IDENTIFYING CLIMATE CHANGE. Aldo Leopold's daughter continues her father's work.

Nina Leopold Bradley walked nimbly along a trail to a weather-beaten cabin that figures prominently in modern environmental history.

Bradley is 89 years old, and using a pair of hiking poles, she pointed out places where she has recorded the arrival of spring for the last 30 years. Her father, the famed ecologist and pioneer of wildlife management Aldo Leopold, had done the same before her.

But spring's advance has been so dramatic that if Leopold were alive today, he'd have to rewrite parts of his seminal book, "A Sand County Almanac."

Identifying as anthropogenic any of the changes remains a challenge.

The higher temperatures coincide with rising levels of carbon dioxide, most of which is the result of burning fossil fuels.

But there are additional explanations for the Earth's warming. Some scientists believe the natural cycles of cooling and the current warming trend have been overlooked.

Edward Hopkins, assistant state climatologist, believes that these natural cycles and rising levels of carbon dioxide are both at work.

"Nobody wants to say that we just don't know," Hopkins said. "I am willing to say that we don't know completely."

Today the majority of scientists believe that an increase of carbon dioxide in the atmosphere is the key reason for why the Earth has warmed so quickly in the past 50 years.

The article details a number of changes in seasonal rhythms, some of which, if continued, are scary. (It will take a few more snow days in April baseball to be evidence of a reverse trend, the way long-term observation works.)