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Goodbye Sunshine

Goodbye Sunshine or a Global Warming very Wildcard

Each year less light reaches the surface of the Earth. No one is sure what's causing 'global dimming' - or what it means for the future. In fact most scientists have never heard of it.

David Adam, Dec 18, 2003 - The Guardian

In 1985, Professor Atsumu Ohmura ,the director of the Zurich-based World Radiation Monitoring Center, the organization that measures the amount of solar radiation hitting the ground around the globe, got the shock of his life. As part of his studies into climate and atmospheric radiation, Ohmura was checking levels of sunlight recorded around Europe when he made an astonishing discovery.


Compared to similar measurements recorded by his predecessors in the 1960's, Ohmura's results suggested that levels of solar radiation striking the Earth's surface had declined by more than 10% in three decades. Sunshine, it seemed, was on the way out.

Professor Atsumu Ohmura has also an interesting talent. Give the Professor a glass of white wine and tell him only its vintage, and he'll swish a mouthful and without referring to legs, bouquets, or mango backgrounds announce where the grapes were grown. (reference)

His trick? The sweetness of white wine grapes is a function of solar radiation. The more sun a grape plant's leaves absorb, the more sugar the plant produces and the more sweetness it infuses into the fruit. So if you pay really close attention to the global meteorological records, and in particular the geographic distribution of solar radiation, then when you sense a wine's sweetness, you can infer its region of origin.

Ohmura and his team reviewed national meteorological records and current publications, copying and pasting solar radiation measurements into a database until they had observations from 1,600 locations around the world, including the very oldest continuous measurement, recorded by the Swedish Professor Anders Angstrom in 1922.

Professor Angstrom is famous for crawling up onto the roof of the University of Stockholm to set up his novel pyranometer, the first device to accurately measure direct and indirect solar radiation, thus initiating the science that would lead Ohmura to the missing sunlight. Never before had anyone drawn such a thorough picture of worldwide solar radiation levels. Their discovery was at first hard to believe. However, the data were extensive. But when this was presented at the 1988 International Radiation Symposium in Lille, France, about the missing chunk of energy? "Oh, they didn't believe it at all," Ohmura says.

These findings were subsequently confirmed by Shabtai Cohen and Gerald Stanhill in 2001, when these Israeli researchers reported the same 10% radiation loss between 1958 and 1992, virtually the same conclusion that Ohmura had reached in 1988. Then, in 2003, Graham Farquhar and Michael Roderick, climatologists at the Australian National University in Canberra, discovered corroborating evidence in the global evaporation record. The going explanation for the loss of sunlight is that particulate pollution such as soot plugs up clouds, so that when it's cloudy, it's darker than before.

Lohmann explains that clouds change as we emit more particles into the atmosphere. Clouds are made of cloud droplets, which form by latching onto tiny particles called condensation nuclei. These occur naturally in the atmosphere, but by emitting more particulate pollution into the atmosphere, humans help make even more condensation nuclei. The result: Instead of fewer, larger water droplets forming, many, smaller water droplets form. In effect, this is like the difference between two sieves, one coarse and the other fine. Like a coarse sieve, the cloud with fewer, larger particles lets more solar radiation through to the ground, whereas like the fine sieve, the cloud with lots of very small particles lets less sunlight pass through. The result is darker days.

Well, hold on a moment, Ohmura says. He's getting ready to unload his next big idea, the one that explains the global warming paradox. For some time, scientists have wondered why, during the '60s and '70s, temperatures remained relatively stable, or even got colder, while at the same time there were plenty of greenhouse gases to crank up the thermostat. To illustrate the point, Ohmura cites the average annual melt rates for each of the last four decades for 40 glaciers around the world:

Ohmura believes that during the 1960's and 1970's, global dimming, caused by particulate pollution, buffered the climate against global warming, caused by greenhouse gases. As the increasing amounts of gases warmed the Earth, the increasing amounts of particulate pollution reduced the sunlight that reached its surface, thereby cooling the planet. In other words, one form of pollution counteracted the other. Hence the lower melt rates and stable temperatures of the 1970s.

But then, scientists realized that particulate pollution was almost entirely responsible for deaths related to air pollution -- pollution that still causes a staggering 135,000 premature deaths in the United States every year. (That's 6 percent of all deaths from any cause.) That may seem like a lot, but consider that by 1990, the U.S. EPA found that if Congress hadn't adopted the Clean Air Act in 1970 and amendments to the act in 1977, particulate matter would have prematurely caused the deaths of 184,000 more Americans per year. Clearly, this type of pollution was (and is) a big problem. So, wisely, the industrialized world began cleaning up its act, curtailing emissions of soot and smoke.

So what about that buffer? As emissions of deadly particulate matter decreased, so did their cooling power. Clouds let the sun shine through and -- behold! -- the greenhouse effect's disguise was cast aside. "Because of this double punch [of more solar radiation and more greenhouse gases], the global temperature increased enormously," Ohmura concludes. Preliminary results of his, based on radiation records from 1992 to present, support this theory. Key monitoring stations show a resurgence of radiation levels during the 1990s -- not to pre-1958 levels, but enough to expose the true warming potential of greenhouse gases.

The implication is that the power of both particulate matter and greenhouse gases on the climate have been underestimated. Traditional global climate models will have to be revised, and although the interactions between climatological components is complex and uncertain, revised models and Ohmura's results will undoubtedly raise challenging questions for the public and for policy makers. For instance, will further reductions in particulate pollution, necessary to alleviate serious, deadly, and widespread respiratory illnesses, mean even more global warming? And if so, do even progressive policies, including the Kyoto Protocol, underestimate the potential damage of climate change?

Whether or not the double-punch theory holds up to a round of scientific scrutiny is impossible to predict, but for now, Ohmura hopes that recalcitrant attitudes will change. "Authority in the natural science is, after all, nature. So unless one observes nature very carefully, there's no anchor in our knowledge. Whether our abstract idea is correct or wrong, we must ask nature, and that is only possible by observation."