The global food supply, as recent events have shown all too clearly, is threatened by many problems. Some of them are man-made; some are natural. The natural ones tend to be obvious—droughts, floods, hurricanes, earthquakes—and, in the past year alone, they have been notably devastating. Searing droughts in Australia and central Europe have squandered wheat supplies; more recently, Cyclone Nargis destroyed rice stocks for millions of people in Myanmar.

Historically, the damage to food supplies by bad weather has been regarded as fleeting: catastrophic in the short term but ultimately remitting. Droughts ease, floodwaters recede, and farmers replant their crops. But as a new government report indicates, such views are increasingly narrow and outdated, in that they fail to acknowledge the creeping reach of global climate change.

The report, released Tuesday, offers one of the most comprehensive looks yet at the impact that climate change is expected to have on U.S. agriculture over the next several decades. Not surprisingly, the prognosis is grim. Temperatures in the United States, scientists say, will rise on average by about 1.2 degrees Celsius by 2040, with carbon dioxide levels up more than 15 percent. The consequences for American-grown food, the report finds, will most likely be far-reaching: Some crop yields are predicted to drop; growing seasons will get longer and use more water; weeds and shrubs will grow faster and spread into new territory, some of it arable farmland; and insect and crop disease outbreaks will become more frequent.

The new report, which was produced by more than a dozen agencies over multiple years and reflects the findings of more than 1,000 scientific studies, offers only predictions, but the predictions reflect a high degree of confidence. In a sense, there is a vein of fatalism among most scientists about what will happen in the next few decades. Government actions, they say, may alter the trajectory of climate change 50 to 100 years from now, but the fate of climate change in the short term has been largely shaped by past behavior, by carbon already released into the atmosphere. The question now is the extent of its impact.

Some agricultural changes are already observable. In the central Great Plains, in states known for their grassy prairies and sprawling row crops, there are new neighbors: trees and large shrubs, often clustering in islands in the middle of fields. In the Southwest, perennial grasses have been largely pushed out by mesquite bushes, those long-rooted staples of the desert. And the invasive kudzu vine, formerly a nuisance only to the South, has advanced steadily northward, forming a staggered line stretching from Connecticut to Illinois. Human practices in all three cases have abetted the turnover, but climate change, scientists say, has been a primary driver, as invasive species reproduce more quickly and expand into areas once deemed too cold for their survival. In turn, high-quality pastureland, once ideal for livestock grazing, has become poor-quality brush, and farmland faces competitors for space.

In the next 30 years these problems will very likely expand and multiply, as an already taxed food system faces threats on multiple fronts. A rise in temperature—even as little as 1 degree Celsius—could cause many plantings to fail, the report indicates, since pollen and seeds are sensitive to slight temperature changes. Yields of corn and rice are expected to decline slightly. Heat-sensitive fruits and vegetables, such as tomatoes, will most likely suffer. Some of the potential damage will be blunted by higher carbon dioxide levels; soybean yields, for instance, will probably improve, because soybeans (and several other crops) thrive from higher carbon inputs. But if temperatures keep rising, the balance will ultimately tip: At some extreme temperature, cells stop dividing, and pollen dies.

High ozone levels, which have risen sixfold in the United States in the past century and are expected to rise further, will suppress yields as well. In fact, ozone levels are already extremely high in the eastern and midwestern regions of the country, rivaled globally only by eastern China (no model of air quality, to be sure) and parts of western Europe. One recent study, for instance, found that high ozone levels significantly suppress yields of soybean, wheat, and peanuts in the Midwest.

Eventually, the effects of climate change, far from being limited to individual plants, could percolate throughout entire ecosystems. If springs become warmer, as predicted, the crop-growing season will expand. Insects and pests, thriving in warmer winters, will reproduce more frequently and spread more rapidly. Many, in fact, are proliferating already, as reflected in reports of abnormally high rates of disease outbreaks in the western half of the United States. Higher temperatures also are usually accompanied by declining rainfall, threatening to slowly transform once lush areas into arid expanses. At the same time, droughts and heavy isolated rainfalls could become more numerous.