Global dimming: the other inconvenient truth

Professor Atsumu Ohmura received the surprise of his life in 1985. Ohmura, the leader of the Global Climate Research Group at the Swiss Federal Institute of Technology, was documenting levels of sunlight striking the Earth's surface when he made the astonishing discovery that the amount of solar radiation had declined by more than 10 percent in three decades.

This discovery went against all scientific thinking. By the mid-80s there was irrefutable evidence that our planet was getting hotter, so the idea of reduced solar radiation seemed impossible. The massive 10 percent shift in only 30 years made this discovery seem even more implausible. Ohmura himself had a hard time accepting it.

He says, "I was shocked. The difference was so big that I just could not believe it."

No one else would believe it either. However, Ohmura's records and data were extensive and undeniable. During the 1988 International Radiation Symposium held in Lille, France, Ohmura told the audience about the decline in solar radiation. He said, "They didn't believe it at all." In the face of scientific disbelief, the phenomenon went virtually unnoticed for an entire decade.

Records show that over the past 50 years the average amount of sunlight reaching the ground has decreased by almost three percent per decade. This amount is too small to notice with the naked eye, but it has implications for everything from climate change to solar power.

To date, "Global Dimming" has generally gone unnoticed by the media and scientific community even though critical records and research papers published by respected scientists worldwide all document that light levels have been falling significantly for decades.

Global Dimming is caused by air pollution. Burning wood and fossil fuels produce not only invisible carbon dioxide, the principal greenhouse gas responsible for global warming, but also tiny airborne particles of soot, ash, sulfur compounds and other pollutants.

This visible air pollution reflects sunlight back into space, preventing sunlight from reaching Earth's surface. The pollution also changes the optical properties of clouds. Because these particles feed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes the clouds more reflective, transmitting the sun's rays away from Earth and back into space.

Scientists are now convinced that Global Dimming, by shielding the oceans from the full power of the sun, is disrupting the pattern of the world's rainfall. There is strong evidence that Global Dimming was responsible for the droughts in sub-Saharan Africa which claimed hundreds of thousands of lives in the 1970s and 1980s.

Global Dimming is also affecting the rainfall and weather patterns in the Pacific Northwest. Dimming is certainly bad news for solar power. The darker skies reduce solar power's meager efficiency even further. The effect on plant and tree growth is much more complicated. Those who are growing tomatoes in a greenhouse are seeing the effects of Global Dimming already. The rule of thumb is that a one percent decrease in solar radiation equals a one percent drop in productivity. Of even greater concern for Central Oregonians is that the disrupted weather patterns are leading to an increased opportunity for the outbreak of wildfires.

However, the most alarming aspect of Global Dimming is that it has led many scientists to underestimate the true dangers of Global Warming. It appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming; in actuality, two of our pollutants are canceling out each other. The result is that the climate is actually much more sensitive to the greenhouse effect than we ever believed.

One of the world's leading climate modelers, Dr. Peter Cox, says the bad news is yet to come. As things stand, carbon dioxide levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is being brought under control. Cox says, "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 major problem for us."

Even the most pessimistic forecasts of Global Warming will now have to be drastically revised upwards because of the impact of Global Dimming. Unless we act urgently to curb our emissions of greenhouse gases, a temperature rise of 18 degrees by 2100 could be possible, raising sea levels and rendering many parts of the world uninhabitable. More information on Global Dimming is available at http://www.realclimate.org.