A federal committee has published a draft of the nation's third climate assessment report, a comprehensive analysis of the latest and best peer-reviewed science on the extent and impacts of global warming on the United States.

None of the body's findings are entirely new, but the report suggests that evidence is now stronger and clearer than ever that the climate is rapidly changing -- primarily as a result of human activities, including the copious burning of fossil fuels. Observed weather extremes are on the rise, and the possible connection between at least some of these events and human-induced climate change is also more strongly supported by the science.

The nation can expect increased impacts on everything from crops to fresh water supplies, and better and broader national plans for adaptation are needed, the assessment noted.

The draft report, which was prepared by the so-called National Climate Assessment Development Advisory Committee and written and amassed by a group of 240 scientists, will be subject to a three-month period of review and public comment.

"Climate change presents a major challenge for society," the committee's leadership said in a letter addressed to the American people. "This report and the sustained assessment process that is being developed represent steps forward in advancing our understanding of that challenge and its far-reaching implications for our nation and the world."

In an emailed statement, Gene Karpinski, the president of the League of Conservation Voters, said the report confirms what many Americans already know. "Hurricane Sandy and the historic droughts, floods and heat waves happening across the country aren't a fluke, but the result of a climate warming much faster than previously thought," he said. "If we put off action on climate change, the costs of addressing its impacts will only rise and this extreme weather will be just the beginning. This report should serve as a wake-up call that it's time to act."

The committee's letter continues:

Summers are longer and hotter, and periods of extreme heat last longer than any living American has ever experienced. Winters are generally shorter and warmer. Rain comes in heavier downpours, though in many regions there are longer dry spells in between.

Other changes are even more dramatic. Residents of some coastal cities see their streets flood more regularly during storms and high tides. Inland cities near large rivers also experience more flooding, especially in the Midwest and Northeast. Hotter and drier weather and earlier snow melt mean that wildfires in the West start earlier in the year, last later into the fall, threaten more homes, cause more evacuations, and burn more acreage. In Alaska, the summer sea ice that once protected the coasts has receded, and fall storms now cause more erosion and damage that is severe enough that some communities are already facing relocation. ...

These and other observed climatic changes are having wide-ranging impacts in every region of our country and most sectors of our economy. Some of these changes can be beneficial, such as longer growing seasons in many regions and a longer shipping season on the Great Lakes. But many more have already proven to be detrimental, largely because society and its infrastructure were designed for the climate of the past, not for the rapidly changing climate of the present or the future.

The report's roots can be traced to the The Global Change Research Act of 1990, which required that a national climate assessment be conducted every four years, with a report issued to the president and Congress. The legislation led to the formation of the U.S. Global Change Research Program, an inter-governmental body involving 13 federal agencies and departments, including the Departments of Commerce, Defense and Energy, as well as the Environmental Protection Agency and the National Science Foundation, among others.

The first such assessment was not published until 2000, however, and it was subsequently attacked by conservative groups who claimed that it exaggerated the climate threat. One group, the free-market Competitive Enterprise Institute, filed multiple lawsuits arguing that the findings were not subjected to federal guidelines for scientific research.

CEI settled its legal challenges with the Bush administration, which subsequently suppressed use of the report by other branches of the federal government in their implementation of policies.

The next full climate assessment was not published until 2009, after President Barack Obama took office.

"This draft report sends a warning to all of us," said Sen. Barbara Boxer, the California Democrat and chairwoman of the Environment and Public Works Committee, in an emailed statement. "We must act in a comprehensive fashion to reduce carbon pollution or expose our people and communities to continuing devastation from extreme weather events and their aftermath."