Global Dimming

Jen Crick

Global Warming is a term that nearly everyone in developed countries is at least familiar with; and if the EU’s new policies are any measure, people are taking it seriously [1]. Many people, including the vast majority of the scientific community view it as a huge threat that requires immediate and decisive action. As if climate change with just the greenhouse gas phenomenon did not pose enough of a threat already, scientists are now saying that we could be having an even bigger impact than we ever thought before. Not only have we been warming the planet with our needs for mass transportation and other modern amenities, but we have been darkening the planet as well [4].

“Global Dimming” is not a new idea or phenomenon. Like Global Warming, Global Dimming stands on decades of assiduous scientific research, and the data is compelling. Dr. David Travis, a climate scientist from the University of Wisconsin, Whitewater, spent fifteen years researching whether airplane condensation trails have a significant impact on the environment [3]. After September 11 th, all but a few military planes were grounded in the US for three days. Over that span of time, Travis measured a one degree Celsius difference in the range of high and low temperatures between Sept 11 th and Sept 14 th. Just three days amounted to one of the quickest climate changes on record. He concluded that contrails have the net effect of darkening the day by reflecting sunlight and warming the nights by acting trapping heat at night. Our pollution is bringing day and night closer together [3].

Other studies also show that global sunlight levels have been decreasing: according to research by climatologists Dr. Beate Liepert and Dr. Gerry Stanhill, “Between the 1950s and the early 1990s the level of solar energy reaching the earth's surface had dropped 9% in Antarctica, 10% in the USA, by almost 30% in Russia. And by 16% in parts of the British Isles [3].” Clouds play by far the biggest role in planetary albedo, the process by which the earth reflects light back into the atmosphere [2]. Creating artificial clouds at the very least disturbs this natural process, and Travis’ research shows that clouds made as a byproduct of human plane travel can have huge impacts. Though the scientists studying the dimming phenomenon are not yet sure whether there is a net cooling or warming effect as a result of increased anthropogenic clouds, the working hypothesis states that Global Dimming has a net warming effect [4]. Yet more clouds causing planetary albedo should mean a global cooling: less sunlight reaching the surface should mean declining temperatures. The obvious fact that this is not the case indicates that we could be counteracting some of the effects of artificial global cooling with pollution from those same planes [2]. If that is the case and the pollution in plane exhaust is enough to counteract all of the artificial planetary albedo we have created, we could be seriously underestimating the totality of our impact on the environment [3].

Dr. Leon Rotstayn of CSIRO Atmospheric Research says that these previously ignored impacts may have caused famines that killed over a million from 1884 – 5 [3]. Ethiopia needs monsoons every season to water the crop supply and keep the country fed [5]. Global Dimming, Rotstayn postulates, simultaneously cooled equatorial waters and blocked needed heat from the sun so that the life-giving rain belt did not move far enough north for a decade. According to the BBC, “If his model is correct, what came out of our exhaust pipes and power stations contributed to the deaths of a million people in Africa, and afflicted 50 million more. But this could be just of taste of what Global Dimming has in store [3].” Other countries that rely on seasonal monsoons for food production, especially those in developing Asia where population growth rates are high, could see just as devastating losses in the future [3].

The only problem with tackling the threat of Global Dimming is that doing so could worsen Global Warming. Letting in more sunlight and reflecting less heat-causing rays back into space will accelerate the current rate of warming [4]. Conversely, treating Global Warming cannot entirely discount the benefits of Global Dimming, because at the moment it is the only tech fix we have. Particles that reflect the sun like the water in contrails are a natural part of the planet’s processes, and Dimming does increase planetary albedo [2]. It’s the other particles in jet exhaust that act as greenhouse gases and those that pose a threat to human health that people see as of most urgent concern. These particles directly contribute to Global Warming and health issues which are both seen as more immediate than the little known Dimming problem. While we cannot indefinitely allow our planet to darken as a trade-off in an attempt to mitigate temperature change, acting on either of the issues separately will pose some major, potentially counterproductive conundrums. Unless humanity addresses all aspects of climate change simultaneously, there is little hope for solving the problem in time without another tech fix. In Global Dimming’s case, it seems that tech fixes might have heftier prices to consider than we had ever before imagined [3].


1. “ Q&A: EU energy plans.” 9 Mar 2007. BBC. <>.

2. “Planetary Albedo.” Dr. DC Griersmith. <>.

3. “Global Dimming.” 5 Dec 2006. BBC Horizons: Science and Nature Programming. <>.

4. “Dimming the Sun.” 4 Sept 2004. NOVA. PBS. <>.

5. “1 984 - 1985 famine in Ethiopia .” Wikipedia. <>.

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last updated 4/14/08