earth and reduce evaporation rates, which in turn would lead to dessication and more dust. When the Earth warms, more vegetation appears and deserts eventually contract once the system reaches an equilibrium, so more sunlight reaches through, increasing the power of the Earth’s hydrological engine.)

Satellite images revealed this global dimming effect was not just limited to India, but also encompassed China extending to the Pacific, Western Europe extending into Africa, the British Isles, etc.

These clouds could alter the world’s rainfall patterns. This may have already led to the first global dimming Holocaust.

There was a major famine in 1984 in Ethiopia, partly caused by a decades-long drought across the Sahel. The area is crucially reliant on a short wet season created by the summer monsoon.

This monsoon depends on the Sun heating the Atlantic north of the Equator, drawing the tropical rain belt northward and bringing rain to the Sahel. This mechanism failed frequently during the 1970’s and 1980’s.

Leon Rotstayn was puzzled by this phenomenon because his climate models indicated that pollution blowing in from Europe and the US over the Atlantic should have little effect on the Sahel’s rainfall patterns. But taking the new Maldives findings into account, he found that the resultant brighter clouds would reflect more sunlight in space, cooling the Atlantic Ocean. Consequently the equatorial rain bands would fail to move as far north, spelling doom for the benighted denizens of the Sahel.

From the 1990’s, there were serious moves towards regulating aerosol emissions in Europe and the US. Scrubbers were installed on factory chimneys, fuel was cleansed of sulfur and cars acquired catalytic converters. The rains returned to the Sahel and the droughts have receded in recent years.

However, the “Asian Brown Cloud” is still growing and as noted earlier the Asian monsoons that sustain 3bn people are crucially dependent on the temperature gradient between land and oceans. These gradients will diminish in the presence of major dimming. Furthermore, could it be that the reason El Nino has increased in recent years (in contrast to the historical record, in which it usually flares up only when the world is colder, i.e. when less sunlight reached the Earth) is due to diminished solar intensity over the west Pacific “fire-stove” off the Indonesian coast that drives this cycle?

From the Frying Pan into the Fire – Accelerated Global Warming

So the world decides to clean up its act. Quite literally. Global dimming eases, the monsoons return to stability and everything will be nice and dandy, right?

Unfortunately not. For global dimming has been masking us from an even greater threat – very fast global warming.

As shown in this satellite photo of the Western US, though contrails are individually small when there are many of them they can blanket the whole sky. Now if according to Travis’ calculations just a three day interruption in air travel can raise the daily temperature range by more than 1.1C – a unprecedentedly sharp jump, then what would happen to global temperatures if all industrial activity were to collapse tomorrow?

The slight global cooling from the 1950’s to the 1970’s may have been due to rapidly rising pollution whose immediate cooling effects overwhelmed the as yet modest effects of global warming (whose impact is not immediate, but stretched over decades with a “half-life” – when the climate system moves half the way to its new equilibrium – of around 30 years). However, since then pollution control in the industrialized world coupled with the end of exponential growth in world hydrocarbons extraction allowed the warming trend to regain the initiative.

The effects are already observable in Europe. During the 1980’s, east-central Europe was an environmental hellhole of hanging smogs, acid rain and wilted forests. The central focus was at the so-called “Black Triangle”, on the borders of Poland, Germany and Czechoslovakia.

The collapse of Communism cleaned away the blight, but nature doesn’t provide free lunches. Europe cutting its pollution may have saved millions of Sahelians and added a few years to the life expectancy of the denizens of Dresden, but temperatures too started rising rapidly – culminating in the ferocious summer heatwave of 2003, which produced 35,000 excess deaths. Within a few decades, this will be the norm; within a few decades more, much of the Mediterranean may become desert.

If global dimming has such a big and immediate impact on temperatures, then this means that global warming is in fact a far stronger beast than previously thought. Furthermore, most aerosol pollutants are washed out of the atmosphere or broken down by hydroxyl within days; CO2 accumulates and stays up there for centuries. In the long run, and absent conscious human intervention, CO2 and global warming will win out.

The Dilemmas of Global Dimming

Once clean air regulations and/or depleting hydrocarbon stocks force a stop to or reversal of “dirty” pollution, which produces a cooling effect, then warming will hit the Earth with full force – by then no doubt accelerated by positive feedbacks like the decreasing ice-albedo effect, ocean acidification, vegetation dieoff and Siberian methane releases.

Global warming will follow the upper end of the IPCC’s projections (6.4C rise by 2100), or even exceed them altogether. We may hit 2C as soon as 2030, initiating the melting of the polar icecaps and dooming the world’s coastal cities. A rise of 4C, perhaps as soon as 2040, will spell the death of the Amazon. The fin de siècle climate may be as much as 10C hotter than today, which implies certain doom for industrial civilization as the (electronic-cyber) map collapses from the assault of the desert of the real.

No wonder then that stratospheric sulfur particulate emissions are one of the leading contenders for geo-engineering plans to “correct” the world’s climate should global warming veer out of control, this idea being proposed by Mikhail Budyko as early as 1974. Or we could try to initiate a hydroxyl collapse so that pollution no longer gets cleansed out and accumulates like CO2, shielding us from the Sun’s wrath.

Of course, both paths – global warming or global dimming – will have catastrophic impacts on world food production. Increasing the global aerosol cover on such a large scale is a huge undertaking in political and social costs. One way to do it is to increase coal burning and to remove the scrubbers from factory chimneys and other such amenities of today’s life. In the future, clean air may become a luxury.

Doing this will be quite cheap – coal is still plentiful, even if the mined ores are constantly declining in energy density, and removing pollution controls will significantly increase its EROEI (energy return on energy invested), which will give a boost to an industrial civilization by now on the verge of collapse. However, embarking on this project will be difficult to explain to citizens already tired of the dead hand of government in their lives, for by now net returns to complexity will be decidedly negative (Tainter). Furthermore, not all nations will benefit or agree to this project, though they will no doubt be bullied into line should the Great Powers reach a common agreement.

However, quite apart from further postponing the inevitable day of reckoning and increasing its magnitude when comes, darkening the world could shut down the Asian monsoon and drastically change the world’s weather patterns.

If global warming is to go unchallenged by global dimming, however, it will be all the faster and more catastrophic. Beyond a 3C rise, the heat will wreck the world’s mid-latitudinal breadbaskets and cause staple crop yields in overpopulated nations like China and India to plummet.

Fire or darkness? That is one hell of a predicament.


  1. I’m a chemist, and found this blog post to be very interesting. Thank you for posting it.