Variation in Solar Radiation

Lucianne.com links to a Natural News article which forcefully makes the point that minor predictable variations in the earth's orbit around the sun account for historically observable climate change. These changes have been going on throughout the earth's history, regardless of human activity.

In 1976, a study published in the journal Science confirmed that Milankovitch’s theory is, in fact, accurate, and that it does correspond to various periods of climate change that have occurred throughout history.

In 1982, six years after this study was published, the National Research Council of the U.S. National Academy of Sciences adopted Milankovitch’s theory as truth, declaring that: “… orbital variations remain the most thoroughly examined mechanism of climatic change on time scales of tens of thousands of years and are by far the clearest case of a direct effect of changing insolation on the lower atmosphere of Earth.”

The biggest factor influencing weather and climate patterns on earth is the sun, period. Depending on the earth’s position to the sun at any given time, climate conditions are going to vary dramatically, and even create drastic abnormalities that defy everything that humans thought they knew about how the earth worked.

Compared to the impact of the sun, the activities of all humans essentially amount to nothing significant on a global scale, though they may have local effect - smog, for instance. It isn't easy for our species - afflicted with hubris - to accept insignificance. See Wikipedia for an explanation of Milankovitch cycles.