The question has taken on new urgency in the aftermath of Hurricane Sandy, which caused coastal flooding that scientists say was almost certainly worsened by the modest rise of sea level over the past century. That kind of storm tide, the experts say, could become routine along American coastlines by late in this century if the ocean rises as fast as they expect.

In previous research, scientists have determined that when the earth warms by only a couple of degrees Fahrenheit, enough polar ice melts, over time, to raise the global sea level by about 25 to 30 feet. But in the coming century, the earth is expected to warm more than that, perhaps four or five degrees, because of human emissions of greenhouse gases.

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Experts say the emissions that may make a huge increase of sea level inevitable are expected to occur in just the next few decades. They fear that because the world’s coasts are so densely settled, the rising oceans will lead to a humanitarian crisis lasting many hundreds of years.

Scientists say it has been difficult to get people to understand or focus on the importance, for future generations, of today’s decisions about greenhouse gases. Their evidence that the gases represent a problem is based not just on computerized forecasts of the future, as is commonly believed, but on what they describe as a growing body of evidence about what occurred in the past.