Nearly everyone who enjoys the out-of-doors has noticed signs of global warming — milder winters, drier summers, fewer snowfalls, fiercer storms — but debate has raged over the cause: Is global warming a natural cycle that in time will reverse itself, or is human activity responsible for changing the climate?

Now scientific evidence demonstrates with a high degree of certainty that a buildup of greenhouse gases is warming the earth and changing the climate. Ice core samples prove that carbon dioxide concentrations are the highest in 400,000 years, and they are increasing at an unprecedented rate — 30 times faster than 10,000 to 20,000 years ago. And the evidence imputes human consumption of fossil fuels as the primary culprit of this dramatic increase.