The changes are unfolding in ways that are likely to produce an uneven national map of harms and benefits, according to the report, released Tuesday and posted online at climatescience.gov.

The authors of the report and some independent experts said the main value of its projections was the level of detail and the high confidence in some conclusions. That confidence comes in part from the report’s emphasis on the next 25 to 50 years, when shifts in emissions are unlikely to make much of a difference in climate trends.

The report also reflects a recent, significant shift by the Bush administration on climate science. During Mr. Bush’s first term, administration officials worked to play down a national assessment of climate effects conducted mainly during the Clinton administration, but released in 2000.

The new report, which includes some findings that are more sobering and definitive than those in the 2000 climate report, holds the signatures of three cabinet secretaries.

According to the report, Western states will face substantial challenges because of growing demand for water and big projected drops in supplies.

From 2040 to 2060, anticipated water flows from rainfall in much of the West are likely to approach a 20 percent decrease in the average from 1901 to 1970, and are likely to be much lower in places like the fast-growing Southwest. In contrast, runoff in much of the Midwest and East is expected to increase that much or more.

Farmers, foresters and ranchers nationwide will face a complicated blend of changes, driven not only by shifting weather patterns but also by the simultaneous spread of nonnative plant and insect pests.

Some invasive grasses, vines and weeds, for example, do better in higher temperatures and carbon dioxide concentrations than do crops and preferred livestock forage plants.

Corn and soybean plants are likely to grow and mature faster, but will be more subject to crop failures from spikes in summer temperatures that can prevent pollination, said one of the authors, Jerry L. Hatfield, a plant physiologist with the , in a conference call with reporters.

David E. Schimel, a lead author and director of a federal system of ecological monitoring stations, said in the call that mitigating emissions in the long run was still important even though not much could be done to change the short-term climate picture.

The 203-page report, “The Effects of Climate Change on Agriculture, Land Resources, Water Resources and Biodiversity in the United States,” is a review of existing studies, including last year’s voluminous quartet of reports from the Intergovernmental Panel on Climate Change. It is part of a continuing assessment of lingering questions related to that was initiated in 2003 by Mr. Bush.

The report did not evaluate how the risk faced by farmers, water-supply managers and others might be reduced if they changed practices or crop and livestock varieties to adjust to changing conditions.

But several authors said that over all, the pace and nature of some of the looming changes would present big challenges in many of the country’s fastest-growing regions.

The West will not only face a dearth of water, but also large shifts in when it is available. Water supplies there will be transformed by midcentury, with mountain snows that provided a steady flow of runoff for irrigation and reservoirs dwindling. That flow will be replaced by rainfall that comes at times and in amounts that make it hard to manage, the report and authors said.

The report also emphasized that the country’s capacity to detect climate shifts and related effects was eroding, as budgets and plans for long-term monitoring of air, water and land changes — both on the ground and from satellites — shrank.

Richard Moss, a vice president of the World Wildlife Fund who previously coordinated federal climate reports under both the Clinton and Bush administrations, said the findings “highlight the urgency of the climate change problem” and provided important new support for action both to limit emissions and adapt to inevitable changes.

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