Observers first began noticing the effect, now called "global dimming," in the 17th Century. Most believed it of generally short duration and isolated to periods after specific events such as large volcanic eruptions. However, recent discoveries show that the reduction of sunlight reaching Earth's surface is becoming more pervasive and that it has been most pronounced in the last several decades.

So, just when climatologists were telling us the world was heating up, other scientists have been noting that the surface has been becoming darker, quite a puzzle since less sunlight reaching us would, according to previous models, have meant an average cooling of the planet's surface, even as, indeed, had earlier been recorded in the first year or so following huge eruptions that for short durations had vastly raised the amount of polluting gases and fine debris carried around the world on the winds of the upper atmosphere.

The long and short of it, though, is that, whereas global warming is thought to have raised the average temperature of the world about a degree, this has occurred at a time when manmade air pollution has been on the increase, filtering out or reflecting back to space enough sunlight that it has had a nearly two degree cooling effect.

Thus, if not for the large amount of pollution being continuously pumped into the air, by now global warming would likely have raised Earth's surface temperature by about three degrees, creating at least two or three times the global warming results.

It may seem, then, that pollution is a boon, saving us from a worse fate. And, for the short-term, that is the overall conclusion. But pollution also has its own negative consequences, of course, and can only be allowed to increase to a finite extent. Some of the pollution is itself contributing to further global warming. Other kinds are having severe health costs and already are resulting in many more illnesses and deaths each year, due to asthma and other major respiratory ailments, for instance, than would be occurring without it.

The bottom line is that we shall have to take steps as a species to limit further pollution. Populations of terribly polluted cities gradually demand it, just as we insisted in this country on reducing the pollution levels in Los Angeles, New York City, Chicago, Houston, etc., in years past. As developing and third world countries follow the examples of the developed nations in cleaning up their acts, for example with enforcement of lower emission standards, the extent of "global dimming" is expected to level off and then decline.

At that point, if scientists' conclusions about the interaction between global warming and "global dimming" are correct, there will be a more rapid increase in the average temperature at Earth's surface, and we shall have greater danger of the warming trend not merely soaring beyond our control but progressing to a degree that our planet's complex natural systems of interactive and counteractive forces will be radically shifted toward a new, relatively permanent state of higher temperatures, several degrees above anything we had known before. Such a transition might occur over a period of a century or so.

More ominously, studies of deep ice cores have shown that such imbalances in the past at times resulted in fairly sudden alterations, happening in only a decade or less, that lead to periods (of up to thousands of years) when the planet was far warmer than anything humans have known since prehistory.